linux-kernel - Re: [PATCH 5/6] drm/vc4: Fix overflow mem unreferencing when the binner runs dry.

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Message-ID: <87bn1kov0b.fsf@eliezer.anholt.net>
Date:	Tue, 26 Jul 2016 16:11:00 -0700
From:	Eric Anholt <eric@...olt.net>
To:	Rob Clark <robdclark@...il.com>
Cc:	"dri-devel\@lists.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH 5/6] drm/vc4: Fix overflow mem unreferencing when the binner runs dry.

Rob Clark <robdclark@...il.com> writes:

> On Tue, Jul 26, 2016 at 4:47 PM, Eric Anholt <eric@...olt.net> wrote:
>> Overflow memory handling is tricky: While it's still referenced by the
>> BPO registers, we want to keep it from being freed.  When we are
>> putting a new set of overflow memory in the registers, we need to
>> assign the old one to the last rendering job using it.
>>
>> We were looking at "what's currently running in the binner", but since
>> the bin/render submission split, we may end up with the binner
>> completing and having no new job while the renderer is still
>> processing.  So, if we don't find a bin job at all, look at the
>> highest-seqno (last) render job to attach our overflow to.
>
> so, drive-by comment.. but can you allocate gem bo's without backing
> them immediately with pages?  If so, just always allocate the bo
> up-front and attach it as a dependency of the batch, and only pin it
> to actual pages when you have to overflow?

The amount of overflow for a given CL is arbitrary, depending on the
geometry submitted, and the overflow pool just gets streamed into by the
hardware as you submit bin jobs.  You'll end up allocating [0,n] new
overflows per bin job.  I don't see where "allocate gem BOs without
backing them immediately with pages" idea would fit into this.

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