linux-kernel - Re: [PATCH 1/1] Support LVDS output on Allwinner A20

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Message-ID: <20200214075835.xugmte6uigmsxqog@gilmour.lan>
Date:   Fri, 14 Feb 2020 08:58:35 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Andrey Lebedev <andrey.lebedev@...il.com>
Cc:     wens@...e.org, airlied@...ux.ie, daniel@...ll.ch,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] Support LVDS output on Allwinner A20

On Thu, Feb 13, 2020 at 08:11:10PM +0200, Andrey Lebedev wrote:
> On Thu, Feb 13, 2020 at 10:24:33AM +0100, Maxime Ripard wrote:
> > > > do you have a board when you have been able to test it?
> > >
> > > Yes, I have the hardware (Cubieboard 2) at hand, but I cannot change the
> > > any physical connections on it. FWIW, it is https://openvario.org, the
> > > device we are (painfully) trying to upgrade from old kernel-3.4 with
> > > proprietary mali drivers to contemporary software.
> >
> > What painpoints do you have?
>
> So, even though proprietary mali drivers worked well for us, they seem
> to hold us back to old kernel-3.4, and it started to get harder to avoid
> various compatibility issues with newer software. Since there seemed to
> be a good progress with OSS lima driver lately, we decided to try to
> replace mali with lima. And that naturally pulled to switch to DRM/KMS.

You can use the proprietary mali drivers with mainline too, but yeah
it's in maintainance mode these days and you should go for lima if
you're (re)starting from scratch.

> The first painpoint is this LVDS support problem: after a week of trial
> and error, I discovered that support was simply not there. But it seemed
> that not that much was actually missing and after couple of evenings
> deep into debugging, here we are.
>
> Another one is the screen rotation: the device is installed into the
> glider' instrument panel, and some pilots prefer it in portrait
> orientation.  Under old mali setup, all that (seemingly) was required
> was setting "fbcon=rotate:n" boot arg, and both linux console and
> graphical app (https://xcsoar.org/) rotated "automagically". With new
> DRM/KMS setup, only console is rotated, all graphical apps seem to see
> the screen in its "natural" landscape orientation. Do you know if there
> is any way to leverage DMS/KMS infrastructure to make screen appear
> rotated for userspace apps (writing to /sys/class/graphics/fb0/rotate
> didn't work)?  There's of course a plan B to teach the app to rotate its
> output, but that leads to problem number 3.

There's a rotation property that can be set by whatever you're using
on top of KMS, DRM_MODE_ROTATE_*, but I'm not sure the driver supports
it at the moment.

> And the 3rd outstanding problem is that app doesn't really work under
> lima module and lima mesa driver. It starts, but renders a black window.
> I haven't dug deeply into this yet, but it is possible that some opengl
> API isn't supported in OSS drivers yet (even though app only renders 2d
> graphics).

I have no idea on this one though, sorry :/

Maxime

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