linux-kernel - Re: [PATCH v2 6/8] drm: Introduce drm_bridge_mode

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Date:   Fri, 12 May 2017 14:01:49 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Archit Taneja <architt@...eaurora.org>
Cc:     dri-devel@...ts.freedesktop.org,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        linux-kernel@...r.kernel.org,
        Carlos Palminha <CARLOS.PALMINHA@...opsys.com>
Subject: Re: [PATCH v2 6/8] drm: Introduce drm_bridge_mode_valid()

Hi Archit,

On Friday 12 May 2017 16:20:07 Archit Taneja wrote:
> On 05/12/2017 03:08 PM, Laurent Pinchart wrote:
> > On Wednesday 10 May 2017 17:14:33 Daniel Vetter wrote:
> >> On Wed, May 10, 2017 at 04:41:09PM +0300, Ville Syrjälä wrote:
> >>> On Tue, May 09, 2017 at 06:00:13PM +0100, Jose Abreu wrote:
> >>>> Introduce a new helper function which calls mode_valid() callback
> >>>> for all bridges in an encoder chain.
> >>>> 
> >>>> Signed-off-by: Jose Abreu <joabreu@...opsys.com>
> >>>> Cc: Carlos Palminha <palminha@...opsys.com>
> >>>> Cc: Alexey Brodkin <abrodkin@...opsys.com>
> >>>> Cc: Ville Syrjälä <ville.syrjala@...ux.intel.com>
> >>>> Cc: Daniel Vetter <daniel.vetter@...ll.ch>
> >>>> Cc: Dave Airlie <airlied@...ux.ie>
> >>>> Cc: Andrzej Hajda <a.hajda@...sung.com>
> >>>> Cc: Archit Taneja <architt@...eaurora.org>
> >>>> ---
> >>>> 
> >>>>  drivers/gpu/drm/drm_bridge.c | 33 +++++++++++++++++++++++++++++++++
> >>>>  include/drm/drm_bridge.h     |  2 ++
> >>>>  2 files changed, 35 insertions(+)
> >>>> 
> >>>> diff --git a/drivers/gpu/drm/drm_bridge.c
> >>>> b/drivers/gpu/drm/drm_bridge.c
> >>>> index 86a7637..dc8cdfe 100644
> >>>> --- a/drivers/gpu/drm/drm_bridge.c
> >>>> +++ b/drivers/gpu/drm/drm_bridge.c
> >>>> @@ -206,6 +206,39 @@ bool drm_bridge_mode_fixup(struct drm_bridge
> >>>> *bridge,
> >>>> 
> >>>>  EXPORT_SYMBOL(drm_bridge_mode_fixup);
> >>>>  
> >>>>  /**
> >>>> 
> >>>> + * drm_bridge_mode_valid - validate the mode against all bridges in
> >>>> the
> >>>> + * 			   encoder chain.
> >>>> + * @bridge: bridge control structure
> >>>> + * @mode: desired mode to be validated
> >>>> + *
> >>>> + * Calls &drm_bridge_funcs.mode_valid for all the bridges in the
> >>>> encoder
> >>>> + * chain, starting from the first bridge to the last. If at least one
> >>>> bridge + * does not accept the mode the function returns the error
> >>>> code.
> >>>> + *
> >>>> + * Note: the bridge passed should be the one closest to the encoder.
> >>>> + *
> >>>> + * RETURNS:
> >>>> + * MODE_OK on success, drm_mode_status Enum error code on failure
> >>>> + */
> >>>> +enum drm_mode_status drm_bridge_mode_valid(struct drm_bridge *bridge,
> >>>> +					   const struct 
drm_display_mode
> > 
> > *mode)
> > 
> >>>> +{
> >>>> +	enum drm_mode_status ret = MODE_OK;
> >>>> +
> >>>> +	if (!bridge)
> >>>> +		return ret;
> >>>> +
> >>>> +	if (bridge->funcs->mode_valid)
> >>>> +		ret = bridge->funcs->mode_valid(bridge, mode);
> >>>> +
> >>>> +	if (ret != MODE_OK)
> >>>> +		return ret;
> >>>> +
> >>>> +	return drm_bridge_mode_valid(bridge->next, mode);
> >>> 
> >>> Looks like it should be pretty trivial to avoid the recursion.
> >>> 
> >>> Am I correct in interpreting this that bridges have some kind of
> >>> a hand rolled linked list implementation? Reusing the standard
> >>> linked lists would allow you to use list_for_each() etc.
> >> 
> >> Yeah it's a hand-rolled list, but current hw also has a bridge nesting
> >> depth of 2, so it really doesn't matter. I guess once we have real long
> >> chains of bridges we can fix this (and just using list_head sounds like a
> >> great idea).
> > 
> > Even if not really needed right now, it's a pretty easy cleanup, if Jose
> > has time to handle it in v3 of this series let's not postpone it ;-)
> 
> jfyi, some of the bridge functions call the ops from the last bridge in the
> chain to first, so we'd need to use list_for_each_entry_prev() (or something
> like that) for them.

And now that I think about it, for some of the operations (especially 
enable/disable) I believe that the bridge should be able to decide whether to 
call the next/previous bridge first or to configure its hardware first. I can 
image bridges that need the previous bridge in the chain to provide a valid 
clock before they get started, as well as bridges that need to be started with 
the incoming video signal stopped.

-- 
Regards,

Laurent Pinchart