Accessing rpmsg_device in sysfs attribute functions

[Pelle Windestam]

> From: Greg KH <greg at kroah.com>
> Sent: Tuesday, 24 March 2020 13:31
> To: Pelle Windestam <Pelle.Windestam at tagmaster.com>
> Cc: kernelnewbies at kernelnewbies.org
> Subject: Re: Accessing rpmsg_device in sysfs attribute functions
> 
> On Tue, Mar 24, 2020 at 12:02:15PM +0000, Pelle Windestam wrote:
> > > On Tue, Mar 24, 2020 at 05:34:44AM +0000, Pelle Windestam wrote:
> > > > Hi,
> > > >
> > > > I am trying to develop a simple driver for the rpmsg bus, in order
> > > > to send
> > > various commands from user space in Linux to a secondary CPU (A Cortex
> M4).
> > > I'm trying to keep things as simple as possible, so my idea was to
> > > create a driver that just has a few attributes which can be set in
> > > /sys which would trigger commands to be sent to the M4 CPU. I have
> > > the communication between the CPU:s up and running, but where I'm
> > > having trouble moving forward is how to access the "struct
> > > rpmsg_device *" that I need in order to communicate with the
> > > endpoint for the M4 CPU from the store/show function of the sysfs
> > > attributes. What my driver does is to register a rpmsg_driver in the
> > > init
> > > function:
> > > >
> > > > register_rpmsg_driver(&pwm_rpmsg_driver);
> > > >
> > > > the device_driver member of my rpmsg_driver struct has its groups
> > > > member
> > > set to my driver attribute groups array:
> > > > static struct rpmsg_driver pwm_rpmsg_driver = {
> > > > 	.probe = pwm_rpmsg_probe,
> > > > 	.remove = pwm_rpmsg_remove,
> > > > 	.callback = pwm_rpmsg_cb,
> > > > 	.id_table = pwm_rpmsg_device_id_table,
> > > > 	.drv = {
> > > > 		.groups = driver_pwm_groups,
> > > > 		.name = "pwm_rpmsg",
> > > > 	},
> > > > };
> > > >
> > > > My issue is that that I am not sure how to access the struct
> > > > "rpmsg_device
> > > *" (i.e. from the probe() function) in the show/store functions for
> > > the sysfs attributes, which have a "struct device_driver *" argument:
> > >
> > > That is because you have created a driver attribute, not a device
> attribute.
> > > Create device attributes and you should be fine, they bind to the
> > > device your driver is passed.
> >
> > Thanks! Changing them to device attributes was a breeze. Now I am
> > slightly confused about the "struct device *" argument to the
> > store/show functions. I was under the impression that this would be
> > the "struct device" in the struct rpmsg_device, (which would let me
> > get the struct rpmsg_device using container_of()?), but it appears to
> > be some completely other device (by looking at the pointer address). I
> > have tried searching the kernel code for similar example, but I have
> > not found anything so far. It feels like I am stumbling a bit in the
> > dark here, looking for my rpmsg_device.
> 
> It's a bit hard to figure out what exactly you are doing here without a
> pointer to the code itself :)
> 
> Are you sure you aren't pointing the platform device accidentally?
> 
> thanks,
> 
> greg k-h

I suppose anything is possible 😊 Here is the actual code if you want to have a look:
https://gist.github.com/iceaway/9900a9c2dd221eb836c5acda49f5d688

//Pelle