Re: [Linux-c6x-dev] [PATCH 3/9] c6x: Provide dma_mmap_coherent() and dma_get_sgtable()

[Geert Uytterhoeven <geert@linux-m68k.org>]

On Tue, Jan 15, 2013 at 5:56 PM, James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
> On Tue, 2013-01-15 at 15:07 +0100, Marek Szyprowski wrote:
>> On 1/15/2013 10:13 AM, Geert Uytterhoeven wrote:
>> > Marek?
>> >
>> > On Tue, Jan 15, 2013 at 5:16 AM, Vineet Gupta
>> > <Vineet.Gupta1@synopsys.com> wrote:
>> > > On Monday 14 January 2013 09:07 PM, Mark Salter wrote:
>> > >> On Sun, 2013-01-13 at 11:44 +0100, Geert Uytterhoeven wrote:
>> > >>> c6x/allmodconfig (assumed):
>> > >>>
>> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c: In function ‘vb2_dc_mmap’:
>> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c:204: error: implicit declaration of function ‘dma_mmap_coherent’
>> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c: In function ‘vb2_dc_get_base_sgt’:
>> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c:387: error: implicit declaration of function ‘dma_get_sgtable’
>> > >>>
>> > >>> For architectures using dma_map_ops, dma_mmap_coherent() and
>> > >>> dma_get_sgtable() are provided in <asm-generic/dma-mapping-common.h>.
>> > >>>
>> > >>> C6x does not use dma_map_ops, hence it should implement them as inline
>> > >>> stubs using dma_common_mmap() and dma_common_get_sgtable().
>> > >>>
>> > >> So are dma_mmap_coherent() and dma_get_sgtable() part of the DMA API
>> > >> now? I don't them in Documentation/DMA*.txt anywhere.
>> > >>
>> > >> Why does the default dma_common_mmap() for !CONFIG_MMU return an
>> > >> error?
>> > >>
>> > >> Wouldn't it be better to provide default implementations that an arch
>> > >> could override rather than having to patch all "no dma_map_ops"
>> > >> architectures?
>> > >>
>> > > Speaking for the still-reviewed ARC Port, I completely agree with Mark.
>>
>> dma_mmap_coherent() was partially in the DMA mapping API for some time, but
>> it was available only on a few architectures (afair ARM, powerpc and avr32).
>> This caused significant problems for writing unified device drivers or some
>> device helper modules which were aimed to work on more than one
>> architecture.
>>
>> dma_get_sgtable() is an extension discussed during the Linaro meetings. It
>> is required to correctly implement buffer sharing between device driver
>> without hacks or any assumptions about memory layout in the device drivers.
>>
>> I have implemented some generic code for both of those two functions,
>> keeping
>> in mind that on some hardware architectures (like already mentioned VIVT)
>> it might be not possible to provide coherent mapping to userspace. It is
>> perfectly fine for those functions to return an error in such case.
>
> It's not possible on VIPT either.  This means that the API is unusable
> on quite a large number of architectures.  Surely, if we're starting to
> write drivers using this, we need to fix the API before more people try
> to use it.
>
> For PA-RISC (and all other VIPT, I assume) I need an API which allows me
> to remap the virtual address of the kernel component (probably using the
> kmap area) so the user space and kernel space addresses are congruent.

So what are we gonna do for 3.8? I'd like to get my allmodconfig build
green again ;-)

Change the API?

Keep the API but do a best-effort fix to unbreak the builds?
  - Apply my patches that got acks (avr32/blackfin/cris/m68k),
  - Use static inlines that return -EINVAL for the rest
(c6x/frv/mn10300/parisc/xtensa).
I still have a few m68k fixes queued for 3.8, for which I've been postponing the
pull request to get this sorted out, so I could include the above.

Any other solution?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds