Re: [PATCH v1.1 2/2] iio: dac: dac5571: Fix chip id detection for OF devices

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Wolfram,

On Tue, Aug 17, 2021 at 10:44:20PM +0200, Wolfram Sang wrote:
> 
> > > When matching an OF device, the match mechanism tries all components of
> > > the compatible property. This can result with a device matched with a
> > > compatible string that isn't the first in the compatible list. For
> > > instance, with a compatible property set to
> > > 
> > >     compatible = "ti,dac081c081", "ti,dac5571";
> > > 
> > > the driver will match the second compatible string, as the first one
> > > isn't listed in the of_device_id table. The device will however be named
> > > "dac081c081" by the I2C core.
> > > 
> > > This causes an issue when identifying the chip. The probe function
> > > receives a i2c_device_id that comes from the module's I2C device ID
> > > table. There is no entry in that table for "dac081c081", which results
> > > in a NULL pointer passed to the probe function.
> > > 
> > > To fix this, add chip_id information in the data field of the OF device
> > > ID table, and retrieve it with of_device_get_match_data() for OF
> > > devices.
> > > 
> > > Signed-off-by: Jose Cazarin <joseespiriki@gmail.com>
> > > Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > > Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > 
> > Interesting problem that I hadn't previously realised could happen.
> > 
> > One request though, can we use device_get_match_data() here rather than
> > the of specific version?  Include property.h as well for that.
> > 
> > That should allow the same issue with compatible to work correctly when
> > using PRP0001 based ACPI methods. 
> > https://elixir.bootlin.com/linux/v5.14-rc1/source/drivers/acpi/bus.c#L891
> > Will result in acpi_of_device_get_match_data() being called which will
> > match to the of_device_id table.
> 
> Couldn't you use the "new" probe_new() callback instead which will drop
> the i2c_device_id? Kieran was interested in such conversions IIRC.

It's a bit unrelated to this patch, but I can add another patch to the
series.

While I have your attention, there's a question for you in the cover
letter :-) Could you please have a look ?

-- 
Regards,

Laurent Pinchart