Re: [RFC 0/1] drm: Add Grain Media GM12U320 kms driver

From: "Marco Diego Aurélio Mesquita" <marcodiegomesquita@gmail.com>
To: Hans de Goede <hdegoede@redhat.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>,
	Emil Velikov <emil.l.velikov@gmail.com>,
	ML dri-devel <dri-devel@lists.freedesktop.org>
Subject: Re: [RFC 0/1] drm: Add Grain Media GM12U320 kms driver
Date: Fri, 9 Jun 2017 17:59:22 -0300	[thread overview]
Message-ID: <CAE_m23kcdhyJBz_srnq=NeMiiPxOV+pPFvCiWQB+8mKrc=kD4w@mail.gmail.com> (raw)
In-Reply-To: <ea0132eb-061f-7d15-0720-be0514366e1d@redhat.com>

Hi Devs!

On Thu, Jun 8, 2017 at 4:08 AM, Hans de Goede <hdegoede@redhat.com> wrote:
> I don't think that using cma for the gm12u320 is a good idea, it will
> typically be used as a secondary GPU output together with a real GPU
> extending the desktop by being a prime display output. So for the memory
> management stuff I would keep the code copied from the udl driver (which
> we may later split out in a separate helper lib for devices where
> the framebuffer is in normal system memory and we have some scather-gather
> capable process copying it to the real device over e.g. USB)
>

I got the PL111 driver and stripped all device specific code. Also, I
added the get_modes and driver usb probe functions from gm12u320. The
resulting code is available in
https://gitlab.com/marcodiego/dummy-display-driver .

The driver compiles, loads, identifies the device when I plug it,
/dev/fb1 is created adequately but it stops there. Gnome monitors tool
does not see it as a new monitor. What I expected was that it would be
seen as a new monitor and that I could even activate it. Instead,
syslog complains: "Cannot find any crtc or sizes".

Anyway, I think this is a first step. My first doubt: where should I
go from now? What should I change in the driver so that gnome monitors
tool sees it as a new monitor and I could activate it, even if it
works as a mere dummy device?

Second doubt is: is this really the simplest/best way? I mean, the
repaper driver that Emil pointed seems a lot simpler, wouldn't it be
better to mimic it? How complicated is it to modify the repaper driver
to build a dummy driver?

By dummy driver, I mean something that gnome monitors tools can
identify as a new monitor and I can activate it. I hope to reach a
point where the update or dirty callback is called. From that point on
it is just a matter of sending the framebuffer through USB the way
gm12u320 does currently.

Thanks!
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