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 > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
