Re: [RFC PATCH v3 2/5] i2c: of: Introduce component probe function

[Andy Shevchenko]

On Tue, Nov 28, 2023 at 04:42:31PM +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.

...

> +/**
> + * i2c_of_probe_component() - probe for devices of "type" on the same i2c bus
> + * @dev: &struct device of the caller, only used for dev_* printk messages
> + * @type: a string to match the device node name prefix to probe for
> + *
> + * Probe for possible I2C components of the same "type" on the same I2C bus
> + * that have their status marked as "fail".

Definitely you haven't run kernel-doc validation.

> + */

...

> +		return dev_err_probe(dev, -ENODEV, "Could not find %s device node\n", type);

I haven't noticed clear statement in the description that this API is only for
the ->probe() stages.

...

> +		if (i2c_smbus_xfer(i2c, addr, 0, I2C_SMBUS_READ, 0, I2C_SMBUS_BYTE, &data) < 0)
> +			continue;

This will require the device to be powered on. Are you sure it will be always
the case?

-- 
With Best Regards,
Andy Shevchenko