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

[James Bottomley]

On Tue, 2013-01-15 at 15:07 +0100, Marek Szyprowski wrote:
> Hello,
> 
> On 1/15/2013 10:13 AM, Geert Uytterhoeven wrote:
> > Marek?
> >
> > On Tue, Jan 15, 2013 at 5:16 AM, Vineet Gupta
> > <Vineet.Gupta1@xxxxxxxxxxxx> 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 âvb2_dc_mmapâ:
> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c:204: error: implicit declaration of function âdma_mmap_coherentâ
> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c: In function âvb2_dc_get_base_sgtâ:
> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c:387: error: implicit declaration of function âdma_get_sgtableâ
> > >>>
> > >>> 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.

James


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