When does of_match_device() return NULL ?

[Himanshu Jha]

Hi,

We know that the only way to probe an OF driver is when a match is found
in the device table. And hence checking for of_match_device is redundant
and instead it is better to use of_device_get_match_data() to get the
matched data in the probe function.

For instance:
https://lkml.org/lkml/2018/4/30/66

 static int mmio_74xx_gpio_probe(struct platform_device *pdev)
 {
-	const struct of_device_id *of_id;
 	struct mmio_74xx_gpio_priv *priv;
 	struct resource *res;
 	void __iomem *dat;
 	int err;
 
-	of_id = of_match_device(mmio_74xx_gpio_ids, &pdev->dev);
-	if (!of_id)
-		return -ENODEV;
-
 	priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
 	if (!priv)
 		return -ENOMEM;
 
+	priv->flags = (uintptr_t)of_device_get_match_data(&pdev->dev);
+
 	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
 	dat = devm_ioremap_resource(&pdev->dev, res);
 	if (IS_ERR(dat))
 		return PTR_ERR(dat);
 
-	priv->flags = (uintptr_t) of_id->data;
-

--------------------------------------------------------------------------

In the above example we know of_match_device() is useless since the
probe occurs *only* when a match is found and therefore checking again
is redundant.

But now my question is if this is the case, then when does
of_match_device() return NULL ? When is the error handling required ?

There are places where it is done as "safe-play" coding as suggested to me
by Jonathan previosuly https://lkml.org/lkml/2018/3/16/1245



-- 
Himanshu Jha
Undergraduate Student
Department of Electronics & Communication
Guru Tegh Bahadur Institute of Technology