Re: [PATCH] alsa-restore.rules: refer to correct attr

[Dave Reisner <d@falconindy.com>]

On Sun, Jan 12, 2014 at 11:15:52AM -0500, Dave Reisner wrote:
> $attr{number} in the RUN rule is an empty expansion. This makes sense,
> because the path doesn't exist -- i.e., it refers to the path:
> 
> /sys/devices/pci0000:00/foo/bar/sound/card0/controlC0/number
> 
> Instead, refer to $attr{device/number}, which does exist.
> 
> Signed-off-by: Dave Reisner <dreisner@archlinux.org>
> ---
> This seems to have been broken when it was first introduced a little over
> three years ago (de7c3eff0e).

I dug into this a little more to figure out why this potentially *would*
have worked and found the following advice from udev(7) about $attr{}:

  If the matching device does not have such an attribute, and a previous
  KERNELS, SUBSYSTEMS, DRIVERS, or ATTRS test selected a parent device,
  then the attribute from that parent device is used.

So, knowing how RUN expansions work, the simple presence of the
following rule causes $attr{number} to expand properly

  SUBSYSTEM=="sound", ATTRS{id}=="M2496"

I named this /etc/udev/rules/91-alsa-foo.rules -- intentionally after
the restore rule.

Of course, this specifically matches my system, but one could imagine
a more general rule which would cause parent matches and thus change the
behavior of the subsequent $attr{number} expansion. I'm not sure if this
is intended behavior in udev since the parent match doesn't occur in the
same rule.

So, $attr{number} is potentially correct (but this seems weird), while
$attr{device/number} should always be correct.

Hope this makes sense...

> Please CC me on replies as I am not subscribed to the list.
> 
>  alsactl/90-alsa-restore.rules.in | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/alsactl/90-alsa-restore.rules.in b/alsactl/90-alsa-restore.rules.in
> index 88e12e0..c68119d 100644
> --- a/alsactl/90-alsa-restore.rules.in
> +++ b/alsactl/90-alsa-restore.rules.in
> @@ -2,7 +2,7 @@ ACTION=="add", SUBSYSTEM=="sound", KERNEL=="controlC*", KERNELS!="card*", GOTO="
>  GOTO="alsa_restore_end"
>  
>  LABEL="alsa_restore_go"
> -TEST!="@daemonswitch@", RUN+="@sbindir@/alsactl restore $attr{number}"
> -TEST=="@daemonswitch@", RUN+="@sbindir@/alsactl nrestore $attr{number}"
> +TEST!="@daemonswitch@", RUN+="@sbindir@/alsactl restore $attr{device/number}"
> +TEST=="@daemonswitch@", RUN+="@sbindir@/alsactl nrestore $attr{device/number}"
>  
>  LABEL="alsa_restore_end"
> -- 
> 1.8.5.2
>