Re: [PATCH v6 08/12] i2c: Introduce OF component probe function

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Wed, Sep 04, 2024 at 05:00:10PM +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.

...

> +int i2c_of_probe_component(struct device *dev, const char *type)
> +{
> +	struct i2c_adapter *i2c;
> +	int ret;
> +
> +	struct device_node *i2c_node __free(device_node) = i2c_of_probe_get_i2c_node(dev, type);
> +	if (IS_ERR(i2c_node))
> +		return PTR_ERR(i2c_node);

> +	for_each_child_of_node_scoped(i2c_node, node) {

Hmm, but can it be for_each_child_of_node_with_prefix_scoped() now?

> +		if (!of_node_name_prefix(node, type))
> +			continue;
> +		if (!of_device_is_available(node))
> +			continue;
> +
> +		/*
> +		 * Device tree has component already enabled. Either the
> +		 * device tree isn't supported or we already probed once.
> +		 */
> +		return 0;
> +	}
> +
> +	i2c = of_get_i2c_adapter_by_node(i2c_node);
> +	if (!i2c)
> +		return dev_err_probe(dev, -EPROBE_DEFER, "Couldn't get I2C adapter\n");
> +
> +	ret = 0;
> +	for_each_child_of_node_scoped(i2c_node, node) {

Ditto.

> +		union i2c_smbus_data data;
> +		u32 addr;
> +
> +		if (!of_node_name_prefix(node, type))
> +			continue;
> +		if (of_property_read_u32(node, "reg", &addr))
> +			continue;
> +		if (i2c_smbus_xfer(i2c, addr, 0, I2C_SMBUS_READ, 0, I2C_SMBUS_BYTE, &data) < 0)
> +			continue;
> +
> +		/* Found a device that is responding */
> +		ret = i2c_of_probe_enable_node(dev, node);
> +		break;
> +	}
> +
> +	i2c_put_adapter(i2c);
> +
> +	return ret;
> +}

...

> +EXPORT_SYMBOL_GPL(i2c_of_probe_component);

Wonder if we may already use namespaced export from day 1.

-- 
With Best Regards,
Andy Shevchenko