Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc_coherent() failures

[Robin Murphy <robin.murphy@arm.com>]

On 12/04/18 09:27, Takashi Iwai wrote:
> On Thu, 12 Apr 2018 10:19:05 +0200,
> Takashi Iwai wrote:
>>
>> On Thu, 12 Apr 2018 10:03:56 +0200,
>> Takashi Iwai wrote:
>>>
>>> On Thu, 12 Apr 2018 08:02:27 +0200,
>>> Christoph Hellwig wrote:
>>>>
>>>> On Wed, Apr 11, 2018 at 09:28:54AM +0200, Takashi Iwai wrote:
>>>>>> But we should try a GFP_DMA32 allocation first, so this is a bit
>>>>>> surprising.
>>>>>
>>>>> Hm, do we really try that?
>>>>> Through a quick glance, dma_alloc_coherent_gfp_flags() gives GFP_DMA32
>>>>> only when coherent mask <= DMA_BIT_MASK(32); in the case of iwlwifi,
>>>>> it's 36bit, so GFP_DMA isn't set.
>>>>
>>>> Oh, yes - it is using an odd dma mask, and amdgpu seems to use an
>>>> just as odd 40-bit dma mask.
>>>>
>>>>> We had a fallback allocation with GFP_DMA32 in the past, but this
>>>>> seems gone long time ago along with cleanups (commit c647c3bb2d16).
>>>>>
>>>>> But I haven't followed about this topic for long time, so I might have
>>>>> missed obviously...
>>>>
>>>> I think a fallback would be much better here rather than relying on the
>>>> limited swiotlb buffer bool.  dma_direct_alloc (which in 4.17 is also
>>>> used for x86) already has a GFP_DMA fallback, so extending this for
>>>> GFP_DMA32 as well would seem reasonable.
>>>>
>>>> Any volunteers?
>>>
>>> Below is a quick attempt, totally untested.  Actually the retry with
>>> GFP_DMA is superfluous for archs without it, so the first patch
>>> corrects it.
>>
>> Gah, scratch this, it doesn't work.  A different check is needed...
> 
> The v2 patches are below, replaced with IS_ENABLED(CONFIG_ZONE_DMA).

That looks pretty reasonable to me - I'd be tempted to try a factoring 
out a helper function to avoid the "goto again" logic, but that's hardly 
crucial.

What I'd really love to see is something like the alloc_pages_mask() 
proposal from years ago to come back such that GFP_DMA32 could die 
entirely, but I don't know anywhere near enough about the mm layer to 
consider taking that on myself :(

Robin.