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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Date:   Fri, 21 Apr 2017 15:53:30 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "devicetree\@vger.kernel.org" <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] drm/vc4: Don't try to initialize FBDEV if we're only bound to V3D.

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?

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