Hi Maximilian,

On Tue, Jul 02, 2019 at 02:37:40AM +0200, Maximilian Luz wrote:

Power and volume button support for 5th and 6th genration Microsoft
Surface devices via soc_button_array.

Note that these devices use the same MSHW0040 device as on the Surface
Pro 4, however the implementation is different (GPIOs vs. ACPI
notifications). Thus some checking is required to ensure we only load
this driver on the correct devices.

When you are saying that Pro 4 and later models use different
notifications, does this mean that Pro 4 does not define any GPIOs? If
so can we use their presence as indicator whether we should be using
this driver or not. I would like to avoid repeating the ACPI parsing
code that you have in the platform driver.

+static int soc_device_check_MSHW0040(struct device *dev)
+{
+	acpi_handle handle = ACPI_HANDLE(dev);
+	union acpi_object *result;
+	u64 oem_platform_rev = 0;
+	int gpios;
+
+	// get OEM platform revision
+	result = acpi_evaluate_dsm_typed(handle, &MSHW0040_DSM_UUID,
+					 MSHW0040_DSM_REVISION,
+					 MSHW0040_DSM_GET_OMPR, NULL,
+					 ACPI_TYPE_INTEGER);
+
+	if (result) {
+		oem_platform_rev = result->integer.value;
+		ACPI_FREE(result);
+	}
+
+	if (oem_platform_rev == 0)
+		return -ENODEV;
+
+	dev_dbg(dev, "OEM Platform Revision %llu\n", oem_platform_rev);
+
+	/*
+	 * We are _really_ expecting GPIOs here. If we do not get any, this
+	 * means the GPIO driver has not been loaded yet (which can happen).
+	 * Try again later.
+	 */
+	gpios = gpiod_count(dev, NULL);
+	if (gpios < 0)
+		return -EAGAIN;

I do not believe -EAGAIN has any special meaning in the driver core;
also when the GPIO controller is not ready gpiod_get() will return
-EPROBE_DEFER, which is the prober way if signalling that some resource
is not yet available and probe should be retries at a later time.

Moreover, I do not believe that gpiod_count() needs GPIO controller to
be ready, the count is taken from board firmware or static board file
definition, so if gpiod_count() returns 0 it should be clear indication
that the driver should not be used with the device.

Thanks.

-- 
Dmitry