Re: [RFC PATCH v3 3/5] platform/chrome: Introduce device tree hardware prober

[Andy Shevchenko]

On Tue, Nov 28, 2023 at 04:42:32PM +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 prober. For any 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.

...

> +#include <linux/array_size.h>
> +#include <linux/i2c.h>
> +#include <linux/of.h>
> +#include <linux/platform_device.h>

init.h for init calls.


> +static int chromeos_of_hw_prober_probe(struct platform_device *pdev)
> +{
> +	for (size_t i = 0; i < ARRAY_SIZE(hw_prober_platforms); i++)
> +		if (of_machine_is_compatible(hw_prober_platforms[i].compatible)) {
> +			int ret;

Perhaps

		if (!of_machine_is_compatible(hw_prober_platforms[i].compatible))
			continue;

?

> +			ret = hw_prober_platforms[i].prober(&pdev->dev,
> +							    hw_prober_platforms[i].data);
> +			if (ret)
> +				return ret;
> +		}
> +
> +	return 0;
> +}

-- 
With Best Regards,
Andy Shevchenko