[PATCH v5 06/10] i2c: Introduce OF component probe function

[Andy Shevchenko]

On Thu, Aug 22, 2024 at 05:19:59PM +0800, Chen-Yu Tsai wrote:
> Some devices are designed and manufactured with some components having
> multiple drop-in replacement options. These components are often
> connected to the mainboard via ribbon cables, having the same signals
> and pin assignments across all options. These may include the display
> panel and touchscreen on laptops and tablets, and the trackpad on
> laptops. Sometimes which component option is used in a particular device
> can be detected by some firmware provided identifier, other times that
> information is not available, and the kernel has to try to probe each
> device.
> 
> This change attempts to make the "probe each device" case cleaner. The
> current approach is to have all options added and enabled in the device
> tree. The kernel would then bind each device and run each driver's probe
> function. This works, but has been broken before due to the introduction
> of asynchronous probing, causing multiple instances requesting "shared"
> resources, such as pinmuxes, GPIO pins, interrupt lines, at the same
> time, with only one instance succeeding. Work arounds for these include
> moving the pinmux to the parent I2C controller, using GPIO hogs or
> pinmux settings to keep the GPIO pins in some fixed configuration, and
> requesting the interrupt line very late. Such configurations can be seen
> on the MT8183 Krane Chromebook tablets, and the Qualcomm sc8280xp-based
> Lenovo Thinkpad 13S.
> 
> Instead of this delicate dance between drivers and device tree quirks,
> this change introduces a simple I2C component probe. function For a
> given class of devices on the same I2C bus, it will go through all of
> them, doing a simple I2C read transfer and see which one of them responds.
> It will then enable the device that responds.
> 
> This requires some minor modifications in the existing device tree. The
> status for all the device nodes for the component options must be set
> to "failed-needs-probe". This makes it clear that some mechanism is
> needed to enable one of them, and also prevents the prober and device
> drivers running at the same time.

...

> --- a/drivers/i2c/Makefile
> +++ b/drivers/i2c/Makefile
> @@ -9,6 +9,7 @@ i2c-core-objs 			:= i2c-core-base.o i2c-core-smbus.o
>  i2c-core-$(CONFIG_ACPI)		+= i2c-core-acpi.o
>  i2c-core-$(CONFIG_I2C_SLAVE) 	+= i2c-core-slave.o
>  i2c-core-$(CONFIG_OF) 		+= i2c-core-of.o
> +i2c-core-$(CONFIG_OF_DYNAMIC) 	+= i2c-core-of-prober.o

Seems like all the above (except ACPI) have the same issue, i.e. TABs/spaces
mixture.

...

> +	ret = of_changeset_apply(ocs);
> +	if (!ret) {

Why not positive conditional?

> +		/*
> +		 * ocs is intentionally kept around as it needs to
> +		 * exist as long as the change is applied.
> +		 */
> +		void *ptr __always_unused = no_free_ptr(ocs);
> +	} else {
> +		/* ocs needs to be explicitly cleaned up before being freed. */
> +		of_changeset_destroy(ocs);
> +	}

-- 
With Best Regards,
Andy Shevchenko