Re: [PATCH 2/2] rtc: x1205: Add DT probing support

[Linus Walleij <linus.walleij@linaro.org>]

On Thu, Jan 31, 2019 at 4:30 PM Alexandre Belloni
<alexandre.belloni@bootlin.com> wrote:
> On 31/01/2019 16:06:42+0100, Linus Walleij wrote:

> > This makes it possible to probe the X1205 RTC from the
> > device tree. This is needed when adding device tree boot
> > support for the IXP4xx-based NSLU2 which has this RTC.
> >
> > Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> > ---
> >  drivers/rtc/rtc-x1205.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/rtc/rtc-x1205.c b/drivers/rtc/rtc-x1205.c
> > index f08f18e4fcdf..c2b64c9363da 100644
> > --- a/drivers/rtc/rtc-x1205.c
> > +++ b/drivers/rtc/rtc-x1205.c
> > @@ -673,9 +673,15 @@ static const struct i2c_device_id x1205_id[] = {
> >  };
> >  MODULE_DEVICE_TABLE(i2c, x1205_id);
> >
> > +static const struct of_device_id x1205_dt_ids[] = {
> > +     { .compatible = "xircom,x1205", },
> > +};
> > +MODULE_DEVICE_TABLE(of, x1205_dt_ids);
> > +
>
> I think this is not necessary and this will probe without the
> of_match_table but it doesn't hurt either.

In the earlier implementations of device tree it was done
such that the device node would match the i2c name and
then the device would probe, such that if I in my device
tree name it:

x1205@6f {
    reg = <>;
};

Then the core will see match that node name to the i2c
device name. (I think this goes way back, possibly to the
earliest Open Firmware.)

However the DT maintainers have more and more pushed
for DT nodes to have functional name, so this would then
be named:

rtc@6f {
    reg = <>;
};

And then it stops working.

I think Lee Jones added the compatible probing to a bunch
of I2C devices for this reason.

Yours,
Linus Walleij