Re: [PATCH v7 0/4] Extend device_get_match_data() to struct bus_type

From: Jonathan Cameron <jic23@kernel.org>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Biju Das <biju.das.jz@bp.renesas.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Peter Rosin <peda@axentia.se>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Daniel Scally <djrscally@gmail.com>,
	Heikki Krogerus <heikki.krogerus@linux.intel.com>,
	Sakari Ailus <sakari.ailus@linux.intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Alexandre Belloni <alexandre.belloni@bootlin.com>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	"linux-acpi@vger.kernel.org" <linux-acpi@vger.kernel.org>,
	Andi Shyti <andi.shyti@kernel.org>, Wolfram Sang <wsa@kernel.org>,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	"linux-rtc@vger.kernel.org" <linux-rtc@vger.kernel.org>,
	"linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
	"linux-i2c@vger.kernel.org" <linux-i2c@vger.kernel.org>,
	"linux-renesas-soc@vger.kernel.org" 
	<linux-renesas-soc@vger.kernel.org>
Subject: Re: [PATCH v7 0/4] Extend device_get_match_data() to struct bus_type
Date: Mon, 28 Aug 2023 14:01:41 +0100	[thread overview]
Message-ID: <20230828140141.220732e0@jic23-huawei> (raw)
In-Reply-To: <CAMuHMdU6LhvnZ5FE_3BxyH7reVi69Rjcircquk=jYZ6-j3cqug@mail.gmail.com>

On Mon, 14 Aug 2023 15:12:24 +0200
Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> Hi Biju,
> 
> On Fri, Aug 11, 2023 at 4:46 PM Biju Das <biju.das.jz@bp.renesas.com> wrote:
> > > Subject: Re: [PATCH v7 0/4] Extend device_get_match_data() to struct
> > > bus_type
> > >
> > > On Fri, Aug 11, 2023 at 01:27:36PM +0000, Biju Das wrote:  
> > > > > On Thu, Aug 10, 2023 at 09:05:10AM +0000, Biju Das wrote:  
> > >
> > > ...
> > >  
> > > > > I'm good with this approach, but make sure you checked the whole
> > > > > kernel source tree for a such.  
> > > >
> > > > Checking against 16 is too short I guess??
> > > >
> > > > drivers/iio/imu/inv_mpu6050/inv_mpu_iio.h has 18 enums.  
> > >
> > > So, what does prevent us from moving that tables to use pointers?  
> >
> > I think that will lead to ABI breakage(client->name vs id->name)
> >
> >         match = device_get_match_data(&client->dev);
> >         if (match) {
> >                 chip_type = (uintptr_t)match;
> >                 name = client->name;
> >         } else if (id) {
> >                 chip_type = (enum inv_devices)
> >                         id->driver_data;
> >                 name = id->name;
> >         } else {
> >                 return -ENOSYS;
> >         }  
> 
> I don't consider that part of the ABI, as e.g. converting from board files
> to DT would change the name.
> In addition, using id->name breaks multiple instances of the same device.

This has always been a mess as I wasn't paying attention a long time back
and we ended up with some client->name entries being used for iio_dev->name
whereas it should be the part number.

Using id->name is correct choice.  This is supposed to be the same for multiple
instances of the same device.  There is label and a bunch of other options
for differentiating them including their parent devices.

Problem is that is exported to userspace and often used as part of the
matching when a userspace tool is trying to find the device.

We could 'give it a go' by setting the name in teh switch statement in the core code
and hope no one notices but it's not ideal
https://elixir.bootlin.com/linux/latest/source/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c#L1597

Jonathan

> 
> I applaud more unification ;-)
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 