Re: [PATCH v2 2/2] i2c: xiic: Support forcing single-master in DT

From: Wolfram Sang <wsa@the-dreams.de>
To: Jaakko Laine <ext-jaakko.laine@vaisala.com>
Cc: michal.simek@xilinx.com, shubhrajyoti.datta@gmail.com,
	linux-arm-kernel@lists.infradead.org, linux-i2c@vger.kernel.org
Subject: Re: [PATCH v2 2/2] i2c: xiic: Support forcing single-master in DT
Date: Tue, 25 Aug 2020 15:24:27 +0200	[thread overview]
Message-ID: <20200825132427.GB991@kunai> (raw)
In-Reply-To: <20200820100241.96866-3-ext-jaakko.laine@vaisala.com>

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On Thu, Aug 20, 2020 at 01:02:41PM +0300, Jaakko Laine wrote:
> I2C master operating in multimaster mode can get stuck
> indefinitely if I2C start is detected on bus, but no master
> has a transaction going.
> 
> This is a weakness in I2C standard, which defines no way
> to recover, since all masters are indefinitely disallowed
> from interrupting the currently operating master. A start
> condition can be created for example by an electromagnetic
> discharge applied near physical I2C lines. Or a already
> operating master could get reset immediately after sending
> a start.
> 
> If it is known during device tree creation that only a single
> I2C master will be present on the bus, this deadlock of the
> I2C bus could be avoided in the driver by ignoring the
> bus_is_busy register of the xiic, since bus can never be
> reserved by any other master.

You could even initiate a recovery procedure if it is a device pulling
SDA low.

> This patch adds this support for detecting single-master flag
> in device tree and when provided, improves I2C reliability by
> ignoring the therefore unnecessary xiic bus_is_busy register.
> 
> Error can be reproduced by pulling I2C SDA -line temporarily low
> by shorting it to ground, while linux I2C master is operating on
> it using the xiic driver. The application using the bus will
> start receiving linux error code 16: "Device or resource busy"
> indefinitely:
> 
> kernel: pca953x 0-0020: failed writing register
> app: Error writing file, error: 16
> 
> With multi-master disabled device will instead receive error
> code 5: "I/O error" while SDA is grounded, but recover normal
> operation once short is removed.
> 
> kernel: pca953x 0-0020: failed reading register
> app: Error reading file, error: 5
> 
> Signed-off-by: Jaakko Laine <ext-jaakko.laine@vaisala.com>

Again, looks good to me and matches what we discussed and prepared.
Let's see what Michal et al. think.

Happy hacking!

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