linux-kernel - Re: [PATCH v3 1/3] HID: i2c-hid: Rely on HID descriptor fetch to probe

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Date: Thu, 2 May 2024 01:09:37 +0200
From: Kenny Levinsen <kl@...wtf>
To: Dmitry Torokhov <dmitry.torokhov@...il.com>
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>,
 Lukasz Majczak <lma@...omium.org>
Subject: Re: [PATCH v3 1/3] HID: i2c-hid: Rely on HID descriptor fetch to
 probe

On 5/1/24 9:09 PM, Dmitry Torokhov wrote:
> Is it possible for a device to be wedged so hard that it refuses to
> acknowledge the address?

A slave is allowed to not acknowledge if not able (e.g., "because it's 
performing some real time function"), but a slave that does not 
acknowledge its address is electrically indistinguishable from a 
disconnected device. In such case the device is impossible to detect 
through I2C operations, and neither smbus probe nor a "real" command 
will see it.

Any logic we have to silence missing devices will also silence entirely 
unresponsive or extremely non-cooperate devices. That is the price to 
pay for avoiding the log message unfortunately.

No other errors from the smbus probe or a real command would be related 
to device presence, and some of them even suggest a device is present 
but broken (arbitration loss, assuming no shorts).