linux-kernel - Re: [PATCH] drm: clarify adjusted_mode for a bridge connected to a crtc

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Message-ID: <20180409075632.GC31310@phenom.ffwll.local>
Date:   Mon, 9 Apr 2018 09:56:32 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Philippe CORNU <philippe.cornu@...com>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        David Airlie <airlied@...ux.ie>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Yannick FERTRE <yannick.fertre@...com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Vincent ABRIOU <vincent.abriou@...com>
Subject: Re: [PATCH] drm: clarify adjusted_mode for a bridge connected to a
 crtc

On Fri, Apr 06, 2018 at 03:28:27PM +0000, Philippe CORNU wrote:
> Hi Laurent,
> 
> On 04/06/2018 04:53 PM, Laurent Pinchart wrote:
> > Hi Philippe,
> > 
> > Thank you for the patch.
> > 
> > On Monday, 26 February 2018 14:16:04 EEST Philippe Cornu wrote:
> >> This patch clarifies the adjusted_mode documentation
> >> for a bridge directly connected to a crtc.
> >>
> >> Signed-off-by: Philippe Cornu <philippe.cornu@...com>
> >> ---
> >> This patch is linked to the discussion https://lkml.org/lkml/2018/1/25/367
> >>
> >>   include/drm/drm_bridge.h | 3 ++-
> >>   1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> >> index 3270fec46979..b5f3c070467c 100644
> >> --- a/include/drm/drm_bridge.h
> >> +++ b/include/drm/drm_bridge.h
> >> @@ -177,7 +177,8 @@ struct drm_bridge_funcs {
> >>   	 * pipeline has been called already. If the bridge is the first element
> >>   	 * then this would be &drm_encoder_helper_funcs.mode_set. The display
> >>   	 * pipe (i.e.  clocks and timing signals) is off when this function is
> >> -	 * called.
> >> +	 * called. If the bridge is connected to the crtc, the adjusted_mode
> >> +	 * parameter is the one defined in &drm_crtc_state.adjusted_mode.
> > 
> > Unless I'm mistaken this will always be the mode stored in
> > &drm_crtc_state.adjusted_mode (at least for atomic drivers), regardless of
> > whether the bridge is the first in the chain (connected to the CRTC) or not.
> > What is important to document is that we have a single adjusted_mode for the
> > whole chain of bridges, and that it corresponds to the mode output by the CRTC
> > for the first bridge. Bridges further in the chain can look at that mode,
> > although there will probably be nothing of interest to them there.
> > 
> > How about the following text ?
> > 
> >      /**
> >       * @mode_set:
> >       *
> >       * This callback should set the given mode on the bridge. It is called
> >       * after the @mode_set callback for the preceding element in the display
> >       * pipeline has been called already. If the bridge is the first element
> >       * then this would be &drm_encoder_helper_funcs.mode_set. The display
> >       * pipe (i.e.  clocks and timing signals) is off when this function is
> >       * called.
> >       *
> >       * The adjusted_mode parameter corresponds to the mode output by the CRTC
> >       * for the first bridge in the chain. It can be different from the mode
> >       * parameter that contains the desired mode for the connector at the end
> >       * of the bridges chain, for instance when the first bridge in the chain
> >       * performs scaling. The adjusted mode is mostly useful for the first
> >       * bridge in the chain and is likely irrelevant for the other bridges.
> >       *
> >       * For atomic drivers the adjusted_mode is the mode stored in
> >       * &drm_crtc_state.adjusted_mode.
> >       *
> >       * NOTE:
> >       *
> >       * If a need arises to store and access modes adjusted for other locations
> >       * than the connection between the CRTC and the first bridge, the DRM
> >       * framework will have to be extended with DRM bridge states.
> >     	 */
> > 
> > Then I think we should also update the documentation of
> > drm_crtc_state.adjusted_mode accordingly:
> > 
> >      /*
> >       * @adjusted_mode:
> >       *
> >       * Internal display timings which can be used by the driver to handle
> >       * differences between the mode requested by userspace in @mode and what
> >       * is actually programmed into the hardware.
> >       *
> >       * For drivers using drm_bridge, this store the hardware display timings
> >       * used between the CRTC and the first bridge. For other drivers, the
> >       * meaning of the adjusted_mode field is purely driver implementation
> >       * defined information, and will usually be used to store the hardware
> >       * display timings used between the CRTC and encoder blocks.
> >       */
> > 
> 
> Your proposal is very clear and understandable. I will make a new patch 
> version based on it.

Just to avoid confusion: Needs to be a fully new patch on top of latest
drm-misc-next, since no rebasing in a group maintained tree.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch