linux-kernel - Re: KMS question

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Message-ID: <CADnq5_OhUKzj76XuDqCL4B_tC8eVifp3S4imkJpXVV+n6dG=Jg@mail.gmail.com>
Date:   Thu, 13 Apr 2017 10:05:41 -0400
From:   Alex Deucher <alexdeucher@...il.com>
To:     Boszormenyi Zoltan <zboszor@...hu>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>
Subject: Re: KMS question

On Thu, Apr 13, 2017 at 9:03 AM, Boszormenyi Zoltan <zboszor@...hu> wrote:
> Hi,
>
> how can I disable the behaviour in the KMS drivers
> that enables all outputs at once?
>
> It is very annoying that on a POS machine with an
> 1024x768 LVDS and a 800x480 secondary monitor (both built-in)
> the KMS driver wakes up both. Then the framebuffer console
> and plymouth use both screens, making the primary output
> very odd with only the top-left part used by the boot splash.
>
> I would like the boot splash to be shown only on the primary
> output at its full resolution instead of on all outputs using
> the smallest common rectangle.
>
> Is there a kernel command line configuration that achieves this?
>
> The device in question uses the gma500 kernel driver but the same
> behaviour is observed with the i915 and radeon drivers.
>

The problem is fbdev is not multi-head aware.  The fbdev emulation in
the KMS drivers attempts to light up all monitors so that something
shows up on all heads.  If you really want different per head
configurations, you need to use the KMS API directly.  As a
workaround, you can use the kernel command line to disable the output
you don't want to be lit up.  See:
https://wiki.archlinux.org/index.php/kernel_mode_setting
for more info.  basically add video=TV-1:d to disable the output in
question.  Replace TV-1 with whatever connector you want to disable.

Alex