linux-kernel - Re: [PATCH 2/2] i2c: designware: abort the transfer if receiving byte count of 0

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Date:   Fri, 11 Aug 2023 17:01:28 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yann Sionneau <yann@...nneau.net>
Cc:     Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yann Sionneau <ysionneau@...ray.eu>,
        Jonathan Borne <jborne@...ray.eu>
Subject: Re: [PATCH 2/2] i2c: designware: abort the transfer if receiving
 byte count of 0

On Fri, Aug 11, 2023 at 02:46:24PM +0200, Yann Sionneau wrote:
> From: Yann Sionneau <ysionneau@...ray.eu>
> 
> Context:
> It's not clear whether Linux SMBus stack supports receiving 0
> as byte count for SMBus read data block.
> 
> Linux supports SMBus v2.0 spec, which says "The byte count may not be 0."
> Which does not seem very clear to me, as a non-native speaker.
> (Note that v3.0 of the spec says "The byte count may be 0.")
> 
> Some drivers explicitly return -EPROTO in case of byte count 0.
> 
> The issue:
> Regardless of whether Linux supports byte count of 0, if this happens
> the i2c-designware driver goes into an unrecoverable state holding
> SCL low if the IP is synthesized with the IC_EMPTYFIFO_HOLD_MASTER_EN
> parameter.
> 
> The fix proposed by this patch:
> Abort the transfer by sending a STOP condition on the bus.
> 
> Another approach would be to ignore the issue and let the driver
> timeout and disable the IP. The IP disabling is fixed by the previous
> patch in this patch series.
> The current patch just makes the recovery faster since Abort is sent
> directly without waiting for the timeout to happen. With this patch,
> disabling the IP is not necessary anymore.

...

> +				if ((tmp <= I2C_SMBUS_BLOCK_MAX) && (tmp != 0))

				if (tmp && tmp <= I2C_SMBUS_BLOCK_MAX)

> +					len = i2c_dw_recv_len(dev, tmp);
> +				else
> +					i2c_dw_abort(dev);

-- 
With Best Regards,
Andy Shevchenko