linux-kernel - Re: [PATCH 01/12 v2] Platform: add a dev_groups pointer to struct platform

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Date:   Sat, 6 Jul 2019 19:19:48 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Johan Hovold <johan@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Richard Gong <richard.gong@...ux.intel.com>,
        Romain Izard <romain.izard.pro@...il.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mans Rullgard <mans@...sr.com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH 01/12 v2] Platform: add a dev_groups pointer to struct
 platform_driver

On Sat, Jul 06, 2019 at 10:04:39AM -0700, Dmitry Torokhov wrote:
> Hi Greg,
> 
> On Sat, Jul 6, 2019 at 1:32 AM Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> 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@...uxfoundation.org> 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.

Sorry, I didn't mean to imply that only platform drivers want to do
this, as you say, many other drivers do as well.

> Drivers for devices attached
> to serio, i2c, USB, spi, etc, etc, all have additional attributes:
> 
> dtor@...r-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.

Yeah, I know, we need to add this type of functionality to those busses
as well.  I don't see a way of doing it other than this bus-by-bus
conversion, do you?

> > > 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.

I agree, but again, this is a kobject change (adding attributes), so
I think the event type I picked here is the correct one.

thanks,

greg k-h