dma-mapping.txt

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			Dynamic DMA mapping
			===================

		 David S. Miller <[e-mail]>
		 Richard Henderson <[e-mail]>
		  Jakub Jelinek <[e-mail]>

Most of the 64bit platforms have special hardware that translates bus
addresses (DMA addresses) into physical addresses.  This is similar to
how page tables and/or a TLB translates virtual addresses to physical
addresses on a cpu.  This is needed so that e.g. PCI devices can
access with a Single Address Cycle (32bit DMA address) any page in the
64bit physical address space.  Previously in Linux those 64bit
platforms had to set artificial limits on the maximum RAM size in the
system, so that the virt_to_bus() static scheme works (the DMA address
translation tables were simply filled on bootup to map each bus
address to the physical page __pa(bus_to_virt())).

So that Linux can use the dynamic DMA mapping, it needs some help from the
drivers, namely it has to take into account that DMA addresses should be
mapped only for the time they are actually used and unmapped after the DMA
transfer.

The following API will work of course even on platforms where no such
hardware exists, see e.g. include/asm-i386/pci.h for how it is implemented on
top of the virt_to_bus interface.

First of all, you should make sure

#include <linux/pci.h>

is in your driver. This file will obtain for you the definition of the
dma_addr_t (which can hold any valid DMA address for the platform)
type which should be used everywhere you hold a DMA (bus) address
returned from the DMA mapping functions.

			 What memory is DMA'able?

The first piece of information you must know is what kernel memory can
be used with the DMA mapping facilitites.  There has been an unwritten
set of rules regarding this, and this text is an attempt to finally
write them down.

If you acquired your memory via the page allocator
(i.e. __get_free_page*()) or the generic memory allocators
(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
that memory using the addresses returned from those routines.

This means specifically that you may _not_ use the memory/addresses
returned from vmalloc() for DMA.  It is possible to DMA to the
_underlying_ memory mapped into a vmalloc() area, but this requires
walking page tables to get the physical addresses, and then
translating each of those pages back to a kernel address using
something like __va().  [ EDIT: Update this when we integrate
Gerd Knorr's generic code which does this. ]

This rule also means that you may not use kernel image addresses
(ie. items in the kernel's data/text/bss segment, or your driver's)
nor may you use kernel stack addresses for DMA.  Both of these items
might be mapped somewhere entirely different than the rest of physical
memory.

Also, this means that you cannot take the return of a kmap()
call and DMA to/from that.  This is similar to vmalloc().

What about block I/O and networking buffers?  The block I/O and
networking subsystems make sure that the buffers they use are valid
for you to DMA from/to.

			DMA addressing limitations

Does your device have any DMA addressing limitations?  For example, is
your device only capable of driving the low order 24-bits of address
on the PCI bus for SAC DMA transfers?  If so, you need to inform the
PCI layer of this fact.

By default, the kernel assumes that your device can address the full
32-bits in a SAC cycle.  For a 64-bit DAC capable device, this needs
to be increased.  And for a device with limitations, as discussed in
the previous paragraph, it needs to be decreased.

For correct operation, you must interrogate the PCI layer in your
device probe routine to see if the PCI controller on the machine can
properly support the DMA addressing limitation your device has.  It is
good style to do this even if your device holds the default setting,
because this shows that you did think about these issues wrt. your
device.

The query is performed via a call to pci_set_dma_mask():

	int pci_set_dma_mask(struct pci_dev *pdev, u64 device_mask);

Here, pdev is a pointer to the PCI device struct of your device, and
device_mask is a bit mask describing which bits of a PCI address your
device supports.  It returns zero if your card can perform DMA
properly on the machine given the address mask you provided.

If it returns non-zero, your device can not perform DMA properly on
this platform, and attempting to do so will result in undefined
behavior.  You must either use a different mask, or not use DMA.

This means that in the failure case, you have three options:

1) Use another DMA mask, if possible (see below).
2) Use some non-DMA mode for data transfer, if possible.
3) Ignore this device and do not initialize it.

It is recommended that your driver print a kernel KERN_WARNING message
when you end up performing either #2 or #2.  In this manner, if a user
of your driver reports that performance is bad or that the device is not
even detected, you can ask them for the kernel messages to find out
exactly why.

The standard 32-bit addressing PCI device would do something like
this:

	if (pci_set_dma_mask(pdev, 0xffffffff)) {
		printk(KERN_WARNING
		       "mydev: No suitable DMA available.\n");
		goto ignore_this_device;
	}

Another common scenario is a 64-bit capable device.  The approach
here is to try for 64-bit DAC addressing, but back down to a
32-bit mask should that fail.  The PCI platform code may fail the
64-bit mask not because the platform is not capable of 64-bit
addressing.  Rather, it may fail in this case simply because
32-bit SAC addressing is done more efficiently than DAC addressing.
Sparc64 is one platform which behaves in this way.

