Re: [PATCH v3 03/13] drm: bridge: Link encoder and bridge in core code

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Archit,

On Wednesday 30 Nov 2016 10:35:02 Archit Taneja wrote:
> On 11/29/2016 11:27 PM, Laurent Pinchart wrote:
> > On Tuesday 29 Nov 2016 15:57:06 Archit Taneja wrote:
> >> On 11/29/2016 02:34 PM, Laurent Pinchart wrote:
> >>> Instead of linking encoders and bridges in every driver (and getting it
> >>> wrong half of the time, as many drivers forget to set the drm_bridge
> >>> encoder pointer), do so in core code. The drm_bridge_attach() function
> >>> needs the encoder and optional previous bridge to perform that task,
> >>> update all the callers.
> >>> 
> >>> Signed-off-by: Laurent Pinchart
> >>> <laurent.pinchart+renesas@ideasonboard.com>
> >>> ---
> >>> 
> >>>  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_output.c   |  4 +-
> >>>  drivers/gpu/drm/bridge/analogix/analogix_dp_core.c |  4 +-
> >>>  drivers/gpu/drm/bridge/dw-hdmi.c                   |  3 +-
> >>>  drivers/gpu/drm/drm_bridge.c                       | 46 ++++++++++-----
> >>>  drivers/gpu/drm/drm_simple_kms_helper.c            |  4 +-
> >>>  drivers/gpu/drm/exynos/exynos_dp.c                 |  5 +--
> >>>  drivers/gpu/drm/exynos/exynos_drm_dsi.c            |  6 +--
> >>>  drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_rgb.c          |  5 +--
> >>>  drivers/gpu/drm/hisilicon/kirin/dw_drm_dsi.c       |  5 +--
> >>>  drivers/gpu/drm/imx/imx-ldb.c                      |  6 +--
> >>>  drivers/gpu/drm/imx/parallel-display.c             |  4 +-
> >>>  drivers/gpu/drm/mediatek/mtk_dpi.c                 |  8 ++--
> >>>  drivers/gpu/drm/mediatek/mtk_dsi.c                 | 24 ++---------
> >>>  drivers/gpu/drm/mediatek/mtk_hdmi.c                | 11 +++---
> >>>  drivers/gpu/drm/msm/dsi/dsi_manager.c              | 17 +++++---
> >>>  drivers/gpu/drm/msm/edp/edp_bridge.c               |  2 +-
> >>>  drivers/gpu/drm/msm/hdmi/hdmi_bridge.c             |  2 +-
> >>>  drivers/gpu/drm/rcar-du/rcar_du_hdmienc.c          |  5 +--
> >>>  drivers/gpu/drm/sti/sti_dvo.c                      |  3 +-
> >>>  drivers/gpu/drm/sti/sti_hda.c                      |  3 +-
> >>>  drivers/gpu/drm/sti/sti_hdmi.c                     |  3 +-
> >>>  drivers/gpu/drm/sun4i/sun4i_rgb.c                  | 13 +++---
> >>>  include/drm/drm_bridge.h                           |  3 +-
> >>>  23 files changed, 83 insertions(+), 103 deletions(-)
> > 
> > [snip]
> > 
> >>> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> >>> index 0ee052b7c21a..850bd6509ef1 100644
> >>> --- a/drivers/gpu/drm/drm_bridge.c
> >>> +++ b/drivers/gpu/drm/drm_bridge.c
> > 
> > [snip]
> > 
> >>> @@ -92,32 +93,53 @@ void drm_bridge_remove(struct drm_bridge *bridge)
> >>>  EXPORT_SYMBOL(drm_bridge_remove);
> >>>  
> >>>  /**
> >>> - * drm_bridge_attach - associate given bridge to our DRM device
> >>> + * drm_bridge_attach - attach the bridge to an encoder's chain
> >>>   *
> >>> - * @dev: DRM device
> >>> - * @bridge: bridge control structure
> >>> + * @encoder: DRM encoder
> >>> + * @bridge: bridge to attach
> >>> + * @previous: previous bridge in the chain (optional)
> >>>   *
> >>> - * Called by a kms driver to link one of our encoder/bridge to the
> >>> given
> >>> - * bridge.
> >>> + * Called by a kms driver to link the bridge to an encoder's chain. The
> >>> previous
> >>> + * argument specifies the previous bridge in the chain. If NULL, the
> >>> bridge is
> >>> + * linked directly at the encoder's output. Otherwise it is linked at
> >>> the
> >>> + * previous bridge's output.
> >>>   *
> >>> - * Note that setting up links between the bridge and our encoder/bridge
> >>> - * objects needs to be handled by the kms driver itself.
> >>> + * If non-NULL the previous bridge must be already attached by a call
> >>> to this
> >>> + * function.
> >>>   *
> >>>   * RETURNS:
> >>>   * Zero on success, error code on failure
> >>>   */
> >>> -int drm_bridge_attach(struct drm_device *dev, struct drm_bridge
> >>> *bridge)
> >>> +int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge
> >>> *bridge,
> >>> +		      struct drm_bridge *previous)
> >>>  {
> >>> -	if (!dev || !bridge)
> >>> +	int ret;
> >>> +
> >>> +	if (!encoder || !bridge)
> >>> +		return -EINVAL;
> >> 
> >> I think we could derive previous from the encoder itself. Something like:
> >> 	previous = encoder->bridge;
> >> 	while (previous && previous->next)
> >> 	
> >> 		previous = previous->next;
> > 
> > That's a very good point. It would however prevent us from catching
> > drivers that attach bridges in the wrong order, which the !previous->dev
> > currently allows us to do (and it should be turned into a WARN_ON as
> > Daniel proposed).
>
> With the simpler API, I don't think we will ever hit the case of
> !previous->dev. The previous bridge (if it exists) in the chain would
> already have a dev attached to it. In other words, we would remove the
> risk of the chance of the 'previous' bridge being unattached.
> 
> I'm a bit unclear about what you mean about the order part. If a kms driver
> wants to create a chain: encoder->bridge1->bridge2, it should ideally do:
> 
> drm_bridge_attach(encoder, bridge1, NULL);
> drm_bridge_attach(encoder, bridge2, bridge1);

