On Mon 11 Apr 2011 at 02:42:13 Mauro Carvalho Chehab wrote: > Em 10-04-2011 19:47, Janusz Krzysztofik escreveu: > > After switching from mem->dma_handle to virt_to_phys(mem->vaddr) > > used for obtaining page frame number passed to remap_pfn_range() > > (commit 35d9f510b67b10338161aba6229d4f55b4000f5b), > > videobuf-dma-contig stopped working on my ARM based board. The ARM > > architecture maintainer, Russell King, confirmed that using > > something like > > virt_to_phys(dma_alloc_coherent()) is not supported on ARM, and can > > be broken on other architectures as well. The author of the > > change, Jiri Slaby, also confirmed that his code may not work on > > all architectures. > > > > The patch takes two different countermeasures against this > > regression: > > > > 1. On architectures which provide dma_mmap_coherent() function (ARM > > for > > > > now), use it instead of just remap_pfn_range(). The code is > > stollen from sound/core/pcm_native.c:snd_pcm_default_mmap(). > > Set vma->vm_pgoff to 0 before calling dma_mmap_coherent(), or it > > fails. > > > > 2. On other architectures, use > > virt_to_phys(bus_to_virt(mem->dma_handle)) > > > > instead of problematic virt_to_phys(mem->vaddr). This should > > work even if those translations would occure inaccurate for DMA > > addresses, since possible errors introduced by both > > calculations, performed in opposite directions, should > > compensate. > > > > Both solutions tested on ARM OMAP1 based Amstrad Delta board. ... > The code is saying that dma_mmap_coherent should be used only on ARM > and PPC architectures, and remap_pfn_range should be used otherwise. > Are you sure that this will work on the other architectures? I > really prefer to have one standard way for doing it, that would be > architecture-independent. Media drivers or core should not have > arch-dependent code inside. More looking at this and making more tests, I found that the dma_mmap_coherent() method, working correctly on OMAP1 which has no countermeasures against unpredictable dma_alloc_coherent() runtime behaviour implemented, may not be compatible with all those dma_declare_coherent_memory() and alike workarounds, still being used, more or less successfully, on other ARM platforms/machines/boards. Under such circumstances, I'd opt for choosing the depreciated, but hopefully working, bi-directional translation method, ie. virt_to_phys(bus_to_virt(mem->dma_handle)), as the regression fix. 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
