Linux Device Drivers, Second Edition

Chapter 13. mmap and DMA

This chapter delves into the area of Linux memory management, with an emphasis on techniques that are useful to the device driver writer. The material in this chapter is somewhat advanced, and not everybody will need a grasp of it. Nonetheless, many tasks can only be done through digging more deeply into the memory management subsystem; it also provides an interesting look into how an important part of the kernel works.

The material in this chapter is divided into three sections. The first covers the implementation of the mmap system call, which allows the mapping of device memory directly into a user process’s address space. We then cover the kernel kiobuf mechanism, which provides direct access to user memory from kernel space. The kiobuf system may be used to implement “raw I/O” for certain kinds of devices. The final section covers direct memory access (DMA) I/O operations, which essentially provide peripherals with direct access to system memory.

Of course, all of these techniques require an understanding of how Linux memory management works, so we start with an overview of that subsystem.

Memory Management in Linux

Rather than describing the theory of memory management in operating systems, this section tries to pinpoint the main features of the Linux implementation of the theory. Although you do not need to be a Linux virtual memory guru to implement mmap, a basic overview of how things work is useful. What follows is a fairly lengthy description of the data structures used by the kernel to manage memory. Once the necessary background has been covered, we can get into working with these structures.

Address Types

Linux is, of course, a virtual memory system, meaning that the addresses seen by user programs do not directly correspond to the physical addresses used by the hardware. Virtual memory introduces a layer of indirection, which allows a number of nice things. With virtual memory, programs running on the system can allocate far more memory than is physically available; indeed, even a single process can have a virtual address space larger than the system’s physical memory. Virtual memory also allows playing a number of tricks with the process’s address space, including mapping in device memory.

Thus far, we have talked about virtual and physical addresses, but a number of the details have been glossed over. The Linux system deals with several types of addresses, each with its own semantics. Unfortunately, the kernel code is not always very clear on exactly which type of address is being used in each situation, so the programmer must be careful.

Figure 13-1. Address types used in Linux

The following is a list of address types used in Linux. Figure 13-1 shows how these address types relate to physical memory.

User virtual addresses

These are the regular addresses seen by user-space programs. User addresses are either 32 or 64 bits in length, depending on the underlying hardware architecture, and each process has its own virtual address space.

Physical addresses

The addresses used between the processor and the system’s memory. Physical addresses are 32- or 64-bit quantities; even 32-bit systems can use 64-bit physical addresses in some situations.

Bus addresses

The addresses used between peripheral buses and memory. Often they are the same as the physical addresses used by the processor, but that is not necessarily the case. Bus addresses are highly architecture dependent, of course.

Kernel logical addresses

These make up the normal address space of the kernel. These addresses map most or all of main memory, and are often treated as if they were physical addresses. On most architectures, logical addresses and their associated physical addresses differ only by a constant offset. Logical addresses use the hardware’s native pointer size, and thus may be unable to address all of physical memory on heavily equipped 32-bit systems. Logical addresses are usually stored in variables of type unsigned long or void *. Memory returned from kmalloc has a logical address.

Kernel virtual addresses

These differ from logical addresses in that they do not necessarily have a direct mapping to physical addresses. All logical addresses are kernel virtual addresses; memory allocated by vmalloc also has a virtual address (but no direct physical mapping). The function kmap, described later in this chapter, also returns virtual addresses. Virtual addresses are usually stored in pointer variables.

If you have a logical address, the macro __pa() (defined in <asm/page.h>) will return its associated physical address. Physical addresses can be mapped back to logical addresses with __va(), but only for low-memory pages.

Different kernel functions require different types of addresses. It would be nice if there were different C types defined so that the required address type were explicit, but we have no such luck. In this chapter, we will be clear on which types of addresses are used where.

High and Low Memory

The difference between logical and kernel virtual addresses is highlighted on 32-bit systems that are equipped with large amounts of memory. With 32 bits, it is possible to address 4 GB of memory. Linux on 32-bit systems has, until recently, been limited to substantially less memory than that, however, because of the way it sets up the virtual address space. The system was unable to handle more memory than it could set up logical addresses for, since it needed directly mapped kernel addresses for all memory.

