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

From: Philippe CORNU
Date: Fri Apr 06 2018 - 11:28:58 EST


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@xxxxxx>
>> ---
>> 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.
Big thanks
Philippe :-)