linux-kernel - Re: [PATCH] drm: remove drm_bridge_hpd_disable() from drm_bridge_connector

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Date:   Tue, 19 Sep 2023 15:53:16 -0700
From:   Abhinav Kumar <quic_abhinavk@...cinc.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC:     <dri-devel@...ts.freedesktop.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        <freedreno@...ts.freedesktop.org>, <quic_jesszhan@...cinc.com>,
        <quic_parellan@...cinc.com>, <andersson@...nel.org>,
        <jani.nikula@...ux.intel.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm: remove drm_bridge_hpd_disable() from
 drm_bridge_connector_destroy()

Hi Laurent

On 9/19/2023 11:12 AM, Laurent Pinchart wrote:
> Hi Abhinav,
> 
> Thank you for the patch.
> 
> On Tue, Sep 19, 2023 at 10:48:12AM -0700, Abhinav Kumar wrote:
>> drm_bridge_hpd_enable()/drm_bridge_hpd_disable() callbacks call into
>> the respective driver's hpd_enable()/hpd_disable() ops. These ops control
>> the HPD enable/disable logic which in some cases like MSM can be a
>> dedicate hardware block to control the HPD.
>>
>> During probe_defer cases, a connector can be initialized and then later
>> destroyed till the probe is retried. During connector destroy in these
>> cases, the hpd_disable() callback gets called without a corresponding
>> hpd_enable() leading to an unbalanced state potentially causing even
>> a crash.
>>
>> This can be avoided by the respective drivers maintaining their own
>> state logic to ensure that a hpd_disable() without a corresponding
>> hpd_enable() just returns without doing anything.
>>
>> However, to have a generic fix it would be better to avoid the
>> hpd_disable() callback from the connector destroy path and let
>> the hpd_enable() / hpd_disable() balance be maintained by the
>> corresponding drm_bridge_connector_enable_hpd() /
>> drm_bridge_connector_disable_hpd() APIs which should get called by
>> drm_kms_helper_disable_hpd().
> 
> The change makes sense to me, but I'm a bit worried this could introduce
> a regression by leaving HPD enabled in some cases.
> 
> I agree that bridges shouldn't track the HPD state, it should be tracked
> by the core and the .enable_hpd() and .disable_hpd() operations should
> be balanced. Their documentation, however, doesn't clearly state this,
> and the documentation of the callers of these operations is also fairly
> unclear.
> 
> Could you perhaps try to improve the documentation ? With that,
> 

Yes, sure, Let me upload another patch to improve the documentation of 
.enable_hpd(), .disable_hpd() and its callers.

> Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> 
> for this patch.
> 

Thanks

Abhinav

>> changes in v2:
>> 	- minor change in commit text (Dmitry)
>>
>> Signed-off-by: Abhinav Kumar <quic_abhinavk@...cinc.com>
>> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
>> ---
>>   drivers/gpu/drm/drm_bridge_connector.c | 6 ------
>>   1 file changed, 6 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_bridge_connector.c b/drivers/gpu/drm/drm_bridge_connector.c
>> index 1da93d5a1f61..c4dba39acfd8 100644
>> --- a/drivers/gpu/drm/drm_bridge_connector.c
>> +++ b/drivers/gpu/drm/drm_bridge_connector.c
>> @@ -187,12 +187,6 @@ static void drm_bridge_connector_destroy(struct drm_connector *connector)
>>   	struct drm_bridge_connector *bridge_connector =
>>   		to_drm_bridge_connector(connector);
>>   
>> -	if (bridge_connector->bridge_hpd) {
>> -		struct drm_bridge *hpd = bridge_connector->bridge_hpd;
>> -
>> -		drm_bridge_hpd_disable(hpd);
>> -	}
>> -
>>   	drm_connector_unregister(connector);
>>   	drm_connector_cleanup(connector);
>>   
>