Re: [PATCH v2 2/2] gpiolib: check the 'ngpios' property in core gpiolib code

On Thu, Nov 18, 2021 at 10:16 PM Andy Shevchenko
[off-list ref] wrote:

On Thu, Nov 18, 2021 at 09:12:59PM +0100, Bartosz Golaszewski wrote:

quoted

On Thu, Nov 18, 2021 at 6:06 PM Andy Shevchenko
[off-list ref] wrote:

quoted

On Thu, Nov 18, 2021 at 02:23:17PM +0100, Bartosz Golaszewski wrote:

quoted

Several drivers read the 'ngpios' device property on their own, but
since it's defined as a standard GPIO property in the device tree bindings
anyway, it's a good candidate for generalization. If the driver didn't
set its gc->ngpio, try to read the 'ngpios' property from the GPIO
device's firmware node before bailing out.

...

quoted

      if (gc->ngpio == 0) {
-             chip_err(gc, "tried to insert a GPIO chip with zero lines\n");
-             ret = -EINVAL;
-             goto err_free_descs;
+             ret = device_property_read_u32(&gdev->dev, "ngpios", &ngpios);
+             if (ret) {
+                     chip_err(gc, "tried to insert a GPIO chip with zero lines\n");
+                     ret = -EINVAL;
+                     goto err_free_descs;
+             }
+
+             gc->ngpio = ngpios;
      }

This should be

        if (gc->ngpio == 0) {
                ret = device_property_read_u32(&gdev->dev, "ngpios", &ngpios);
                if (ret)
                        return ret;

But device_property_read_u32() returning -ENODATA means there's no
such property, which should actually be converted to -EINVAL as the
caller wanting to create the chip provided invalid configuration - in
this case: a chip with 0 lines. In case of the non-array variant of
read_u32 that's also the only error that can be returned so this bit
looks right to me.

So, what is so special about -EINVAL? Why -ENODATA is not good enough which
will exactly explain to the caller what's going on, no?

Let's imagine the user sets gc->ngpio = 0 incorrectly thinking it'll
make gpiolib set it to some sane default. Then gpiochip_add_data()
returns -ENODATA (No data available). This is confusing IMO. But if we
convert it to -EINVAL, it now says "Invalid value" which points to the
wrong configuration.

ENODATA means "device tree property is not present" in this case but
the problem is that user supplies the gpiolib with invalid
configuration. EINVAL is the right error here.

Bart

quoted

quoted

                gc->ngpio = ngpios;
        }

        if (gc->ngpio == 0) {
                chip_err(gc, "tried to insert a GPIO chip with zero lines\n");
                ret = -EINVAL;
                goto err_free_descs;

When the caller intended to create a chip with 0 GPIOs they will get an error
as you wish with an error message.

Yes, as it was before.

Bart

quoted

quoted

        }

--
With Best Regards,
Andy Shevchenko