Recent developments have eliminated the limitations on memory, and 32-bit systems can now work with well over 4 GB of system memory (assuming, of course, that the processor itself can address that much memory). The limitation on how much memory can be directly mapped with logical addresses remains, however. Only the lowest portion of memory (up to 1 or 2 GB, depending on the hardware and the kernel configuration) has logical addresses; the rest (high memory) does not. High memory can require 64-bit physical addresses, and the kernel must set up explicit virtual address mappings to manipulate it. Thus, many kernel functions are limited to low memory only; high memory tends to be reserved for user-space process pages.

The term “high memory” can be confusing to some, especially since it has other meanings in the PC world. So, to make things clear, we’ll define the terms here:

Low memory

Memory for which logical addresses exist in kernel space. On almost every system you will likely encounter, all memory is low memory.

High memory

Memory for which logical addresses do not exist, because the system contains more physical memory than can be addressed with 32 bits.

On i386 systems, the boundary between low and high memory is usually set at just under 1 GB. This boundary is not related in any way to the old 640 KB limit found on the original PC. It is, instead, a limit set by the kernel itself as it splits the 32-bit address space between kernel and user space.

We will point out high-memory limitations as we come to them in this chapter.

The Memory Map and struct page

Historically, the kernel has used logical addresses to refer to explicit pages of memory. The addition of high-memory support, however, has exposed an obvious problem with that approach—logical addresses are not available for high memory. Thus kernel functions that deal with memory are increasingly using pointers to struct page instead. This data structure is used to keep track of just about everything the kernel needs to know about physical memory; there is one struct page for each physical page on the system. Some of the fields of this structure include the following:

atomic_t count;

The number of references there are to this page. When the count drops to zero, the page is returned to the free list.

wait_queue_head_t wait;

A list of processes waiting on this page. Processes can wait on a page when a kernel function has locked it for some reason; drivers need not normally worry about waiting on pages, though.

void *virtual;

The kernel virtual address of the page, if it is mapped; NULL, otherwise. Low-memory pages are always mapped; high-memory pages usually are not.

unsigned long flags;

A set of bit flags describing the status of the page. These include PG_locked, which indicates that the page has been locked in memory, and PG_reserved, which prevents the memory management system from working with the page at all.

There is much more information within struct page, but it is part of the deeper black magic of memory management and is not of concern to driver writers.

The kernel maintains one or more arrays of struct page entries, which track all of the physical memory on the system. On most systems, there is a single array, called mem_map. On some systems, however, the situation is more complicated. Nonuniform memory access (NUMA) systems and those with widely discontiguous physical memory may have more than one memory map array, so code that is meant to be portable should avoid direct access to the array whenever possible. Fortunately, it is usually quite easy to just work with struct page pointers without worrying about where they come from.

Some functions and macros are defined for translating between struct page pointers and virtual addresses:

struct page *virt_to_page(void *kaddr);

This macro, defined in <asm/page.h>, takes a kernel logical address and returns its associated struct page pointer. Since it requires a logical address, it will not work with memory from vmalloc or high memory.

void *page_address(struct page *page);

Returns the kernel virtual address of this page, if such an address exists. For high memory, that address exists only if the page has been mapped.

#include <linux/highmem.h> , void *kmap(struct page *page); , void kunmap(struct page *page);

kmap returns a kernel virtual address for any page in the system. For low-memory pages, it just returns the logical address of the page; for high-memory pages, kmap creates a special mapping. Mappings created with kmap should always be freed with kunmap; a limited number of such mappings is available, so it is better not to hold on to them for too long. kmap calls are additive, so if two or more functions both call kmap on the same page the right thing happens. Note also that kmap can sleep if no mappings are available.

We will see some uses of these functions when we get into the example code later in this chapter.

Page Tables

When a program looks up a virtual address, the CPU must convert the address to a physical address in order to access physical memory. The step is usually performed by splitting the address into bitfields. Each bitfield is used as an index into an array, called a page table, to retrieve either the address of the next table or the address of the physical page that holds the virtual address.

The Linux kernel manages three levels of page tables in order to map virtual addresses to physical addresses. The multiple levels allow the memory range to be sparsely populated; modern systems will spread a process out across a large range of virtual memory. It makes sense to do things that way; it allows for runtime flexibility in how things are laid out.

Note that Linux uses a three-level system even on hardware that only supports two levels of page tables or hardware that uses a different way to map virtual addresses to physical ones. The use of three levels in a processor-independent implementation allows Linux to support both two-level and three-level processors without clobbering the code with a lot of #ifdef statements. This kind of conservative coding doesn’t lead to additional overhead when the kernel runs on two-level processors, because the compiler actually optimizes out the unused level.

