linux-kernel - Re: [PATCH 7/7] drm/fsl-dcu: use mode flags for hsync/vsync pixelclk polarity

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Message-ID: <8d3ce0adf415c792f1156bd9a18528e4@agner.ch>
Date:	Wed, 03 Feb 2016 15:18:47 -0800
From:	Stefan Agner <stefan@...er.ch>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	airlied@...il.com, alison.wang@...escale.com,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	airlied@...ux.ie, daniel.vetter@...ll.ch,
	jianwei.wang.chn@...il.com, Shawn Guo <shawnguo@...nel.org>
Subject: Re: [PATCH 7/7] drm/fsl-dcu: use mode flags for hsync/vsync pixelclk
 polarity

On 2016-02-03 06:00, Thierry Reding wrote:
> On Wed, Jan 27, 2016 at 06:46:50PM -0800, Stefan Agner wrote:
> [...]
>> > diff --git a/drivers/gpu/drm/panel/panel-simple.c
>> > b/drivers/gpu/drm/panel/panel-simple.c
>> > index f97b73e..fa68b56 100644
>> > --- a/drivers/gpu/drm/panel/panel-simple.c
>> > +++ b/drivers/gpu/drm/panel/panel-simple.c
>> > @@ -960,6 +960,8 @@ static const struct drm_display_mode
>> > nec_nl4827hc19_05b_mode = {
>> >  	.vsync_end = 272 + 2 + 4,
>> >  	.vtotal = 272 + 2 + 4 + 2,
>> >  	.vrefresh = 74,
>> > +	.flags = DRM_MODE_FLAG_NVSYNC | DRM_MODE_FLAG_NHSYNC |
>> > +		 DISPLAY_FLAGS_PIXDATA_POSEDGE,
> 
> It doesn't seem like these two types of flags should be mixed because
> they overlap. DISPLAY_FLAGS_PIXDATA_POSEDGE has the same value as the
> DRM_MODE_FLAG_CSYNC define.

There are other entries such as hannstar_hsd100pxn1_timing which make
use of DISPLAY_FLAGS too, hence I did not bother further. Having a
conflict is definitely not what we want.

> The definition of the DISPLAY_FLAGS_PIXDATA_POSEDGE is also not very
> clear to me. I don't think we have an equivalent DRM_MODE_FLAG_* but
> we could add one if there's really a need.

E.g. assuming a parallel video signal, the pixel data signals could be
changing on positive or negative edge relative to the clock signal. Most
displays _sample_ the data on rising edge, hence controllers should
normally drive data on falling edge.

However, as always, there are exceptions to the rule, and one of the
exception is Freescales default display for the Tower evaluation board.
It samples data on falling edge, hence the controller should drive data
on rising edge...

Yeah I also did not found an equivalent in DRM_MODE_FLAG_*. I have here
also other displays where we would need this flag. The display-timings
binds and enum display_flags support it too, hence I guess we should
have it here too.

I will split out this patch from the patchset (I already applied the
rest), add another patch to add the flag, make use of the flag in this
patch and resend it as v2.

--
Stefan