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

[Marek Szyprowski <m.szyprowski@samsung.com>]

On 1/21/2013 9:00 PM, Geert Uytterhoeven wrote:
> 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.

I think this would be the best solution for fixing v3.8. I would like to put
those patches in linux-next asap and then after a week or so send a pull
request for merging them to v3.8-rc6.

EINVAL approach can be later fixed/extended with real implementation (if
there are any driver candidates, which will use such interface).

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center