linux-kernel - Re: [PATCH] drm/sun4i: Avoid failing to init fbdev without any connector

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Message-ID: <20180820095657.rcs3wxikxprty55d@flea>
Date:   Mon, 20 Aug 2018 11:56:57 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Paul Kocialkowski <contact@...lk.fr>
Cc:     Daniel Vetter <daniel@...ll.ch>, dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Airlie <airlied@...ux.ie>, Chen-Yu Tsai <wens@...e.org>,
        linux-sunxi@...glegroups.com
Subject: Re: [PATCH] drm/sun4i: Avoid failing to init fbdev without any
 connector

On Tue, Aug 07, 2018 at 10:31:30PM +0200, Paul Kocialkowski wrote:
> Hi,
> 
> Le mardi 07 août 2018 à 22:18 +0200, Daniel Vetter a écrit :
> > On Tue, Aug 07, 2018 at 09:39:19PM +0200, Paul Kocialkowski wrote:
> > > Initializing and registering fbdev requires at least one DRM connector
> > > and will fail otherwise. In order to support headless setups (for
> > > instance for GPU rendering with the GBM backend, where a DRI card node
> > > is required to provide GEM memory reservation), add a check on the
> > > number of registered connectors before initializing fbdev.
> > 
> > sun4i is a pure kms driver, why exactly do you need it for gbm backed
> > rendering? What exactly is rendering here, and why does it insist on a
> > display card node, even if that display card node is 100% defunct?
> 
> The non-free Mali blobs provide GPU support with a GBM interface that
> takes a DRM fd in order to reserve the memory it needs for its rendering
> targets. This uses the GEM interface available through the DRI card node
> (and there is no render node with the non-free Mali driver). Allwinner
> has a minimalistic out-of-tree driver pretty much only for this purpose.
> 
> I crafted this patch when someone on IRC tried to get the out-of-tree
> Mali GPU driver going with the mainline kernel in order to do rendering
> in a GBM buffer, since it's apparently the only option for headless
> rendering with the blob.
> 
> In my opinion, supporting the pipeline of an out-of-tree GPU driver that
> only works with a non-free userspace blob is not a valid reason to do
> anything. Still, it felt like adding support for the headless use-case
> wouldn't hurt, whatever the underlying use case for it may be.

If you start considering the DMA constraints and the API in itself, it
does hurt. You'll get buffers that are not meant for the GPU, and
might not be accessible, and from another device without using
dma-buf, which again works because the hardware is dumb enough, but is
breaking a lot of expectations in the process.

If one wants to do headless rendering, mali-drm or lima looks like a
better solution.

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

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