Here is how you would handle a 64-bit capable device which can drive
all 64-bits during a DAC cycle:

	int using_dac;

	if (!pci_set_dma_mask(pdev, 0xffffffffffffffff)) {
		using_dac = 1;
	} else if (!pci_set_dma_mask(pdev, 0xffffffff)) {
		using_dac = 0;
	} else {
		printk(KERN_WARNING
		       "mydev: No suitable DMA available.\n");
		goto ignore_this_device;
	}

If your 64-bit device is going to be an enormous consumer of DMA
mappings, this can be problematic since the DMA mappings are a
finite resource on many platforms.  Please see the "DAC Addressing
for Address Space Hungry Devices" setion near the end of this
document for how to handle this case.

Finally, if your device can only drive the low 24-bits of
address during PCI bus mastering you might do something like:

	if (pci_set_dma_mask(pdev, 0x00ffffff)) {
		printk(KERN_WARNING
		       "mydev: 24-bit DMA addressing not available.\n");
		goto ignore_this_device;
	}

When pci_set_dma_mask() is successful, and returns zero, the PCI layer
saves away this mask you have provided.  The PCI layer will use this
information later when you make DMA mappings.

There is a case which we are aware of at this time, which is worth
mentioning in this documentation.  If your device supports multiple
functions (for example a sound card provides playback and record
functions) and the various different functions have _different_
DMA addressing limitations, you may wish to probe each mask and
only provide the functionality which the machine can handle.  It
is important that the last call to pci_set_dma_mask() be for the 
most specific mask.

Here is pseudo-code showing how this might be done:

	#define PLAYBACK_ADDRESS_BITS	0xffffffff
	#define RECORD_ADDRESS_BITS	0x00ffffff

	struct my_sound_card *card;
	struct pci_dev *pdev;

	...
	if (pci_set_dma_mask(pdev, PLAYBACK_ADDRESS_BITS)) {
		card->playback_enabled = 1;
	} else {
		card->playback_enabled = 0;
		printk(KERN_WARN "%s: Playback disabled due to DMA limitations.\n",
		       card->name);
	}
	if (pci_set_dma_mask(pdev, RECORD_ADDRESS_BITS)) {
		card->record_enabled = 1;
	} else {
		card->record_enabled = 0;
		printk(KERN_WARN "%s: Record disabled due to DMA limitations.\n",
		       card->name);
	}

A sound card was used as an example here because this genre of PCI
devices seems to be littered with ISA chips given a PCI front end,
and thus retaining the 16MB DMA addressing limitations of ISA.

			Types of DMA mappings

There are two types of DMA mappings:

- Consistent DMA mappings which are usually mapped at driver
  initialization, unmapped at the end and for which the hardware should
  guarantee that the device and the cpu can access the data
  in parallel and will see updates made by each other without any
  explicit software flushing.

  Think of "consistent" as "synchronous" or "coherent".

  Consistent DMA mappings are always SAC addressable.  That is
  to say, consistent DMA addresses given to the driver will always
  be in the low 32-bits of the PCI bus space.

  Good examples of what to use consistent mappings for are:

	- Network card DMA ring descriptors.
	- SCSI adapter mailbox command data structures.
	- Device firmware microcode executed out of
	  main memory.

  The invariant these examples all require is that any cpu store
  to memory is immediately visible to the device, and vice
  versa.  Consistent mappings guarantee this.

  IMPORTANT: Consistent DMA memory does not preclude the usage of
             proper memory barriers.  The cpu may reorder stores to
	     consistent memory just as it may normal memory.  Example:
	     if it is important for the device to see the first word
	     of a descriptor updated before the second, you must do
	     something like:

		desc->word0 = address;
		wmb();
		desc->word1 = DESC_VALID;

             in order to get correct behavior on all platforms.

- Streaming DMA mappings which are usually mapped for one DMA transfer,
  unmapped right after it (unless you use pci_dma_sync below) and for which
  hardware can optimize for sequential accesses.

  This of "streaming" as "asynchronous" or "outside the coherency
  domain".

  Good examples of what to use streaming mappings for are:

	- Networking buffers transmitted/received by a device.
	- Filesystem buffers written/read by a SCSI device.

  The interfaces for using this type of mapping were designed in
  such a way that an implementation can make whatever performance
  optimizations the hardware allows.  To this end, when using
  such mappings you must be explicit about what you want to happen.

Neither type of DMA mapping has alignment restrictions that come
from PCI, although some devices may have such restrictions.

		 Using Consistent DMA mappings.

To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
you should do:

	dma_addr_t dma_handle;

	cpu_addr = pci_alloc_consistent(dev, size, &dma_handle);

where dev is a struct pci_dev *. You should pass NULL for PCI like buses
where devices don't have struct pci_dev (like ISA, EISA).  This may be
called in interrupt context. 

