linux-kernel - Re: [RFC v2 8/8] drm: tegra: Add gr2d device

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Message-ID: <50B63AB5.4090801@wwwdotorg.org>
Date:	Wed, 28 Nov 2012 09:24:21 -0700
From:	Stephen Warren <swarren@...dotorg.org>
To:	Terje Bergström <tbergstrom@...dia.com>
CC:	Lucas Stach <dev@...xeye.de>, Dave Airlie <airlied@...il.com>,
	Thierry Reding <thierry.reding@...onic-design.de>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arto Merilainen <amerilainen@...dia.com>
Subject: Re: [RFC v2 8/8] drm: tegra: Add gr2d device

On 11/28/2012 07:45 AM, Terje Bergström wrote:
> On 28.11.2012 16:06, Lucas Stach wrote:
>> Why do even need/use dma-buf for this use case? This is all one DRM
>> device, even if we separate host1x and gr2d as implementation modules.
> 
> I didn't want to implement dependency to drm gem objects in nvhost, but
> we have thought about doing that. dma-buf brings quite a lot of
> overhead, so implementing support for gem buffers would make the
> sequence a bit leaner.
> 
> nvhost already has infra to support multiple memory managers.
> 
>> So standard way of doing this is:
>> 1. create gem object for pushbuffer
>> 2. create fake mmap offset for gem obj
>> 3. map pushbuf using the fake offset on the drm device
>> 4. at submit time zap the mapping
>>
>> You need this logic anyway, as normally we don't rely on userspace to
>> sync gpu and cpu, but use the kernel to handle the concurrency issues.
> 
> Taking a step back - 2D streams are actually very short, in the order of
> <100 bytes. Just copying them to kernel space would actually be faster
> than doing MMU operations.

I'm not sure it's a good idea to have one buffer submission mechanism
for the 2D class and another for the 3D/... class, nor to bet that 2D
streams will always be short.
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