Messages in this thread | | | Subject | Re: [PATCH] instmem/gk20a: do not use non-portable dma_to_phys() | From | Guenter Roeck <> | Date | Tue, 10 Nov 2015 22:52:03 -0800 |
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On 11/10/2015 10:27 PM, Alexandre Courbot wrote: > On Wed, Nov 11, 2015 at 7:41 AM, Andrew Morton > <akpm@linux-foundation.org> wrote: >> On Tue, 10 Nov 2015 14:10:47 +0900 Alexandre Courbot <acourbot@nvidia.com> wrote: >> >>> dma_to_phys() is not guaranteed to be available on all platforms and >>> should not be used outside of arch/. Replace it with what it is expected >>> to do in our case: simply cast the DMA handle to a physical address. >> >> mainline i386 allmodconfig is now busted. > > How? I just managed to build it both with and without this patch (note > that this is not to dispute the uglyness of this patch). >
Good for you. For me, it is
ERROR: "dma_to_phys" [drivers/gpu/drm/nouveau/nouveau.ko] undefined!
as recorded in
http://server.roeck-us.net:8010/builders/hwmon-i386-master/builds/338/steps/buildcommand/logs/stdio http://server.roeck-us.net:8010/builders/hwmon-i386-master/builds/339/steps/buildcommand/logs/stdio
Guenter
>> >>> --- a/drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c >>> +++ b/drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c >>> @@ -134,13 +134,17 @@ static void __iomem * >>> gk20a_instobj_cpu_map_dma(struct nvkm_memory *memory) >>> { >>> struct gk20a_instobj_dma *node = gk20a_instobj_dma(memory); >>> - struct device *dev = node->base.imem->base.subdev.device->dev; >>> int npages = nvkm_memory_size(memory) >> 12; >>> struct page *pages[npages]; >>> int i; >>> >>> - /* phys_to_page does not exist on all platforms... */ >>> - pages[0] = pfn_to_page(dma_to_phys(dev, node->handle) >> PAGE_SHIFT); >>> + /* >>> + * Ideally we would have a function to translate a handle to a physical >>> + * address, but there is no portable way of doing this. However since we >>> + * always use the DMA API without an IOMMU, we can assume that handles >>> + * are actual physical addresses. >>> + */ >>> + pages[0] = pfn_to_page(((phys_addr_t)node->handle) >> PAGE_SHIFT); >> >> This looks ugly. > > It's what dma_to_phys() does. :) > >> >> What's actually going on here? Why is this driver doing something which >> no other driver appears to need to do? > > So this code is actually a fallback for the case where no IOMMU is > available. In such cases, we are using the DMA API to obtain > contiguous memory for the GPU. > > Sometimes the CPU needs to touch this memory as well, and thus we need > a memory mapping. However since this is rather occasional we do not > want to clutter the CPU's memory space by using the permanent mapping > that DMA API proposes - instead we allocate that memory with the > DMA_ATTR_NO_KERNEL_MAPPING flag in order to manage the CPU mapping > ourselves. > > What we want to do here is obtain the list of physical pages of the > buffer so we can map it using vmap. However AFAIK there is no function > that gives us the pages used by a buffer allocated using the DMA API. > Since we know that in this case there is no IOMMU, we rely on the fact > that that the handle points to the beginning of the physical address > of the contiguous buffer. > >> >> Is it the driver which is broken, or are the core kernel APIs inadequate? >> >> If the latter, what can we do to fix them up? >> >> IOW, how do we fix this properly? > > I guess a reasonable fix would be to have explicit CPU > mapping/unmapping functions in the DMA API, instead of the current > all-or-nothing situation (either a mapping that lasts as long as the > buffer does, or the buffer is invisible to the CPU). This may take a > while if it happens at all, as I suspect the current behavior must be > driven by some limitation. > > For now the most urgent is to remove this use of dma_to_phys() outside > of arch/, which was the purpose of this patch. Dave's fix does the > trick too, but I'm concerned that it may make other people think that > it is ok to call this function from drivers. > > Let's do the following instead: we drop this patch, and I will change > that code to use the permanent mapping created by the DMA API. It is a > fallback anyway, so it is not too much of a concern if it is not > optimal. Besides it should be safer, as we know ARM does not like > multiple CPU mappings. > > David, I will send you the proper patch ASAP, ideally later today. > > Apologies for breaking mainline. >_<; >
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