Re: [PATCH v6 01/24] drm: Add ddc link in sysfs created by drm_connector

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

Hi Andrzej,

On Sun, Aug 04, 2019 at 03:04:37PM +0300, Laurent Pinchart wrote:
> Hi Andrzej,
> 
> Thank you for the patch, and sorry for the late review (I've been
> travelling for the past few weeks).
> 
> On Fri, Jul 26, 2019 at 07:22:55PM +0200, Andrzej Pietrasiewicz wrote:
> > Add generic code which creates symbolic links in sysfs, pointing to ddc
> > interface used by a particular video output. For example:
> > 
> > ls -l /sys/class/drm/card0-HDMI-A-1/ddc
> > lrwxrwxrwx 1 root root 0 Jun 24 10:42 /sys/class/drm/card0-HDMI-A-1/ddc \
> > 	-> ../../../../soc/13880000.i2c/i2c-2
> > 
> > This makes it easy for user to associate a display with its ddc adapter
> > and use e.g. ddcutil to control the chosen monitor.
> > 
> > This patch adds an i2c_adapter pointer to struct drm_connector. Particular
> > drivers can then use it instead of using their own private instance. If a
> > connector contains a ddc, then create a symbolic link in sysfs.
> > 
> > Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
> > Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
> > ---
> >  drivers/gpu/drm/drm_sysfs.c |  8 ++++++++
> >  include/drm/drm_connector.h | 11 +++++++++++
> >  2 files changed, 19 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_sysfs.c b/drivers/gpu/drm/drm_sysfs.c
> > index ad10810bc972..e962a9d45f7e 100644
> > --- a/drivers/gpu/drm/drm_sysfs.c
> > +++ b/drivers/gpu/drm/drm_sysfs.c
> > @@ -14,6 +14,7 @@
> >  #include <linux/err.h>
> >  #include <linux/export.h>
> >  #include <linux/gfp.h>
> > +#include <linux/i2c.h>
> >  #include <linux/kdev_t.h>
> >  #include <linux/slab.h>
> >  
> > @@ -294,6 +295,9 @@ int drm_sysfs_connector_add(struct drm_connector *connector)
> >  	/* Let userspace know we have a new connector */
> >  	drm_sysfs_hotplug_event(dev);
> >  
> > +	if (connector->ddc)
> > +		return sysfs_create_link(&connector->kdev->kobj,
> > +				 &connector->ddc->dev.kobj, "ddc");
> >  	return 0;
> >  }
> >  
> > @@ -301,6 +305,10 @@ void drm_sysfs_connector_remove(struct drm_connector *connector)
> >  {
> >  	if (!connector->kdev)
> >  		return;
> > +
> > +	if (connector->ddc)
> > +		sysfs_remove_link(&connector->kdev->kobj, "ddc");
> > +
> >  	DRM_DEBUG("removing \"%s\" from sysfs\n",
> >  		  connector->name);
> >  
> > diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> > index 4c30d751487a..33a6fff85fdb 100644
> > --- a/include/drm/drm_connector.h
> > +++ b/include/drm/drm_connector.h
> > @@ -41,6 +41,7 @@ struct drm_property;
> >  struct drm_property_blob;
> >  struct drm_printer;
> >  struct edid;
> > +struct i2c_adapter;
> >  
> >  enum drm_connector_force {
> >  	DRM_FORCE_UNSPECIFIED,
> > @@ -1311,6 +1312,16 @@ struct drm_connector {
> >  	 * [0]: progressive, [1]: interlaced
> >  	 */
> >  	int audio_latency[2];
> > +
> > +	/**
> > +	 * @ddc: associated ddc adapter.
> > +	 * A connector usually has its associated ddc adapter. If a driver uses
> > +	 * this field, then an appropriate symbolic link is created in connector
> > +	 * sysfs directory to make it easy for the user to tell which i2c
> > +	 * adapter is for a particular display.
> 
> The first sentence isn't very clear. The rest is mixing "ddc adapter"
> and "i2c adapter". How about the following ?
> 
> "When the connector carries DDC signals, this field points to the I2C
> adapter connected to the DDC signals, if any. When this field is not
> NULL a symbolic link is created in the connector's sysfs directory to
> expose the I2C adapter used by the connector."
> 
> Should we also mention that the field isn't meant to be set directly,
> but shall be set with drm_connector_init_with_ddc() ?
> 
> "This field shall not be set directly by drivers, use
> drm_connector_init_with_ddc() instead."

I should have read patch 02/24 before answering this :-)

> I'm also slightly concerned about the lifetime of this pointer, and what
> would happen if the I2C adapter disappears while the connector is still
> exposed to userspace, but I suppose this isn't a new issue, existing
> code likely suffers from this.
> 
> With the updated documentation,
> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

I've just realised that this patch has been applied already. I'll send
the above as a documentation update patch.

> > +	 */
> > +	struct i2c_adapter *ddc;
> > +
> >  	/**
> >  	 * @null_edid_counter: track sinks that give us all zeros for the EDID.
> >  	 * Needed to workaround some HW bugs where we get all 0s

-- 
Regards,

Laurent Pinchart
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