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Date: Tue, 9 Apr 2024 17:46:15 +0200
From: Johan Hovold <johan@kernel.org>
To: =?utf-8?Q?=C5=81ukasz?= Majczak <lma@chromium.org>
Cc: Jiri Kosina <jikos@kernel.org>, Dmitry Torokhov <dtor@chromium.org>,
	Benjamin Tissoires <benjamin.tissoires@redhat.com>,
	Douglas Anderson <dianders@chromium.org>,
	Hans de Goede <hdegoede@redhat.com>,
	Maxime Ripard <mripard@kernel.org>,
	Kai-Heng Feng <kai.heng.feng@canonical.com>,
	Johan Hovold <johan+linaro@kernel.org>, linux-input@vger.kernel.org,
	linux-kernel@vger.kernel.org, Radoslaw Biernacki <rad@chromium.org>
Subject: Re: [PATCH v2] HID: i2c-hid: wait for i2c touchpad deep-sleep to
 power-up transition
Message-ID: <ZhVix-HJrqQbiPrB@hovoldconsulting.com>
References: <20240405102436.3479210-1-lma@chromium.org>
 <ZhOccGFkTFkUkRUI@hovoldconsulting.com>
 <CAE5UKNqufWZfKLAXLcpBYKQpJEVt6jPD4Xtr=Nesh34VkNOETg@mail.gmail.com>
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On Tue, Apr 09, 2024 at 12:53:43PM +0200, Łukasz Majczak wrote:

> > Can you please explain why this would not a problem for all future
> > transactions as well?

> 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.

Sure, but what about other transactions that are initiated by the host
(e.g. SET_POWER)?

Perhaps this hack at probe is enough for your use case, but is an
incomplete hack and at a minimum you'd need to add a comment explaining
why it is there.

Johan