linux-kernel - Re: [PATCH v11 3/7] i2c: Introduce OF component probe function

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Message-ID: <d0d07673-389c-49de-bdd1-4c5a697ec332@collabora.com>
Date: Thu, 14 Nov 2024 11:37:02 +0100
From: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
To: Chen-Yu Tsai <wenst@...omium.org>,
 Matthias Brugger <matthias.bgg@...il.com>, Wolfram Sang <wsa@...nel.org>
Cc: Rob Herring <robh@...nel.org>, Saravana Kannan <saravanak@...gle.com>,
 Benson Leung <bleung@...omium.org>, Tzung-Bi Shih <tzungbi@...nel.org>,
 chrome-platform@...ts.linux.dev, devicetree@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org,
 linux-kernel@...r.kernel.org, Douglas Anderson <dianders@...omium.org>,
 Johan Hovold <johan@...nel.org>, Jiri Kosina <jikos@...nel.org>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 linux-i2c@...r.kernel.org, Andrey Skvortsov <andrej.skvortzov@...il.com>
Subject: Re: [PATCH v11 3/7] i2c: Introduce OF component probe function

Il 06/11/24 10:33, Chen-Yu Tsai ha scritto:
> 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 probe function. For a
> 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 "fail-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.
> 
> Signed-off-by: Chen-Yu Tsai <wenst@...omium.org>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>