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

[Alexandre Courbot]

On Tue, Jun 24, 2014 at 10:58 PM, Lucas Stach <l.stach@xxxxxxxxxxxxxx> wrote:
> Am Dienstag, den 24.06.2014, 22:52 +0900 schrieb Alexandre Courbot:
>> On Tue, Jun 24, 2014 at 10:25 PM, Lucas Stach <l.stach@xxxxxxxxxxxxxx> wrote:
>> > Am Dienstag, den 24.06.2014, 14:27 +0200 schrieb Maarten Lankhorst:
>> >> op 24-06-14 14:23, Alexandre Courbot schreef:
>> >> > On Tue, Jun 24, 2014 at 7:55 PM, Alexandre Courbot <acourbot@xxxxxxxxxx> 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@xxxxxxxxxx>
>> >> >>>>>
>> >> >>>>> 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
>> >> >>>
>> >> >>> (âÂâÂïâï âââ)
>> >> >>
>> >> >> *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?
>> >> Convert TTM to use the dma api? :-)
>> >
>> > Actually TTM already has a page alloc backend using the DMA API. It's
>> > just not used for the standard case right now.
>>
>> Indeed, and Nouveau even already makes use of it if CONFIG_SWIOTLB is
>> set apparently.
>>
>> > I would argue that we should just use this page allocator (which has the
>> > side effect of getting pages from CMA if available -> you are actually
>> > free to change the caching) and do away with the other allocator in the
>> > ARM case.
>>
>> Mm? Does it mean that CMA memory is not mapped into lowmem? That would
>> certainly help in the present case, but I wonder how useful it will be
>> once the iommu support is in place. Will also need to consider
>> performance of such coherent memory for e.g. user-space mappings.
>>
>> Anyway, I will experiment a bit with this tomorrow, thanks!
>
> CMA memory is reserved before the lowmem section mapping is set up. It
> is then mapped with individual 4k pages before giving it back to the
> buddy allocator.
> This means CMA pages in use by the kernel are mapped into lowmem, but
> they are actually unmapped from lowmem once you allocate them as DMA
> memory.

Thanks for the explanation. I really need to spend more time studying
the DMA allocator. I wonder if all this is already explained somewhere
in Documentation/ ?
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