linux-kernel - Re: [PATCH v4 02/11] drm/bridge: dw-hdmi: add max bpc connector property

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Message-ID: <6eede520-cbd8-51e5-6b56-afd727d50ab0@baylibre.com>
Date:   Mon, 2 Mar 2020 16:55:37 +0100
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Jernej Škrabec <jernej.skrabec@...l.net>,
        a.hajda@...sung.com, jonas@...boo.se,
        boris.brezillon@...labora.com, linux-amlogic@...ts.infradead.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v4 02/11] drm/bridge: dw-hdmi: add max bpc connector
 property

On 02/03/2020 10:18, Laurent Pinchart wrote:
> Hi Neil and Jonas,
> 
> (CC'ing Daniel for a framework question)
> 
> Thank you for the patch.
> 
> On Fri, Feb 21, 2020 at 09:50:18AM +0100, Neil Armstrong wrote:
>> On 17/02/2020 07:38, Jernej Škrabec wrote:
>>> Dne četrtek, 06. februar 2020 ob 20:18:25 CET je Neil Armstrong napisal(a):
>>>> From: Jonas Karlman <jonas@...boo.se>
>>>>
>>>> Add the max_bpc property to the dw-hdmi connector to prepare support
>>>> for 10, 12 & 16bit output support.
>>>>
>>>> Signed-off-by: Jonas Karlman <jonas@...boo.se>
>>>> Signed-off-by: Neil Armstrong <narmstrong@...libre.com>
>>>> ---
>>>>  drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 4 ++++
>>>>  1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
>>>> b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c index
>>>> 9e0927d22db6..051001f77dd4 100644
>>>> --- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
>>>> +++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
>>>> @@ -2406,6 +2406,10 @@ static int dw_hdmi_bridge_attach(struct drm_bridge
>>>> *bridge) DRM_MODE_CONNECTOR_HDMIA,
>>>>  				    hdmi->ddc);
>>>>
>>>> +	drm_atomic_helper_connector_reset(connector);
>>>
>>> Why is this reset needed?
>>
>> I assume it's to allocate a new connector state to attach a the bpc propery.
>>
>> But indeed, this helper is never used here, but only as callback to the drm_connector_funcs->reset.
>>
>> But, amdgpu calls :
>> 	/*
>> 	 * Some of the properties below require access to state, like bpc.
>> 	 * Allocate some default initial connector state with our reset helper.
>> 	 */
>> 	if (aconnector->base.funcs->reset)
>> 		aconnector->base.funcs->reset(&aconnector->base);
>>
>> which is the same.
> 
> A comment would be useful:
> 
> 	/*
> 	 * drm_connector_attach_max_bpc_property() requires the
> 	 * connector to have a state.
> 	 */
> 	drm_atomic_helper_connector_reset(connector);
> 
> 	drm_connector_attach_max_bpc_property(connector, 8, 16);
> 

Done

> I don't like this much though, it feels like the initial reset performed
> by drm_mode_config_reset() should set default values for all state
> members that are related to properties. Daniel, what's the rationale
> behind the current implementation ?
> 
> This is a DRM core issue that shouldn't block this patch though, so

I'll investigate why, but I haven't found out how the intel driver got the
connector state initialized since they don't use the atomic helpers....

> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> 
>>>> +
>>>> +	drm_connector_attach_max_bpc_property(connector, 8, 16);
>>>> +
>>>>  	if (hdmi->version >= 0x200a && hdmi->plat_data->use_drm_infoframe)
>>>>  		drm_object_attach_property(&connector->base,
>>>>  			connector->dev-
>>>> mode_config.hdr_output_metadata_property, 0);
>