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

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Date:	Tue, 24 Jun 2014 15:25:38 +0200
From:	Lucas Stach <l.stach@...gutronix.de>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
Cc:	Alexandre Courbot <gnurou@...il.com>,
	Alexandre Courbot <acourbot@...dia.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Ben Skeggs <bskeggs@...hat.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Nouveau] [PATCH v2 2/3] drm/ttm: introduce dma cache sync
 helpers

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@...dia.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@...xeye.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?
> 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.

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.

Regards,
Lucas
-- 
Pengutronix e.K.             | Lucas Stach                 |
Industrial Linux Solutions   | http://www.pengutronix.de/  |

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