Re: [PATCH v3 1/4] remoteproc: core: Move cdev add before device add

[Siddharth Gupta]

On 6/23/2021 12:27 AM, Greg KH wrote:
> On Wed, Jun 16, 2021 at 11:47:01AM -0700, Siddharth Gupta wrote:
> > On 6/15/2021 10:58 PM, Greg KH wrote:
> > > On Tue, Jun 15, 2021 at 12:03:26PM -0700, Siddharth Gupta wrote:
> > > > On 6/14/2021 9:56 PM, Greg KH wrote:
> > > > > On Mon, Jun 14, 2021 at 07:21:08PM -0700, Siddharth Gupta wrote:
> > > > > > When cdev_add is called after device_add has been called there is no
> > > > > > way for the userspace to know about the addition of a cdev as cdev_add
> > > > > > itself doesn't trigger a uevent notification, or for the kernel to
> > > > > > know about the change to devt. This results in two problems:
> > > > > >     - mknod is never called for the cdev and hence no cdev appears on
> > > > > >       devtmpfs.
> > > > > >     - sysfs links to the new cdev are not established.
> > > > > >
> > > > > > The cdev needs to be added and devt assigned before device_add() is
> > > > > > called in order for the relevant sysfs and devtmpfs entries to be
> > > > > > created and the uevent to be properly populated.
> > > > > So this means no one ever ran this code on a system that used devtmpfs?
> > > > >
> > > > > How was it ever tested?
> > > > My testing was done with toybox + Android's ueventd ramdisk.
> > > > As I mentioned in the discussion, the race became evident
> > > > recently. I will make sure to test all such changes without
> > > > systemd/ueventd in the future.
> > > It isn't an issue of systemd/ueventd, those do not control /dev on a
> > > normal system, that is what devtmpfs is for.
> > I am not fully aware of when devtmpfs is enabled or not, but in
> > case it is not - systemd/ueventd will create these files with
> > mknod, right?
> No, systemd does not create device nodes, and neither does udev.  Hasn't
> done so for well over 10 years now.
Oh okay. I thought ueventd does it because it allows setting
the node permissions through ueventd.rc:
https://android.googlesource.com/platform/system/core/+/master/rootdir/ueventd.rc
> > I was even manually able to call mknod from the
> > terminal when some of the remoteproc character device entries
> > showed up (using major number from there, and minor number being
> > the remoteproc id), and that allowed me to boot up the
> > remoteprocs as well.
> Yes, that is fine, but that also means that this was not working from
> the very beginning :(
Right. To clarify, I did this after we started seeing the problem
on our devices, which led me to believe there was a race between
ueventd and cdev_add(). Not sure anymore if that is not the case.
> > > And devtmpfs nodes are only created if you create a struct device
> > > somewhere with a proper major/minor, which you were not doing here, so
> > > you must have had a static /dev on your test systems, right?
> > I am not sure of what you mean by a static /dev? Could you
> > explain? In case you mean the character device would be
> > non-functional, that is not the case. They have been working
> > for us since the beginning.
> /dev on modern systems is managed by devtmpfs, which knows to create the
> device nodes when you properly register the device with the driver core.
> A "static" /dev is managed by mknod from userspace, like you did "by
> hand", and that is usually only done by older systems.
Thanks for the explanation! As I mentioned earlier - I was under
the impression that ueventd does it. I will go through our older
builds where this was working (without this patch) and try to see
how the dev nodes were being populated.

Thanks,
Sid
> thanks,
>
> greg k-h