When does of_match_device() return NULL ?

[Himanshu Jha <himanshujha199640@gmail.com>]

Hi,



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 driver specific 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 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 Cameron previosuly https://lkml.org/lkml/2018/3/16/1245

I am working on a Coccinelle SmPL rule with the help of Julia Lawall to
replace of_match_device() + error handling with
of_device_get_match_data() simply.

I am testing the refactoring changes and it is at:
https://github.com/himanshujha199640/linux-next/commit/efb7ed923bd00c86fe0a4e67e2ddb636ff0e0ff4

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

Some statistics found by Julia Lawall:


f_match_device with check: 257/283
  no of tbl: 39, of tbl only: 148, other tbl only: 4, multi tbl: 66
  has of_match_ptr: 87, no of_match_ptr: 170 

of_match_device without check: 26/283
  no of tbl: 14, of tbl only: 11, other tbl only: 0, multi tbl: 1
  has of_match_ptr: 6, no of_match_ptr: 20

of_device_get_match_data with check: 158/212
  no of tbl: 29, of tbl only: 78, other tbl only: 0, multi tbl: 51
  has of_match_ptr: 53, no of_match_ptr: 105 

of_device_get_match_data without check: 54/212
  no of tbl: 13, of tbl only: 40, other tbl only: 0, multi tbl: 1
  has of_match_ptr: 8, no of_match_ptr: 46

Multi table files without an of_match_device check
  drivers/mmc/host/mxs-mmc.c

Multi table files without an of_device_get_match_data check
  drivers/clk/clk-versaclock5.c

of_match_ptr files without an of_match_device check
  drivers/pinctrl/mvebu/pinctrl-orion.c
  drivers/tty/serial/mvebu-uart.c
  drivers/misc/eeprom/eeprom_93xx46.c
  arch/arm/plat-pxa/ssp.c
  drivers/soc/mediatek/mtk-scpsys.c
  drivers/pinctrl/mvebu/pinctrl-armada-cp110.c
of_match_ptr files without an of_device_get_match_data check
  drivers/gpu/drm/nouveau/nouveau_platform.c
  drivers/gpio/gpio-pxa.c
  drivers/gpio/gpio-rcar.c
  drivers/iommu/arm-smmu.c
  drivers/usb/gadget/udc/renesas_usb3.c
  drivers/i2c/busses/i2c-mt65xx.c
  drivers/iommu/ipmmu-vmsa.c
  drivers/mtd/nand/raw/mtk_ecc.c

other table only (no of_device_id matching)
  drivers/mfd/wm831x-i2c.c
  drivers/macintosh/macio_asic.c
  drivers/mfd/max14577.c
  drivers/mfd/wm831x-spi.c

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


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