This argument is needed because the DMA translations may be bus
specific (and often is private to the bus which the device is attached
to).

Size is the length of the region you want to allocate, in bytes.

This routine will allocate RAM for that region, so it acts similarly to
__get_free_pages (but takes size instead of a page order).  If your
driver needs regions sized smaller than a page, you may prefer using
the pci_pool interface, described below.

The consistent DMA mapping interfaces, for non-NULL dev, will always
return a DMA address which is SAC (Single Address Cycle) addressible.
Even if the device indicates (via PCI dma mask) that it may address
the upper 32-bits and thus perform DAC cycles, consistent allocation
will still only return 32-bit PCI addresses for DMA.  This is true
of the pci_pool interface as well.

In fact, as mentioned above, all consistent memory provided by the
kernel DMA APIs are always SAC addressable.

pci_alloc_consistent returns two values: the virtual address which you
can use to access it from the CPU and dma_handle which you pass to the
card.

The cpu return address and the DMA bus master address are both
guaranteed to be aligned to the smallest PAGE_SIZE order which
is greater than or equal to the requested size.  This invariant
exists (for example) to guarantee that if you allocate a chunk
which is smaller than or equal to 64 kilobytes, the extent of the
buffer you receive will not cross a 64K boundary.

To unmap and free such a DMA region, you call:

	pci_free_consistent(dev, size, cpu_addr, dma_handle);

where dev, size are the same as in the above call and cpu_addr and
dma_handle are the values pci_alloc_consistent returned to you.
This function may not be called in interrupt context.

If your driver needs lots of smaller memory regions, you can write
custom code to subdivide pages returned by pci_alloc_consistent,
or you can use the pci_pool API to do that.  A pci_pool is like
a kmem_cache, but it uses pci_alloc_consistent not __get_free_pages.
Also, it understands common hardware constraints for alignment,
like queue heads needing to be aligned on N byte boundaries.

Create a pci_pool like this:

	struct pci_pool *pool;

	pool = pci_pool_create(name, dev, size, align, alloc, flags);

The "name" is for diagnostics (like a kmem_cache name); dev and size
are as above.  The device's hardware alignment requirement for this
type of data is "align" (which is expressed in bytes, and must be a
power of two).  The flags are SLAB_ flags as you'd pass to
kmem_cache_create.  Not all flags are understood, but SLAB_POISON may
help you find driver bugs.  If you call this in a non- sleeping
context (f.e. in_interrupt is true or while holding SMP locks), pass
SLAB_ATOMIC.  If your device has no boundary crossing restrictions,
pass 0 for alloc; passing 4096 says memory allocated from this pool
must not cross 4KByte boundaries (but at that time it may be better to
go for pci_alloc_consistent directly instead).

Allocate memory from a pci pool like this:

	cpu_addr = pci_pool_alloc(pool, flags, &dma_handle);

flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
holding SMP locks), SLAB_ATOMIC otherwise.  Like pci_alloc_consistent,
this returns two values, cpu_addr and dma_handle.

Free memory that was allocated from a pci_pool like this:

	pci_pool_free(pool, cpu_addr, dma_handle);

where pool is what you passed to pci_pool_alloc, and cpu_addr and
dma_handle are the values pci_pool_alloc returned. This function
may be called in interrupt context.

Destroy a pci_pool by calling:

	pci_pool_destroy(pool);

Make sure you've called pci_pool_free for all memory allocated
from a pool before you destroy the pool. This function may not
be called in interrupt context.

			DMA Direction

The interfaces described in subsequent portions of this document
take a DMA direction argument, which is an integer and takes on
one of the following values:

 PCI_DMA_BIDIRECTIONAL
 PCI_DMA_TODEVICE
 PCI_DMA_FROMDEVICE
 PCI_DMA_NONE

One should provide the exact DMA direction if you know it.

PCI_DMA_TODEVICE means "from main memory to the PCI device"
PCI_DMA_FROMDEVICE means "from the PCI device to main memory"
It is the direction in which the data moves during the DMA
transfer.

You are _strongly_ encouraged to specify this as precisely
as you possibly can.

If you absolutely cannot know the direction of the DMA transfer,
specify PCI_DMA_BIDIRECTIONAL.  It means that the DMA can go in
either direction.  The platform guarantees that you may legally
specify this, and that it will work, but this may be at the
cost of performance for example.

The value PCI_DMA_NONE is to be used for debugging.  One can
hold this in a data structure before you come to know the
precise direction, and this will help catch cases where your
direction tracking logic has failed to set things up properly.

Another advantage of specifying this value precisely (outside of
potential platform-specific optimizations of such) is for debugging.
Some platforms actually have a write permission boolean which DMA
mappings can be marked with, much like page protections in the user
program address space.  Such platforms can and do report errors in the