It is time to take a look at the data structures used to implement the paging system. The following list summarizes the implementation of the three levels in Linux, and Figure 13-2 depicts them.

Figure 13-2. The three levels of Linux page tables

Page Directory (PGD)

The top-level page table. The PGD is an array of pgd_t items, each of which points to a second-level page table. Each process has its own page directory, and there is one for kernel space as well. You can think of the page directory as a page-aligned array of pgd_ts.

Page mid-level Directory (PMD)

The second-level table. The PMD is a page-aligned array of pmd_t items. A pmd_t is a pointer to the third-level page table. Two-level processors have no physical PMD; they declare their PMD as an array with a single element, whose value is the PMD itself—we’ll see in a while how this is handled in C and how the compiler optimizes this level away.

Page Table

A page-aligned array of items, each of which is called a Page Table Entry. The kernel uses the pte_t type for the items. A pte_t contains the physical address of the data page.

The types introduced in this list are defined in <asm/page.h>, which must be included by every source file that plays with paging.

The kernel doesn’t need to worry about doing page-table lookups during normal program execution, because they are done by the hardware. Nonetheless, the kernel must arrange things so that the hardware can do its work. It must build the page tables and look them up whenever the processor reports a page fault, that is, whenever the page associated with a virtual address needed by the processor is not present in memory. Device drivers, too, must be able to build page tables and handle faults when implementing mmap.

It’s interesting to note how software memory management exploits the same page tables that are used by the CPU itself. Whenever a CPU doesn’t implement page tables, the difference is only hidden in the lowest levels of architecture-specific code. In Linux memory management, therefore, you always talk about three-level page tables irrespective of whether they are known to the hardware or not. An example of a CPU family that doesn’t use page tables is the PowerPC. PowerPC designers implemented a hash algorithm that maps virtual addresses into a one-level page table. When accessing a page that is already in memory but whose physical address has expired from the CPU caches, the CPU needs to read memory only once, as opposed to the two or three accesses required by a multilevel page table approach. The hash algorithm, like multilevel tables, makes it possible to reduce use of memory in mapping virtual addresses to physical ones.

Irrespective of the mechanisms used by the CPU, the Linux software implementation is based on three-level page tables, and the following symbols are used to access them. Both <asm/page.h> and <asm/pgtable.h> must be included for all of them to be accessible.

PTRS_PER_PGD , PTRS_PER_PMD , PTRS_PER_PTE

The size of each table. Two-level processors set PTRS_PER_PMD to 1, to avoid dealing with the middle level.

unsigned pgd_val(pgd_t pgd) , unsigned pmd_val(pmd_t pmd) , unsigned pte_val(pte_t pte)

These three macros are used to retrieve the unsigned value from the typed data item. The actual type used varies depending on the underlying architecture and kernel configuration options; it is usually either unsigned long or, on 32-bit processors supporting high memory, unsigned long long. SPARC64 processors use unsigned int. The macros help in using strict data typing in source code without introducing computational overhead.

pgd_t * pgd_offset(struct mm_struct * mm, unsigned long address) , pmd_t * pmd_offset(pgd_t * dir, unsigned long address) , pte_t * pte_offset(pmd_t * dir, unsigned long address)

These inline functions^[50] are used to retrieve the pgd, pmd, and pte entries associated with address. Page-table lookup begins with a pointer to struct mm_struct. The pointer associated with the memory map of the current process is current->mm, while the pointer to kernel space is described by &init_mm. Two-level processors define pmd_offset(dir,add) as (pmd_t *)dir, thus folding the pmd over the pgd. Functions that scan page tables are always declared as inline, and the compiler optimizes out any pmd lookup.

struct page *pte_page(pte_t pte)

This function returns a pointer to the struct page entry for the page in this page-table entry. Code that deals with page-tables will generally want to use pte_page rather than pte_val, since pte_page deals with the processor-dependent format of the page-table entry and returns the struct page pointer, which is usually what’s needed.

pte_present(pte_t pte)

This macro returns a boolean value that indicates whether the data page is currently in memory. This is the most used of several functions that access the low bits in the pte—the bits that are discarded by pte_page. Pages may be absent, of course, if the kernel has swapped them to disk (or if they have never been loaded). The page tables themselves, however, are always present in the current Linux implementation. Keeping page tables in memory simplifies the kernel code because pgd_offset and friends never fail; on the other hand, even a process with a “resident storage size” of zero keeps its page tables in real RAM, wasting some memory that might be better used elsewhere.

