linux-kernel - Re: [PATCH RFC] i2c: Use ID table to detect ACPI I2C devices

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Message-ID: <20150520074708.GQ1490@lahna.fi.intel.com>
Date:	Wed, 20 May 2015 10:47:08 +0300
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	Robert Dolca <robert.dolca@...el.com>
Cc:	linux-i2c@...r.kernel.org, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org, Wolfram Sang <wsa@...-dreams.de>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	Daniel Baluta <daniel.baluta@...el.com>
Subject: Re: [PATCH RFC] i2c: Use ID table to detect ACPI I2C devices

On Tue, May 19, 2015 at 05:03:29PM +0300, Robert Dolca wrote:
> For i2c devices enumerated with ACPI you need to declare both
> acpi_match_table and id_table. When using ACPI, the i2c_device_id structure
> supplied to the probe function is null and you have to handle this case
> in the driver.
> 
> The current name for the i2c client when using ACPI is "HID:UID" where the
> UID has 7 or 8 characters and the UID has 2 characters. The UID is not
> relevant for identifying the chip so it does not have any practical
> purpose.

First of all, it is not "HID:UID" since the number after ":" is actually
increasing number assigned by the ACPI core. Nothing to do with _UID.

Secondly we do not list "_HID:nn" in drivers acpi_match_tables but
instead it is either "HID" or "CID", no ":nn" there.

> Modifying i2c_match_id we make the comparison by ignoring the UID from the
> client name when the device was discovered using ACPI. The comparison is
> case insensitive because the ACPI names are uppercase and the DT and ID
> table names are lowercase. It would not make sense to have two different
> chips with the same name and the only diference being the capitalized
> letters.
> 
> With these changes the probe function gets a valid i2c_device_id and the
> driver doesn't have to declare acpi_match_table.

No. We don't do that for DT and we definitely don't want to mix ACPI
identifiers with arbitrary I2C device names.

You are not supposed to put ACPI identifiers into i2c_device_id table.
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