Message-ID: <1358849633.2387.11.camel@dabdike.int.hansenpartnership.com>
Date:	Tue, 22 Jan 2013 10:13:53 +0000
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Marek Szyprowski <m.szyprowski@...sung.com>,
	Mark Salter <msalter@...hat.com>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	linux-arch@...r.kernel.org, linux-c6x-dev@...ux-c6x.org,
	linux-kernel@...r.kernel.org
Subject: Re: [Linux-c6x-dev] [PATCH 3/9] c6x: Provide dma_mmap_coherent()
 and dma_get_sgtable()

On Mon, 2013-01-21 at 22:59 +0000, James Bottomley wrote:
> On Mon, 2013-01-21 at 21:00 +0100, Geert Uytterhoeven wrote:
> > On Tue, Jan 15, 2013 at 5:56 PM, James Bottomley
> > <James.Bottomley@...senpartnership.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@...opsys.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?
> 
> Well, if we want the API to work universally, we have to.  As I said,
> for VIPT systems, the only coherency mechanism we have is the virtual
> address ... we have to fix that in the kernel to be congruent with the
> userspace virtual address if we want coherency between the kernel and
> userspace.
> 
> However, if it only needs to work on ARM and x86, it can stay the way it
> is and we could just pull it out of the generic core.
> 
> Who actually wants to use this API, and what for?
> 
> > 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?
> 
> If it's an API that only works on ARM and x86, there's not much point
> pretending it's universal, so we should remove it from the generic arch
> code and allow only those architectures which can use it.

There might be a simple solution:  just replace void *cpu_addr with void
**cpu_addr in the API.  This is a bit nasty since compilers think that
void ** to void * conversion is quite legal, so it would be hard to pick
up misuse of this (uint8_t ** would be better).  That way VIPT could
remap the kernel pages to a coherent address.  This would probably have
to change in the dma_mmap_attr() and dma_ops structures.

All consumers would have to expect cpu_addr might change, but that seems
doable.

James


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