linux-kernel - Re: [PATCH v7 11/12] drm/atomic-helper: Re-order bridge chain pre-enable and post-disable

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Message-ID: <20250114-hypersonic-amiable-seagull-2d9f8d@houat>
Date: Tue, 14 Jan 2025 17:43:06 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Aradhya Bhatia <aradhya.bhatia@...ux.dev>
Cc: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>, 
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, 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>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, 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 v7 11/12] drm/atomic-helper: Re-order bridge chain
 pre-enable and post-disable

On Tue, Jan 14, 2025 at 10:05:56PM +0530, Aradhya Bhatia wrote:
> 
> 
> On 1/14/25 18:34, Tomi Valkeinen wrote:
> > Hi,
> > 
> > On 14/01/2025 13:24, Dmitry Baryshkov wrote:
> >> On Tue, Jan 14, 2025 at 11:26:25AM +0530, Aradhya Bhatia wrote:
> >>> Move the bridge pre_enable call before crtc enable, and the bridge
> >>> post_disable call after the crtc disable.
> >>>
> >>> The sequence of enable after this patch will look like:
> >>>
> >>>     bridge[n]_pre_enable
> >>>     ...
> >>>     bridge[1]_pre_enable
> >>>
> >>>     crtc_enable
> >>>     encoder_enable
> >>>
> >>>     bridge[1]_enable
> >>>     ...
> >>>     bridge[n]_enable
> >>>
> >>> And, the disable sequence for the display pipeline will look like:
> >>>
> >>>     bridge[n]_disable
> >>>     ...
> >>>     bridge[1]_disable
> >>>
> >>>     encoder_disable
> >>>     crtc_disable
> >>>
> >>>     bridge[1]_post_disable
> >>>     ...
> >>>     bridge[n]_post_disable
> >>>
> >>> The definition of bridge pre_enable hook says that,
> >>> "The display pipe (i.e. clocks and timing signals) feeding this bridge
> >>> will not yet be running when this callback is called".
> >>>
> >>> Since CRTC is also a source feeding the bridge, it should not be enabled
> >>> before the bridges in the pipeline are pre_enabled. Fix that by
> >>> re-ordering the sequence of bridge pre_enable and bridge post_disable.
> >>
> >> The patch contains both refactoring of the corresponding functions and
> >> changing of the order. Please split it into two separate commits.
> >>
> >>>
> >>> Signed-off-by: Aradhya Bhatia <a-bhatia1@...com>
> >>> Signed-off-by: Aradhya Bhatia <aradhya.bhatia@...ux.dev>
> >>> ---
> >>>   drivers/gpu/drm/drm_atomic_helper.c | 300 +++++++++++++++++-----------
> >>>   1 file changed, 181 insertions(+), 119 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/drm_atomic_helper.c
> >>> b/drivers/gpu/drm/drm_atomic_helper.c
> >>> index 5186d2114a50..ad6290a4a528 100644
> >>> --- a/drivers/gpu/drm/drm_atomic_helper.c
> >>> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> >>> @@ -74,6 +74,12 @@
> >>>    * also shares the &struct drm_plane_helper_funcs function table
> >>> with the plane
> >>>    * helpers.
> >>>    */
> >>> +
> >>> +enum bridge_chain_operation_type {
> >>> +    DRM_BRIDGE_PRE_ENABLE_OR_POST_DISABLE,
> >>> +    DRM_BRIDGE_ENABLE_OR_DISABLE,
> >>> +};
> >>> +
> >>
> >> I have mixed feelings towards this approach. I doubt that it actually
> >> helps. Would you mind replacing it with just 'bool pre_enable' / 'bool
> >> post_disable' arguments?
> > 
> > If my memory serves, I suggested the enum. I don't like it too much
> > either. But neither do I like the boolean that much, as this is not a
> > yes/no, on/off case... Then again, maybe boolean is fine here, as the
> > "off" case is the "normal/default" case so it's still ok-ish.
> > 
> > But this doesn't matter much, I think. It's internal, and can be
> > trivially adjusted later.
> > 
> 
> Alright! I will drop the enum reference entirely, and just use the
> booleans.

Whatever you do, I think that we're at a point where the bridge chain
traversal is complicated enough that we'll want to have tests for all
cases.

Maxime

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