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

[Archit Taneja <architt@codeaurora.org>]

On 11/29/2016 11:27 PM, Laurent Pinchart wrote:
> Hi Archit,
>
> 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);

We can't do much if the kms driver does the opposite:

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


> 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.

Thanks,
Archit

>
>>> +
>>> +	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>
>

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