Each process in the system has a struct mm_struct structure, which contains its page tables and a great many other things. It also contains a spinlock called page_table_lock, which should be held while traversing or modifying the page tables.

Just seeing the list of these functions is not enough for you to be proficient in the Linux memory management algorithms; real memory management is much more complex and must deal with other complications, like cache coherence. The previous list should nonetheless be sufficient to give you a feel for how page management is implemented; it is also about all that you will need to know, as a device driver writer, to work occasionally with page tables. You can get more information from the include/asm and mm subtrees of the kernel source.

Virtual Memory Areas

Although paging sits at the lowest level of memory management, something more is necessary before you can use the computer’s resources efficiently. The kernel needs a higher-level mechanism to handle the way a process sees its memory. This mechanism is implemented in Linux by means of virtual memory areas, which are typically referred to as areas or VMAs.

An area is a homogeneous region in the virtual memory of a process, a contiguous range of addresses with the same permission flags. It corresponds loosely to the concept of a “segment,” although it is better described as “a memory object with its own properties.” The memory map of a process is made up of the following:

• An area for the program’s executable code (often called text).

• One area each for data, including initialized data (that which has an explicitly assigned value at the beginning of execution), uninitialized data (BSS),^[51] and the program stack.

• One area for each active memory mapping.

The memory areas of a process can be seen by looking in /proc/ pid /maps (where pid, of course, is replaced by a process ID). /proc/self is a special case of /proc/ pid, because it always refers to the current process. As an example, here are a couple of memory maps, to which we have added short comments after a sharp sign:

morgana.root#cat /proc/1/maps   # look at init
08048000-0804e000 r-xp 00000000 08:01 51297      /sbin/init  # text
0804e000-08050000 rw-p 00005000 08:01 51297      /sbin/init  # data
08050000-08054000 rwxp 00000000 00:00 0          # zero-mapped bss
40000000-40013000 r-xp 00000000 08:01 39003      /lib/ld-2.1.3.so # text
40013000-40014000 rw-p 00012000 08:01 39003      /lib/ld-2.1.3.so # data
40014000-40015000 rw-p 00000000 00:00 0          # bss for ld.so
4001b000-40108000 r-xp 00000000 08:01 39006      /lib/libc-2.1.3.so # text
40108000-4010c000 rw-p 000ec000 08:01 39006      /lib/libc-2.1.3.so # data
4010c000-40110000 rw-p 00000000 00:00 0          # bss for libc.so
bfffe000-c0000000 rwxp fffff000 00:00 0          # zero-mapped stack

morgana.root# rsh wolf head /proc/self/maps  #### alpha-axp: static ecoff
000000011fffe000-0000000120000000 rwxp 0000000000000000 00:00 0     # stack
0000000120000000-0000000120014000 r-xp 0000000000000000 08:03 2844  # text
0000000140000000-0000000140002000 rwxp 0000000000014000 08:03 2844  # data
0000000140002000-0000000140008000 rwxp 0000000000000000 00:00 0     # bss

The fields in each line are as follows:

start-end perm offset major:minor inode image.

Each field in /proc/*/maps (except the image name) corresponds to a field in struct vm_area_struct, and is described in the following list.

start , end

The beginning and ending virtual addresses for this memory area.

perm

A bit mask with the memory area’s read, write, and execute permissions. This field describes what the process is allowed to do with pages belonging to the area. The last character in the field is either p for “private” or s for “shared.”

offset

Where the memory area begins in the file that it is mapped to. An offset of zero, of course, means that the first page of the memory area corresponds to the first page of the file.

major , minor

The major and minor numbers of the device holding the file that has been mapped. Confusingly, for device mappings, the major and minor numbers refer to the disk partition holding the device special file that was opened by the user, and not the device itself.

inode

The inode number of the mapped file.

image

The name of the file (usually an executable image) that has been mapped.

A driver that implements the mmap method needs to fill a VMA structure in the address space of the process mapping the device. The driver writer should therefore have at least a minimal understanding of VMAs in order to use them.