Correct.

> We can't do much if the kms driver does the opposite:
> 
> drm_bridge_attach(encoder, bridge2, NULL);
> drm_bridge_attach(encoder, bridge2, bridge1);

That would certainly be a very stupid thing for a driver to do :-) The problem 
that we could catch with my current proposal is

drm_bridge_attach(encoder, bridge2, bridge1);
...
drm_bridge_attach(encoder, bridge1, NULL);

which I expect to happen from time to time as the two bridge can be attached 
through separate code paths sometimes a bit difficult to trace. It's not a big 
deal though, you could convince me that the advantages of a simpler API exceed 
its drawbacks.

> > I'm fine losing that ability, as your proposal makes the API simpler. I'll
> > let you decide, which option do you prefer ?
> 
> I prefer the simpler API. I guess the main aim of the patch was to prevent
> the driver setting up the encoder<->bridge links, which will be done
> anyway.

If you still prefer the simpler API after reading the above, I'll update my 
patch.

> >>> +
> >>> +	if (previous && (!previous->dev || previous->encoder != encoder))
> >>>  		return -EINVAL;
> >>>  	
> >>>  	if (bridge->dev)
> >>>  		return -EBUSY;
> >>> 
> >>> -	bridge->dev = dev;
> >>> +	bridge->dev = encoder->dev;
> >>> +	bridge->encoder = encoder;
> >>> +
> >>> +	if (bridge->funcs->attach) {
> >>> +		ret = bridge->funcs->attach(bridge);
> >>> +		if (ret < 0) {
> >>> +			bridge->dev = NULL;
> >>> +			bridge->encoder = NULL;
> >>> +			return ret;
> >>> +		}
> >>> +	}
> >>> 
> >>> -	if (bridge->funcs->attach)
> >>> -		return bridge->funcs->attach(bridge);
> >>> +	if (previous)
> >>> +		previous->next = bridge;
> >>> +	else
> >>> +		encoder->bridge = bridge;
> >>> 
> >>>  	return 0;
> >>>  }
> >> 
> >> <snip>

-- 
Regards,

Laurent Pinchart