[PATCH v2 2/3] drm/ttm: introduce dma cache sync helpers

[Alexandre Courbot]

On Tue, Jun 24, 2014 at 7:55 PM, Alexandre Courbot <acourbot at nvidia.com> wrote:
> On 06/24/2014 07:33 PM, Alexandre Courbot wrote:
>>
>> On 06/24/2014 07:02 PM, Russell King - ARM Linux wrote:
>>>
>>> On Tue, Jun 24, 2014 at 06:54:26PM +0900, Alexandre Courbot wrote:
>>>>
>>>> From: Lucas Stach <dev at lynxeye.de>
>>>>
>>>> On architectures for which access to GPU memory is non-coherent,
>>>> caches need to be flushed and invalidated explicitly at the
>>>> appropriate places. Introduce two small helpers to make things
>>>> easy for TTM-based drivers.
>>>
>>>
>>> Have you run this with DMA API debugging enabled?  I suspect you haven't,
>>> and I recommend that you do.
>>
>>
>> # cat /sys/kernel/debug/dma-api/error_count
>> 162621
>>
>> (╯°□°）╯︵ ┻━┻)
>
>
> *puts table back on its feet*
>
> So, yeah - TTM memory is not allocated using the DMA API, hence we cannot
> use the DMA API to sync it. Thanks Russell for pointing it out.
>
> The only alternative I see here is to flush the CPU caches when syncing for
> the device, and invalidate them for the other direction. Of course if the
> device has caches on its side as well the opposite operation must also be
> done for it. Guess the only way is to handle it all by ourselves here. :/

... and it really sucks. Basically if we cannot use the DMA API here
we will lose the convenience of having a portable API that does just
the right thing for the underlying platform. Without it we would have
to duplicate arm_iommu_sync_single_for_cpu/device() and we would only
have support for ARM.

The usage of the DMA API that we are doing might be illegal, but in
essence it does exactly what we need - at least for ARM. What are the
alternatives?