Let’s look at the most important fields in struct vm_area_struct (defined in <linux/mm.h>). These fields may be used by device drivers in their mmap implementation. Note that the kernel maintains lists and trees of VMAs to optimize area lookup, and several fields of vm_area_struct are used to maintain this organization. VMAs thus can’t be created at will by a driver, or the structures will break. The main fields of VMAs are as follows (note the similarity between these fields and the /proc output we just saw):

unsigned long vm_start; , unsigned long vm_end;

The virtual address range covered by this VMA. These fields are the first two fields shown in /proc/*/maps.

struct file *vm_file;

A pointer to the struct file structure associated with this area (if any).

unsigned long vm_pgoff;

The offset of the area in the file, in pages. When a file or device is mapped, this is the file position of the first page mapped in this area.

unsigned long vm_flags;

A set of flags describing this area. The flags of the most interest to device driver writers are VM_IO and VM_RESERVED. VM_IO marks a VMA as being a memory-mapped I/O region. Among other things, the VM_IO flag will prevent the region from being included in process core dumps. VM_RESERVED tells the memory management system not to attempt to swap out this VMA; it should be set in most device mappings.

struct vm_operations_struct *vm_ops;

A set of functions that the kernel may invoke to operate on this memory area. Its presence indicates that the memory area is a kernel “object” like the struct file we have been using throughout the book.

void *vm_private_data;

A field that may be used by the driver to store its own information.

Like struct vm_area_struct, the vm_operations_struct is defined in <linux/mm.h>; it includes the operations listed next. These operations are the only ones needed to handle the process’s memory needs, and they are listed in the order they are declared. Later in this chapter, some of these functions will be implemented; they will be described more completely at that point.

void (*open)(struct vm_area_struct *vma);

The open method is called by the kernel to allow the subsystem implementing the VMA to initialize the area, adjust reference counts, and so forth. This method will be invoked any time that a new reference to the VMA is made (when a process forks, for example). The one exception happens when the VMA is first created by mmap; in this case, the driver’s mmap method is called instead.

void (*close)(struct vm_area_struct *vma);

When an area is destroyed, the kernel calls its close operation. Note that there’s no usage count associated with VMAs; the area is opened and closed exactly once by each process that uses it.

void (*unmap)(struct vm_area_struct *vma, unsigned long addr, size_t len);

The kernel calls this method to “unmap” part or all of an area. If the entire area is unmapped, then the kernel calls vm_ops->close as soon as vm_ops->unmap returns.

void (*protect)(struct vm_area_struct *vma, unsigned long, size_t, unsigned int newprot);

This method is intended to change the protection on a memory area, but is currently not used. Memory protection is handled by the page tables, and the kernel sets up the page-table entries separately.

int (*sync)(struct vm_area_struct *vma, unsigned long, size_t, unsigned int flags);

This method is called by the msync system call to save a dirty memory region to the storage medium. The return value is expected to be 0 to indicate success and negative if there was an error.

struct page *(*nopage)(struct vm_area_struct *vma, unsigned long address, int write_access);

When a process tries to access a page that belongs to a valid VMA, but that is currently not in memory, the nopage method is called (if it is defined) for the related area. The method returns the struct page pointer for the physical page, after, perhaps, having read it in from secondary storage. If the nopage method isn’t defined for the area, an empty page is allocated by the kernel. The third argument, write_access, counts as “no-share”: a nonzero value means the page must be owned by the current process, whereas 0 means that sharing is possible.

struct page *(*wppage)(struct vm_area_struct *vma, unsigned long address, struct page *page);

This method handles write-protected page faults but is currently unused. The kernel handles attempts to write over a protected page without invoking the area-specific callback. Write-protect faults are used to implement copy-on-write. A private page can be shared across processes until one process writes to it. When that happens, the page is cloned, and the process writes on its own copy of the page. If the whole area is marked as read-only, a SIGSEGV is sent to the process, and the copy-on-write is not performed.

int (*swapout)(struct page *page, struct file *file);

This method is called when a page is selected to be swapped out. A return value of 0 signals success; any other value signals an error. In case of error, the process owning the page is sent a SIGBUS. It is highly unlikely that a driver will ever need to implement swapout; device mappings are not something that the kernel can just write to disk.

That concludes our overview of Linux memory management data structures. With that out of the way, we can now proceed to the implementation of the mmap system call.

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^[50] On 32-bit SPARC processors, the functions are not inline but rather real extern functions, which are not exported to modularized code. Therefore you won’t be able to use these functions in a module running on the SPARC, but you won’t usually need to.

^[51] The name BSS is a historical relic, from an old assembly operator meaning “Block started by symbol.” The BSS segment of executable files isn’t stored on disk, and the kernel maps the zero page to the BSS address range.