Re: [PATCH] instmem/gk20a: do not use non-portable dma_to_phys()

From: Alexandre Courbot
Date: Wed Nov 11 2015 - 01:28:47 EST

On Wed, Nov 11, 2015 at 7:41 AM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 10 Nov 2015 14:10:47 +0900 Alexandre Courbot <acourbot@xxxxxxxxxx> 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).

>> --- 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

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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