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

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>,
	Biju Das <biju.das.jz@bp.renesas.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: Tue, 8 Aug 2023 15:18:52 +0300	[thread overview]
Message-ID: <ZNIyrG/2h/PeS9Oz@smile.fi.intel.com> (raw)
In-Reply-To: <ZNFV+C1HCIRJpbdC@google.com>

On Mon, Aug 07, 2023 at 01:37:12PM -0700, Dmitry Torokhov wrote:
> On Mon, Aug 07, 2023 at 05:54:07PM +0300, Andy Shevchenko wrote:
> > On Sun, Aug 06, 2023 at 02:29:50PM +0100, Jonathan Cameron wrote:
> > > On Sat, 5 Aug 2023 17:42:21 +0000
> > > Biju Das <biju.das.jz@bp.renesas.com> wrote:
> > > > > On Fri,  4 Aug 2023 17:17:24 +0100
> > > > > Biju Das <biju.das.jz@bp.renesas.com> wrote:

...

> > > > + * Besides the fact that some drivers abuse the device ID driver_data type
> > > > + * and claim it to be integer, for the bus specific ID tables the driver_data
> > > > + * may be defined as kernel_ulong_t. For these tables 0 is a valid response,
> > > > + * but not for this function. It's recommended to convert those either to avoid
> > > > + * 0 or use a real pointer to the predefined driver data.
> > 
> > > We still need to maintain consistency across the two tables, which
> > > is a stronger requirement than avoiding 0.
> > 
> > True. Any suggestion how to amend the above comment? Because the documentation
> > makes sense on its own (may be split from the series?).
> > 
> > > Some drivers already do that by forcing the enum used to start at 1 which
> > > doesn't solver the different data types issue.
> > 
> > And some maintainers do not want to see non-enum values in i2c ID table.
> > *Shrug*.
> 
> So in legacy ID lookup path we can safely assume that values below 4096
> are scalars and return NULL from the new device_get_match_data(). This
> way current drivers using the values as indices or doing direct
> comparisons against them can continue doing manual look up and using
> them as they see fit. And we can convert the drivers at our leisure.

It's a good idea, but I believe will be received as hack.
But why not to try? We indeed have an error pointers for the last page
and NULL (which is only up to 16 IIRC) and reserved space in the first
page. To be more robust I would check all enums that are being used
in I2C ID tables for maximum value and if that is less than 16, use
ZERO_OR_NULL_PTR() instead of custom stuff.

-- 
With Best Regards,
Andy Shevchenko