Re: [PATCH 03/10] drm/sysfs: Add mstpath attribute to connector devices

["Ville Syrjälä" <ville.syrjala-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Thu, Jul 04, 2019 at 03:05:12PM -0400, sunpeng.li@amd.com wrote:
> From: Leo Li <sunpeng.li@amd.com>
> 
> This can be used to create more descriptive symlinks for MST aux
> devices. Consider the following udev rule:
> 
> SUBSYSTEM=="drm_dp_aux_dev", SUBSYSTEMS=="drm", ATTRS{mstpath}=="?*",
> 	SYMLINK+="drm_dp_aux/by-path/$attr{mstpath}"
> 
> The following symlinks will be created (depending on your MST topology):
> 
> $ ls /dev/drm_dp_aux/by-path/
> card0-mst:0-1  card0-mst:0-1-1  card0-mst:0-1-8  card0-mst:0-8
> 
> Signed-off-by: Leo Li <sunpeng.li@amd.com>
> ---
>  drivers/gpu/drm/drm_sysfs.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_sysfs.c b/drivers/gpu/drm/drm_sysfs.c
> index ad10810bc972..53fd1f71e900 100644
> --- a/drivers/gpu/drm/drm_sysfs.c
> +++ b/drivers/gpu/drm/drm_sysfs.c
> @@ -236,16 +236,39 @@ static ssize_t modes_show(struct device *device,
>  	return written;
>  }
>  
> +static ssize_t mstpath_show(struct device *device,
> +			    struct device_attribute *attr,
> +			    char *buf)
> +{
> +	struct drm_connector *connector = to_drm_connector(device);
> +	ssize_t ret = 0;
> +	char *path;
> +
> +	mutex_lock(&connector->dev->mode_config.mutex);

The blob is populated when the connector is created so I don't think
this lock is actually doing anything for us.

One would also hope that device_unregister() protects us from racing
with the removal of the attribute. Eg. if you hold a file descriptor
open to the sysfs file does device_unregister() block until the fd is
closed?

> +	if (!connector->path_blob_ptr)
> +		goto unlock;
> +
> +	path = connector->path_blob_ptr->data;
> +	ret = snprintf(buf, PAGE_SIZE, "card%d-%s\n",
> +		       connector->dev->primary->index, path);
> +
> +unlock:
> +	mutex_unlock(&connector->dev->mode_config.mutex);
> +	return ret;
> +}
> +
>  static DEVICE_ATTR_RW(status);
>  static DEVICE_ATTR_RO(enabled);
>  static DEVICE_ATTR_RO(dpms);
>  static DEVICE_ATTR_RO(modes);
> +static DEVICE_ATTR_RO(mstpath);
>  
>  static struct attribute *connector_dev_attrs[] = {
>  	&dev_attr_status.attr,
>  	&dev_attr_enabled.attr,
>  	&dev_attr_dpms.attr,
>  	&dev_attr_modes.attr,
> +	&dev_attr_mstpath.attr,
>  	NULL
>  };
>  
> -- 
> 2.22.0

-- 
Ville Syrjälä
Intel
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