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

From: Dave Airlie <airlied@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexandre Courbot <acourbot@nvidia.com>,
	Dave Airlie <airlied@linux.ie>, Ben Skeggs <bskeggs@redhat.com>,
	Guenter Roeck <linux@roeck-us.net>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	linux-next <linux-next@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Alexandre Courbot <gnurou@gmail.com>
Subject: Re: [PATCH] instmem/gk20a: do not use non-portable dma_to_phys()
Date: Wed, 11 Nov 2015 12:29:18 +1000	[thread overview]
Message-ID: <CAPM=9twRiQFTk8ccW_aaE_C4LM3gSnqsNJxAnMKZkvb_ns1TZA@mail.gmail.com> (raw)
In-Reply-To: <20151110144144.39c2a4109c26e91f4f3fb47b@linux-foundation.org>

On 11 November 2015 at 08:41, 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.

I'll push my hack to make things build into drm-fixes while we discuss
the subtlety.

Dave.
>
>> --- 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.
>
> What's actually going on here?  Why is this driver doing something which
> no other driver appears to need to do?
>
> 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?
> --
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