Re: [PATCH 2/2] HID: i2c-hid: Do not bind to CHPN0001 touchscreen

[Hans de Goede <hdegoede@redhat.com>]

Hi again,

I realized I did not answer 1 of your questions:

On 17-08-17 21:39, Wolfram Sang wrote:
> Hey guys,
> 
> Sorry, I don't understand some of the stuff here. But I'd like to
> understand it before I add something to the I2C core. Especially as it
> feels a bit a the edge of the driver model to me.
> 
> On Sat, Jul 22, 2017 at 08:55:37PM +0200, Hans de Goede wrote:
>> The CHPN0001 ACPI device has a _CID of PNP0C50 but is not HID compatible,
>> it uses its own protocol which is handled by the chipone_icn8318 driver.
>>
>> If the i2c_hid_driver's probe functon gets called it will fail with a
>> "hid_descr_cmd failed" error.
> 
> That sounds like it fails pretty late. I'd assume we could check the
> blacklist right at the beginning of probe and bail out immediately?
> 
>> Worse, after the probe failure the i2c / ACPI core code will put the ACPI
>> device in D3 state
> 
> Where does that happen? Sorry, I can't find it. Would it be an idea to
> add a flag somewhere telling the device should not be put into D3?

It is already possible to do this and my patches for the icn8318 driver
do this:

	struct acpi_device *adev;

	adev = ACPI_COMPANION(dev);

	/*
	 * Disable ACPI power management the _PS3 method is empty, so
	 * there is no powersaving when using ACPI power management.
	 * The _PS0 method resets the controller causing it to loose its
	 * firmware, which has been loaded by the BIOS and we do not
	 * know how to restore the firmware.
	 */
	adev->flags.power_manageable = 0;

The problem is that this happens in the probe() from the icn8318 driver
and if the i2c-hid drivers probe() executes first we end up in the
dev_pm_domain_detach() path of i2c_device_probe() and after that the
touchscreen-controller no longer works (*), iow after that it is too late
to disable acpi pm for the device.

*) Unless we find a way to reload the firmware, which technically is
doable, but then we get into the problem of now having to distribute the
firmware in linux-firmware

Regards,

Hans