linux-kernel - Re: [PATCH 2/3] i2c: designware: Enable transfer with different target addresses

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Message-ID: <5779084.ZASKD2KPVS@benoit.monin>
Date: Wed, 22 Oct 2025 10:36:28 +0200
From: Benoît Monin <benoit.monin@...tlin.com>
To: Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc: Andi Shyti <andi.shyti@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>,
 Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
 Mika Westerberg <mika.westerberg@...ux.intel.com>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Jan Dabros <jsd@...ihalf.com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
 Hans Verkuil <hverkuil+cisco@...nel.org>,
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
 Gregory CLEMENT <gregory.clement@...tlin.com>,
 Théo Lebrun <theo.lebrun@...tlin.com>,
 Tawfik Bayouk <tawfik.bayouk@...ileye.com>,
 Vladimir Kondratiev <vladimir.kondratiev@...ileye.com>,
 Dmitry Guzman <dmitry.guzman@...ileye.com>, linux-i2c@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev
Subject:
 Re: [PATCH 2/3] i2c: designware: Enable transfer with different target
 addresses

On Monday, 20 October 2025 at 21:52:01 CEST, Wolfram Sang wrote:
> 
> > For your particular case, that will not help reaching the other segments as
> > we wait for a STOP before changing the target address. So, it should not
> > fail but do a write to segment 0 in your eeprom.
> 
> So, this patch replaces a repeated start with a stop + start
> combination? Please don't do this. It will give users a false impression
> that proper repeated start is supported. Honestly reporting that the HW
> does not support is the better option, so the user can decide what to do
> then.
> 
This patch replaces a -EINVAL in the middle of the transfer by a
STOP-then-START, but you are right, the expectation is to have a single
STOP at the end of a combined transfer. I somehow overlooked that part.
 
Maybe I could add support for the I2C_M_STOP flag instead? Or does an
adapter has to support all the protocol mangling if flagged with
I2C_FUNC_PROTOCOL_MANGLING?
 
That would still allow to group multiple accesses to device that support a
STOP in a transaction when done via i2c_dev I2C_RDWR ioctl, in a single-master
configuration.

Best regards,
-- 
Benoît Monin, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com