linux-kernel - Re: [PATCH v2 26/26] drm/bridge: establish a link between the bridge supplier and consumer

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Date:   Fri, 11 May 2018 09:37:47 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Andrzej Hajda <a.hajda@...sung.com>, linux-kernel@...r.kernel.org
Cc:     Archit Taneja <architt@...eaurora.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        David Airlie <airlied@...ux.ie>,
        Peter Senna Tschudin <peter.senna@...labora.com>,
        Martin Donnelly <martin.donnelly@...com>,
        Martyn Welch <martyn.welch@...labora.co.uk>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        Inki Dae <inki.dae@...sung.com>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        CK Hu <ck.hu@...iatek.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Rob Clark <robdclark@...il.com>,
        Sandy Huang <hjc@...k-chips.com>,
        Heiko Stübner <heiko@...ech.de>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Vincent Abriou <vincent.abriou@...com>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, linux-arm-msm@...r.kernel.org,
        freedreno@...ts.freedesktop.org, linux-renesas-soc@...r.kernel.org,
        linux-rockchip@...ts.infradead.org, Jyri Sarha <jsarha@...com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v2 26/26] drm/bridge: establish a link between the bridge
 supplier and consumer

On 2018-05-10 10:10, Andrzej Hajda wrote:
> On 04.05.2018 15:52, Peter Rosin wrote:
>> If the bridge supplier is unbound, this will bring the bridge consumer
>> down along with the bridge. Thus, there will no longer linger any
>> dangling pointers from the bridge consumer (the drm_device) to some
>> non-existent bridge supplier.
>>
>> Signed-off-by: Peter Rosin <peda@...ntia.se>
>> ---
>>  drivers/gpu/drm/drm_bridge.c | 18 ++++++++++++++++++
>>  include/drm/drm_bridge.h     |  2 ++
>>  2 files changed, 20 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
>> index 78d186b6831b..0259f0a3ff27 100644
>> --- a/drivers/gpu/drm/drm_bridge.c
>> +++ b/drivers/gpu/drm/drm_bridge.c
>> @@ -26,6 +26,7 @@
>>  #include <linux/mutex.h>
>>  
>>  #include <drm/drm_bridge.h>
>> +#include <drm/drm_device.h>
>>  #include <drm/drm_encoder.h>
>>  
>>  #include "drm_crtc_internal.h"
>> @@ -127,12 +128,25 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
>>  	if (bridge->dev)
>>  		return -EBUSY;
>>  
>> +	if (encoder->dev->dev != bridge->odev) {
> 
> I wonder why device_link_add does not handle this case (self dependency)
> silently as noop, as it seems to be a correct behavior.

It's kind-of a silly corner-case though, so perfectly understandable
that it isn't handled.

>> +		bridge->link = device_link_add(encoder->dev->dev,
>> +					       bridge->odev, 0);
>> +		if (!bridge->link) {
>> +			dev_err(bridge->odev, "failed to link bridge to %s\n",
>> +				dev_name(encoder->dev->dev));
>> +			return -EINVAL;
>> +		}
>> +	}
>> +
>>  	bridge->dev = encoder->dev;
>>  	bridge->encoder = encoder;
>>  
>>  	if (bridge->funcs->attach) {
>>  		ret = bridge->funcs->attach(bridge);
>>  		if (ret < 0) {
>> +			if (bridge->link)
>> +				device_link_del(bridge->link);
>> +			bridge->link = NULL;
>>  			bridge->dev = NULL;
>>  			bridge->encoder = NULL;
>>  			return ret;
>> @@ -159,6 +173,10 @@ void drm_bridge_detach(struct drm_bridge *bridge)
>>  	if (bridge->funcs->detach)
>>  		bridge->funcs->detach(bridge);
>>  
>> +	if (bridge->link)
>> +		device_link_del(bridge->link);
>> +	bridge->link = NULL;
>> +
>>  	bridge->dev = NULL;
>>  }
>>  
>> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
>> index b656e505d11e..804189c63a4c 100644
>> --- a/include/drm/drm_bridge.h
>> +++ b/include/drm/drm_bridge.h
>> @@ -261,6 +261,7 @@ struct drm_bridge_timings {
>>   * @list: to keep track of all added bridges
>>   * @timings: the timing specification for the bridge, if any (may
>>   * be NULL)
>> + * @link: drm consumer <-> bridge supplier
> 
> Nitpick: "<->" suggests symmetry, maybe "device link from drm consumer
> to the bridge" would be better.

I meant "<->" to indicate that the link is bidirectional, not that the
relationship is in any way symmetric. I wasn't aware of any implication
of a symmetric relationship when using "<->", do you have a reference?
But I guess the different arrow notations in math are somewhat overloaded
and that someone at some point must have used "<->" to indicate a
symmetric relationship...

> Anyway:
> Reviewed-by: Andrzej Hajda <a.hajda@...sung.com>

Thanks!

Cheers,
Peter

>  --
> Regards
> Andrzej
> 
>>   * @funcs: control functions
>>   * @driver_private: pointer to the bridge driver's internal context
>>   */
>> @@ -271,6 +272,7 @@ struct drm_bridge {
>>  	struct drm_bridge *next;
>>  	struct list_head list;
>>  	const struct drm_bridge_timings *timings;
>> +	struct device_link *link;
>>  
>>  	const struct drm_bridge_funcs *funcs;
>>  	void *driver_private;
> 
>