[PATCH 2.6.39 v2] V4L: videobuf-dma-contig: fix mmap_mapper broken on ARM
jkrzyszt at tis.icnet.pl
Wed Apr 13 06:52:31 EDT 2011
On Tue 12 Apr 2011 at 23:40:11 Russell King - ARM Linux wrote:
> On Tue, Apr 12, 2011 at 11:06:34PM +0200, Janusz Krzysztofik wrote:
> > The patch tries to solve this regression by using
> > virt_to_phys(bus_to_virt(mem->dma_handle)) instead of problematic
> > virt_to_phys(mem->vaddr).
> Who says that DMA handles are bus addresses in the strictest sense?
> DMA handles on ARM are the bus address to program 'dev' with in order
> for it to access the memory mapped by dma_alloc_coherent(). On some
> ARM platforms, this bus address is dependent on 'dev' - such as
> platforms with more than one root PCI bus, and so bus_to_virt() just
> doesn't hack it.
Taking into account that I'm just trying to fix a regression, and not
invent a new, long term solution: are you able to name an ARM based
board which a) is already supported in 2.6.39, b) is (or can be)
equipped with a device supported by a V4L driver which uses videobuf-
dma-config susbsystem, c) has a bus structure with which
virt_to_phys(bus_to_virt(dma_handle)) is not equal dma_handle?
If there is one, then I agree that my short-term fix is wrong.
> What is really needed is for this problem - the mapping of DMA
> coherent memory into userspace - to be solved with a proper arch API
> rather than all these horrible hacks which subsystems instead
> invent. That's something I tried to do with the dma_mmap_coherent()
> stuff but it was shot down by linux-arch as (iirc) PA-RISC objected
> to it.
> Hence why ARM only implements it.
I thought so too, but missed the fact that PowerPC implements it
actually, even defining the ARCH_HAS_DMA_MMAP_COHERENT symbol, which ARM
doesn't so far.
> Maybe the video drivers should try to resurect the idea, maybe only
> allowing this support for architectures which provide
AFAICT, ARM implementation of dma_mmap_coherent() is not compatible with
dma_declare_coherent_memory(), is it? If I'm wrong, please correct me,
I'll get back to the idea presented in v1 of the fix.
Otherwise, I think that switching to dma_mmap_coherent() is not an
option for the videobuf-dma-contig subsystem as long as there is no good
solution to the problem of dma_alloc_coherent() not guaranteed to
succeed with high-order allocations at any time. Any chance for your
or perhaps a new, better solution ever finding its way to the mainline
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