linux-kernel - Re: [5/5] ARM: tegra: jetson-tk1: enable GK20A GPU

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Date:	Thu, 25 Sep 2014 12:07:01 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Thierry Reding <thierry.reding@...il.com>
CC:	Sjoerd Simons <sjoerd@...n.net>,
	Alexandre Courbot <acourbot@...dia.com>,
	Ben Skeggs <bskeggs@...hat.com>, nouveau@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org, gnurou@...il.com
Subject: Re: [5/5] ARM: tegra: jetson-tk1: enable GK20A GPU

On 09/25/2014 10:41 AM, Thierry Reding wrote:
> On Thu, Sep 25, 2014 at 09:48:01AM -0600, Stephen Warren wrote:
>> On 09/25/2014 07:27 AM, Sjoerd Simons wrote:
>>> Playing a bit with todays linux-next on my jetson, it seems this patch is
>>> still required for enabling the GPU. Is there anything blocking it (firmware
>>> not available yet in liux-firmware?)
>>
>> I think initially I was waiting for the DRM patch "drm/nouvea: support for
>> probing platform devices" to be applied, but it looks like that's been
>> applied already, so only patches 4 and 5 in this series are still
>> outstanding.
>>
>> Alex, wasn't there also some issue where the VPR register had to be
>> programmed, and if it wasn't there'd be a hang when the GPU registers were
>> touched? If we've added code to Nouveau/tegradrm to detect that and avoid
>> the problem, then I guess we can commit these last two patches for 3.19. A
>> resend after the 3.18 merge window might help.
>
> A patch that programs VPR was merged into U-Boot (though I don't think
> it's made it into master yet). I'm not sure we can reasonably check for
> that in Nouveau, given that the register is somewhere completely
> unrelated. In fact I think the U-Boot patch was triggered by some
> discussion about how to solve this and it was decided that it shouldn't
> be done in the kernel, but U-Boot should set it up.
>
> That said, perhaps one solution would be to make U-Boot enable the gk20a
> device if it's set up the VPR and disable it otherwise?

For that to work, we'd need the DT to say status="disabled" by default 
for the GPU, and for the fixed U-Boot (and indeed every other 
bootloader...) to enable the GPU node. This would allow people with old 
versions of U-Boot (or other bootloaders) to continue to boot. This 
means bootloaders would only have to set status="okay", but never have 
to set status="disabled", which at least simplifies them a tiny bit.
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