Re: [PATCH] staging: vchiq: convert to use a miscdevice

On Fri, Sep 10, 2021 at 02:05:19PM +0200, Greg Kroah-Hartman wrote:

On Fri, Sep 10, 2021 at 05:10:04PM +0530, Ojaswin Mujoo wrote:

quoted

On Tue, Sep 07, 2021 at 01:50:45PM +0200, Greg Kroah-Hartman wrote:

quoted

Using a struct class, a cdev, and another device just for a single minor
device is total overkill.  Just use a dynamic misc device instead,
saving lots of logic and memory.

Hello Greg,

I got some time to test this out at my end. This seems to work correctly
however there's a small change in permissions applied to /dev/vchiq that
is causing tests to break.

* Permissions before the patch *
$ ls -l /dev/vchiq 
crw-rw---- 1 root video 235, 0 May  7 17:33 vchiq

* Permissions after the patch *
$ ls -l /dev/vchiq 
crw------- 1 root root 10, 125 May  7 17:30 vchiq

As seen above, after the patch, the cdev is only accessible by root user,
which is causing the tests ($ vchiq_test -f 10) to fail when run as
non-root.

Ah, that's not under the kernel's control, but as you point out, it's a
udev issue.

quoted

I believe assigning the permission and "video" group to /dev/vchiq is
handled by udev, in the downstream pi OS, as seen in this line in
/lib/udev/rules.d/10-local-rpi.rules file:

SUBSYSTEM=="vchiq", GROUP="video", MODE="0660" 

I'm not completely sure how the SUBSYSTEM part is passed to udev from
the kernel modules, however seems like the miscdevice is not notifying
udev correctly (?).

No, the SUBSYSTEM for this device has changed from "vchiq" to "misc".

Having a whole subsystem for just one character device is crazy, which
is why I did the kernel change.

Try changing the line in the udev file to:

	NAME=="vchiq", GROUP="video", MODE="0660"

(SUBSYSTEM changes to NAME) and see if that works both on the newer
kernel, and on the older ones as
well.

Hello, thanks for the explanation and pointers.

The "NAME==vchiq" change doesn't seem to work but I was able to get it
correctly working by using "KERNEL=vchiq" instead:

   KERNEL=="vchiq", GROUP="video", MODE="0660"

I tested this with and without this patch and it works as expected. 

Regards,
Ojaswin

thanks,

greg k-h

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