linux-kernel - Re: [PATCH v5 13/13] drm/bridge: cdns-dsi: Use pre_enable/post

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Message-ID: <CAA8EJprWwKyyh5JvsmXnJC7QbybxEmwnAceMBgUr96uGzEJMtQ@mail.gmail.com>
Date: Mon, 21 Oct 2024 21:39:17 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Aradhya Bhatia <aradhya.bhatia@...ux.dev>
Cc: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>, 
	Laurent Pinchart <laurent.pinchart@...asonboard.com>, Andrzej Hajda <andrzej.hajda@...el.com>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
	Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Dominik Haller <d.haller@...tec.de>, Sam Ravnborg <sam@...nborg.org>, 
	Kieran Bingham <kieran.bingham+renesas@...asonboard.com>, Nishanth Menon <nm@...com>, 
	Vignesh Raghavendra <vigneshr@...com>, Devarsh Thakkar <devarsht@...com>, Praneeth Bajjuri <praneeth@...com>, 
	Udit Kumar <u-kumar1@...com>, Jayesh Choudhary <j-choudhary@...com>, 
	DRI Development List <dri-devel@...ts.freedesktop.org>, 
	Linux Kernel List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 13/13] drm/bridge: cdns-dsi: Use pre_enable/post_disable
 to enable/disable

On Mon, 21 Oct 2024 at 20:07, Aradhya Bhatia <aradhya.bhatia@...ux.dev> wrote:
>
> Hi Dmitry,
>
> Thank you for reviewing the patches!
>
> On 10/20/24 17:27, Dmitry Baryshkov wrote:
> > On Sun, Oct 20, 2024 at 01:35:30AM +0530, Aradhya Bhatia wrote:
> >> From: Aradhya Bhatia <a-bhatia1@...com>
> >>
> >> The cdns-dsi controller requires that it be turned on completely before
> >> the input DPI's source has begun streaming[0]. Not having that, allows
> >> for a small window before cdns-dsi enable and after cdns-dsi disable
> >> where the previous entity (in this case tidss's videoport) to continue
> >> streaming DPI video signals. This small window where cdns-dsi is
> >> disabled but is still receiving signals causes the input FIFO of
> >> cdns-dsi to get corrupted. This causes the colors to shift on the output
> >> display. The colors can either shift by one color component (R->G, G->B,
> >> B->R), or by two color components (R->B, G->R, B->G).
> >>
> >> Since tidss's videoport starts streaming via crtc enable hooks, we need
> >> cdns-dsi to be up and running before that. Now that the bridges are
> >> pre_enabled before crtc is enabled, and post_disabled after crtc is
> >> disabled, use the pre_enable and post_disable hooks to get cdns-dsi
> >> ready and running before the tidss videoport to get pass the color shift
> >> issues.
> >>
> >
> > Not being an expert in the TI DSS driver, would it be more proper to
> > handle that in the TI driver instead? I mean, sending out DPI signals
> > isn't a part of the CRTC setup, it's a job of the encoder.
>
> I haven't done a feasibility analysis of moving the CRTC bits of TIDSS
> into the encoder, but even if it were possible, it wouldn't solve the
> issue.
>
> The bridge_enable() sequence gets called _after_ the encoder has been
> enabled - causing the TIDSS's DPI (enabled from encoder) to still be
> up and running before the DSI has had a chance to be ready.

But then you can move CDSI setup to pre_enable, so that all setup
happens before encoder's (= DPI) being enabled.


-- 
With best wishes
Dmitry