linux-kernel - Re: [PATCH v2] HID: i2c-hid: wait for i2c touchpad deep-sleep to power-up transition

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Date: Tue, 9 Apr 2024 12:53:43 +0200
From: Łukasz Majczak <lma@...omium.org>
To: Johan Hovold <johan@...nel.org>
Cc: Jiri Kosina <jikos@...nel.org>, Dmitry Torokhov <dtor@...omium.org>, 
	Benjamin Tissoires <benjamin.tissoires@...hat.com>, Douglas Anderson <dianders@...omium.org>, 
	Hans de Goede <hdegoede@...hat.com>, Maxime Ripard <mripard@...nel.org>, 
	Kai-Heng Feng <kai.heng.feng@...onical.com>, Johan Hovold <johan+linaro@...nel.org>, 
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Radoslaw Biernacki <rad@...omium.org>
Subject: Re: [PATCH v2] HID: i2c-hid: wait for i2c touchpad deep-sleep to
 power-up transition

Hi Johan,
>
> Can you please explain why this would not a problem for all future
> transactions as well?
>
> If it is, then it sounds like this needs to be addressed in the i2c
> driver. If not, then perhaps the problem is really that you just need a
> delay after enabling the power supplies?
>

The problem is that the probe function calling i2c_smbus_read_byte()
is not aware that
uC on the other end is in a deep sleep state so the first read will
fail and so the whole probe.

In a normal scenario, when a user touches the touchpad (when it is in
a deep sleep), the touch will first wake up the uC and
then generate an interrupt to AP, so in this case the touchpad is
fully awake and operational.

> Nit: move above the 'ret' declaration to maintain reverse xmas style
> ordering.
>
> Johan

Will do so.

Kind regards,
Lukasz