Re: [PATCH 01/12 v2] Platform: add a dev_groups pointer to struct platform_driver

[Dmitry Torokhov]

Hi Greg,

On Sat, Jul 6, 2019 at 1:32 AM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Jul 04, 2019 at 02:17:22PM -0700, Dmitry Torokhov wrote:
> > Hi Greg,
> >
> > On Thu, Jul 4, 2019 at 5:15 AM Greg Kroah-Hartman
> > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > Platform drivers like to add sysfs groups to their device, but right now
> > > they have to do it "by hand".  The driver core should handle this for
> > > them, but there is no way to get to the bus-default attribute groups as
> > > all platform devices are "special and unique" one-off drivers/devices.
> > >
> > > To combat this, add a dev_groups pointer to platform_driver which allows
> > > a platform driver to set up a list of default attributes that will be
> > > properly created and removed by the platform driver core when a probe()
> > > function is successful and removed right before the device is unbound.
> >
> > Why is this limited to platform bus? Drivers for other buses also
> > often want to augment list of their attributes during probe(). I'd
> > move it to generic probe handling.
>
> This is not limited to the platform at all, the driver core supports
> this for any bus type today, but it's then up to the bus-specific code
> to pass that on to the driver core.  That's usually set for the
> bus-specific attributes that they want exposed for all devices of that
> bus type (see the bus_groups, dev_groups, and drv_groups pointers in
> struct bus_type).
>
> For the platform devices, the problem is that this is something that the
> individual drivers want after they bind to the device.  And as all
> platform devices are "different" they can't be a "common" set of
> attributes, so they need to be created after the device is bound to the
> driver.

I believe that your assertion that only platform devices want to
install custom attributes is incorrect. Drivers for devices attached
to serio, i2c, USB, spi, etc, etc, all have additional attributes:

dtor@dtor-ws:~/kernel/work (master *)$ grep -l '\(i2c\|usb\|spi\)'
`git grep -l '\(device_add_group\|sysfs_create_group\)' -- drivers` |
wc -l
170

I am pretty sure some of this count is false positives, but majority
is actually proper hits.

...

> >
> > We already emit KOBJ_BIND when we finish binding device to a driver,
> > regardless of the bus. I know we still need to teach systemd to handle
> > it properly, but I think it is better than sprinkling KOBJ_CHANGE
> > around.
>
> But the object's attributes did just change, which is what KOBJ_CHANGE
> tells userspace, so this should be the correct thing to say to
> userspace.
>
> And yes, ideally KOBJ_BIND would be handled, and it will be sent once
> the device's probe function succeeds, but we have to deal with old
> userspaces as well, right?

Not for the new functionality, I do not think so. Newer kernels should
be compatible with older userspace as it not breaking it, but new
functionality is not guaranteed to be available with older userspace.

Thanks.

-- 
Dmitry