[PATCH 2.6.39 v2] V4L: videobuf-dma-contig: fix mmap_mapper broken on ARM

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:

quoted

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
dma_mmap_coherent().

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 
already proposed 
(http://lists.infradead.org/pipermail/linux-arm-kernel/2010-December/036463.html, 
http://lists.infradead.org/pipermail/linux-arm-kernel/2011-January/036809.html), 
or perhaps a new, better solution ever finding its way to the mainline 
tree?

Thanks,
Janusz