Date:   Thu, 25 Jul 2019 22:18:31 +0300
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Richard Gong <richard.gong@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johan Hovold <johan@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        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

Hi Richard,

On Thu, Jul 25, 2019 at 02:02:03PM -0500, Richard Gong wrote:
> Hi Greg,
> 
> On 7/25/19 8:44 AM, Greg Kroah-Hartman wrote:
> > On Sat, Jul 20, 2019 at 07:38:57AM +0300, Dmitry Torokhov wrote:
> > > On Fri, Jul 19, 2019 at 08:52:20PM +0900, Greg Kroah-Hartman wrote:
> > > > On Sat, Jul 06, 2019 at 10:39:38AM -0700, Dmitry Torokhov wrote:
> > > > > 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()?
> > > > 
> > > > Sorry for the delay, got busy with the merge window...
> > > > 
> > > > Anyway, no, we can't call this then, because driver_sysfs_add() is
> > > > called before probe() is called.  So if probe() fails, we don't bind the
> > > > device to the driver.  We also should not be creating sysfs files for a
> > > > driver that has not had probe() called yet, as internal structures will
> > > > not be set up at that time.
> > > 
> > > Ah, yes, I got confused by the fact that driver_sysfs_remove is called
> > > early. Anyway, I think you want something like this:
> > 
> > Ah, nice, this looks good.  Let me try this and see how it goes...
> > 
> 
> I tried Dmitry's patch on Intel Stratix10 platform and it works.
> 
> I added one minor change on the top of Dmitry's patch, since I think we need
> add one additional check prior to device_add_groups().

sysfs_create_groups() returns success when NULL is passed, and
device_add_groups() is a simple wrapper around it.

If anything I'd prefer to codify the behavior of
device_add_groups()/sysfs_create_groups() via comments rather than
adding unneeded checks.

Thanks.

-- 
Dmitry

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