linux-kernel - Re: [PATCH 2/3] drm/vc4: Don't try to initialize FBDEV if we're only bound to V3D.

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Message-ID: <CADnq5_OEX2kzLJ5ABfBs6fZnmhFmJAPbg1XjJWx9Pt_pu=bTEA@mail.gmail.com>
Date:   Mon, 24 Apr 2017 10:26:45 -0400
From:   Alex Deucher <alexdeucher@...il.com>
To:     Eric Anholt <eric@...olt.net>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Mark Rutland <mark.rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH 2/3] drm/vc4: Don't try to initialize FBDEV if we're only
 bound to V3D.

On Fri, Apr 21, 2017 at 6:53 PM, Eric Anholt <eric@...olt.net> wrote:
> Daniel Vetter <daniel@...ll.ch> writes:
>
>> On Wed, Apr 19, 2017 at 7:55 PM, Eric Anholt <eric@...olt.net> wrote:
>>> Daniel Vetter <daniel@...ll.ch> writes:
>>>> On Tue, Apr 18, 2017 at 9:11 PM, Eric Anholt <eric@...olt.net> wrote:
>>>>> The FBDEV initialization would throw an error in dmesg, when we just
>>>>> want to silently not initialize fbdev on a V3D-only VC4 instance.
>>>>>
>>>>> Signed-off-by: Eric Anholt <eric@...olt.net>
>>>>
>>>> Hm, this shouldn't be an error really, you might want to hotplug more
>>>> connectors later on. What exactly complains?
>>>
>>> drm_fb_helper_init() throws an error if the passed in connector count is
>>> 0, so drm_fb_cma_helper() printks an error.
>>
>> Oh, _that_ thing. The error in there is correct, but (almost) everyone
>> gets this parameter wrong. This isn't the max number of connectors the
>> fb helper will light up, but just the max number of connectors _per_
>> crtc when driving in hw clone mode. There's two problems with that:
>> - fb helpers don't support hw clone mode, we select 1:1 crtcs for each
>> active connector
>> - I mentioned that everyone gets this wrong?
>>
>> If you're moderately bored it'd be great to nuke the max_connector
>> argument from drm_fb_helper_init, and hard-code it to 1 (with a big
>> comment explaining that this needs to be changed, probably with
>> dynamic reallocation, once someone gets around to implementing hw
>> clone mode).
>>
>> If you're less bored, just hardcode this to 1 in vc4 and done. Plus a
>> TODO.rst entry would be great in that case.
>
> If I'm driving a GPU with no display subsystem at all, it seems like I
> shouldn't initialize fbdev for it, right?

That's what we do for radeon/amdgpu on hw without display blocks.

Alex