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 10:39:38 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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 6, 2019 at 10:19 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> 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?

Can't you push the **dev_groups from platform driver down to the
generic driver structure and handle them in driver_sysfs_add()?

>
> > > > 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.

I guess once you push it all into core you'll end up with 2 uevents
being emitted back-to-back and this seems inefficient.

If you really want KOBJ_CHANGE maybe have some additional attribute
like "CHANGE=driver-specific-attrs" in it? It's all quite ugly though.

Thanks.

-- 
Dmitry