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Re: pci_* interface

To: linux-origin@xxxxxxxxxxx
Subject: Re: pci_* interface
From: "Leo Dagum" <dagum@xxxxxxxxxxxxxxxxxxx>
Date: Mon, 31 Jan 2000 11:08:17 -0800
In-reply-to: Len Widra <len@xxxxxxx> "Re: pci_* interface" (Jan 31, 11:00am)
References: <20000131193615.I12102@xxxxxxxxxxxxxx> <3895DBC2.89FDF88E@xxxxxxx>
Sender: owner-linux-origin@xxxxxxxxxxx
On Jan 31, 11:00am, Len Widra wrote:
> Subject: Re: pci_* interface
> > Seems others also aren't really hapy with the interface.  On Friday
> I just joined this mail list.  Who's not happy with what?

People are unhappy with the dynamic DMA mapping interface proposed and
included in release 2.3.41.  Below is a document outlining the interface.
(Apologies to the people who've gotten this 3 times already from me..)

- leo

patchex: extracting file DMA-mapping.txt from patch-2.3.41 to stdout
diff -u --recursive --new-file v2.3.40/linux/Documentation/DMA-mapping.txt
--- v2.3.40/linux/Documentation/DMA-mapping.txt Wed Dec 31 16:00:00 1969
--- v2.3.40/linux/Documentation/DMA-mapping.txt Wed Dec 31 16:00:00 1969
++ linux/Documentation/DMA-mapping.txt  Thu Jan 27 08:58:15 2000
@@ -0,0 1,143 @@
                        Dynamic DMA mapping

                 David S. Miller <davem@xxxxxxxxxx>
                 Richard Henderson <rth@xxxxxxxxxx>
                  Jakub Jelinek <jakub@xxxxxxxxxx>

Most of the 64bit platforms have special hardware that translates bus
addresses (DMA addresses) to physical addresses similarly to how page
tables and/or TLB translate virtual addresses to physical addresses.
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

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 defines a dma_addr_t type which should be
used everywhere you hold a DMA (bus) address returned from the DMA mapping

There are two types of DMA mappings:
- static DMA mappings which are usually mapped at driver initialization,
  unmapped at the end and for which the hardware should not assume
  sequential accesses (from both the DMA engine in the card and CPU).
- 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.

To allocate and map a static DMA region, 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 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.
This routine will allocate RAM for that region, so it acts similarly to
__get_free_pages (but takes size instead of page order).
It 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 return address is guaranteed to be page aligned.

To unmap and free such 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.

The streaming DMA mapping routines can be called from interrupt context.
There are two versions of each map/unmap, one which map/unmap a single
memory region, one which map/unmap a scatterlist.

To map a single region, you do:

        dma_addr_t dma_handle;

        dma_handle = pci_map_single(dev, addr, size);

and to unmap it:

        pci_unmap_single(dev, dma_handle, size);

You should call pci_unmap_single when the DMA activity is finished, e.g.
from interrupt which told you the DMA transfer is done.

Similarly with scatterlists, you map a region gathered from several regions by:

        int i, count = pci_map_sg(dev, sglist, nents);
        struct scatterlist *sg;

        for (i = 0, sg = sglist; i < count; i++, sg++) {
                hw_address[i] = sg_dma_address(sg);
                hw_len[i] = sg_dma_len(sg);

where nents is the number of entries in the sglist.
The implementation is free to merge several consecutive sglist entries
into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
consecutive sglist entries can be merged into one provided the first one
ends and the second one starts on a page boundary - in fact this is a huge
advantage for cards which either cannot do scatter-gather or have very
limited number of scatter-gather entries) and returns the actual number
of sg entries it mapped them too.
Then you should loop count times (note: this can be less than nents times)
and use sg_dma_address() and sg_dma_length() macros where you previously
accessed sg->address and sg->length as shown above.

To unmap a scatterlist, just call:

        pci_unmap_sg(dev, sglist, nents);

Again, make sure DMA activity finished.
Every pci_map_{single,sg} call should have its pci_unmap_{single,sg}
counterpart, because the bus address space is a shared resource (although
in some ports the mapping is per each BUS so less devices contend for the
same bus address space) and you could render the machine unusable by eating
all bus addresses.

If you need to use the same streaming DMA region multiple times and touch
the data in between the DMA transfers, just map it
with pci_map_{single,sg}, after each DMA transfer call either:

        pci_dma_sync_single(dev, dma_handle, size);


        pci_dma_sync_sg(dev, sglist, nents);

and after the last DMA transfer call one of the DMA unmap routines
pci_unmap_{single,sg}. If you don't touch the data from the first pci_map_*
call till pci_unmap_*, then you don't have to call pci_sync_* routines.

Drivers converted fully to this interface should not use virt_to_bus any
longer, nor should they use bus_to_virt. Some drivers have to be changed a
little bit, because there is no longer an equivalent to bus_to_virt in the
dynamic DMA mapping scheme - you have to always store the DMA addresses
returned by the pci_alloc_consistent and pci_map_single calls (pci_map_sg
stores them in the scatterlist itself if the platform supports dynamic DMA
mapping in hardware) in your driver structures and/or in the card registers.

For PCI cards which recognize fewer address lines than 32 in Single
Address Cycle, you should set corresponding pci_dev's dma_mask field to a
different mask. The dma mapping routines then should either honour your request
and allocate the DMA only with the bus address with bits set in your
dma_mask or should complain that the device is not supported on that platform.

Leo Dagum    SGI  Mountain View, CA 94043 (650-933-2179)

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