Re: [PATCH] drm/sun4i: Only count TCON endpoints as valid outputs

From: Chen-Yu Tsai
Date: Wed Nov 23 2016 - 09:31:39 EST

On Tue, Nov 22, 2016 at 11:37 PM, Maxime Ripard
<maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote:
> Hi,
> On Fri, Nov 18, 2016 at 10:22:40AM +0800, Chen-Yu Tsai wrote:
>> On Fri, Nov 18, 2016 at 3:02 AM, Maxime Ripard
>> <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote:
>> > On Wed, Nov 16, 2016 at 05:37:31PM +0800, Chen-Yu Tsai wrote:
>> >> The sun4i DRM driver counts the number of endpoints it found and
>> >> registers the whole DRM pipeline if any endpoints are found.
>> >>
>> >> However, if the TCON and its child endpoints (LCD panels, TV encoder,
>> >> HDMI encoder, MIPI DSI encoder, etc.) aren't found, that means we
>> >> don't have any usable CRTCs, and the display pipeline is incomplete
>> >> and useless.
>> >
>> > If some node set as available is not probed, then yes, it does, but
>> > I'm not really sure how it's a problem. Quite the opposite actually.
>> Actually the problem occurs when the TCON is _not_ available, but
>> the other endpoints preceding it are.
> By preceding, you mean the display engine or the HDMI or TV encoders?

The display engine.

>> >> The debug message "Queued %d outputs on pipeline %d\n" is also telling.
>> >>
>> >> This patch makes the driver only count enabled TCON endpoints. If
>> >> none are found, the DRM pipeline is not used. This avoids screwing
>> >> up the simple framebuffer provided by the bootloader in cases where
>> >> we aren't able to support the display with the DRM subsystem, due
>> >> to lack of panel or bridge drivers, or just lack of progress.
>> >
>> > The framebuffer is removed only at bind time, which means that all the
>> > drivers have probed already. Lack of progress isn't an issue here,
>> > since the node simply won't be there, and we wouldn't have it in the
>> > component lists. And lack of drivers shouldn't be an issue either,
>> > since in order for bind to be called, all the drivers would have
>> > gone through their probe.
>> >
>> > So I'm not really sure what it fixes.
>> To recap, on sun6i I had enabled the display engine node by default
>> in the dtsi, along with the backend and drc. The tcon is disabled
>> by default, so it doesn't get added to the list of components.
>> The available components get probed, binded, and simplefb gets
>> pushed out.
>> I suppose disabling the display engine by default would be better?
>> At least simplefb still works.
> Yep, that works for me.

OK. I'll send out a patch.