[PATCH] mdev: create devices from /sys/dev

[Isaac Dunham]

On Mon, Apr 11, 2016 at 11:09:41AM +0200, Linus Walleij wrote:
> Currently some new devices that have a bus but no class will
> be missed by mdev coldplug device creation after boot. This
> happens because mdev recursively searches /sys/class which will
> by definition only find class devices.
> 
> Some important devices such as iio and gpiochip does not have
> a class. But users will need them.
> 
> This switches from using /sys/class as the place to look for
> devices to create to using /sys/dev where all char and block
> devices are listed. We keep /sys/class as a fallback if /sys/dev
> is nonexisting.
> 
> Tested on kernel v4.6-rc2 with a bunch of devices, including
> some IIO and gpiochip devices.

In general, I think this is a good idea (especially considering that
module loading is not done by mdev -s).
However, I suspect that this patch will break $SUBSYSTEM rules--for
example, this rule to change the group of sound devices:
SUBSYSTEM=sound;.*	root:audio	0660

The problem here is that dirAction is setting the subsystem based on
the first directory under /sys/class/, so only block devices would have
the right SUBSYSTEM.
In /sys/dev/char, there's no way to get subsystem from the directory,
so you would need to set it in fileAction; I'm not sure what file is
guaranteed to be readable if present and contains the subsystem off
the top of my head. (You would want to use openat() rather than accepting
a file in addition to ./dev, though.)

> Cc: Isaac Dunham <ibid.ag at gmail.com>
> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> Cc: Jonathan Cameron <jic23 at cam.ac.uk>
> Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
> ---
> If you prefer that I delete the code searching for devices in
> /sys/class with sys/block as fallback, I will happily do so.

If I recall correctly, that would only be problematic for some early
2.6.2x kernels.

> ---
>  util-linux/mdev.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/util-linux/mdev.c b/util-linux/mdev.c
> index 37fa56827520..f833d9730ea5 100644
> --- a/util-linux/mdev.c
> +++ b/util-linux/mdev.c
> @@ -1079,9 +1079,16 @@ int mdev_main(int argc UNUSED_PARAM, char **argv)
>  				ACTION_RECURSE | ACTION_FOLLOWLINKS | ACTION_QUIET,
>  				fileAction, dirAction, temp, 0);
>  		}
> -		recursive_action("/sys/class",
> -			ACTION_RECURSE | ACTION_FOLLOWLINKS,
> -			fileAction, dirAction, temp, 0);
> +		/* If /sys/dev does not exist, use /sys/class */
> +		if (access("/sys/dev", F_OK) != 0) {
> +			recursive_action("/sys/class",
> +					 ACTION_RECURSE | ACTION_FOLLOWLINKS,
> +					 fileAction, dirAction, temp, 0);
> +		}
> +		/* By default create all devices from /sys/dev hierarchy */
> +		recursive_action("/sys/dev",
> +				 ACTION_RECURSE | ACTION_FOLLOWLINKS,
> +				 fileAction, dirAction, temp, 0);
>  	} else {
>  		char *fw;
>  		char *seq;
> -- 
> 2.4.3
>