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Date:   Thu, 9 Sep 2021 15:13:20 +0200
From:   Jean Delvare <jdelvare@suse.de>
To:     Sasha Levin <sashal@kernel.org>
Cc:     linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Heiner Kallweit <hkallweit1@gmail.com>,
        Wolfram Sang <wsa@kernel.org>, linux-i2c@vger.kernel.org
Subject: Re: [PATCH AUTOSEL 5.14 088/252] i2c: i801: Fix handling
 SMBHSTCNT_PEC_EN
Message-ID: <20210909151320.7bddd134@endymion>
In-Reply-To: <20210909114106.141462-88-sashal@kernel.org>
References: <20210909114106.141462-1-sashal@kernel.org>
        <20210909114106.141462-88-sashal@kernel.org>
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Hi Sascha,

On Thu,  9 Sep 2021 07:38:22 -0400, Sasha Levin wrote:
> From: Heiner Kallweit <hkallweit1@gmail.com>
> 
> [ Upstream commit a6b8bb6a813a6621c75ceacd1fa604c0229e9624 ]
> 
> Bit SMBHSTCNT_PEC_EN is used only if software calculates the CRC and
> uses register SMBPEC. This is not supported by the driver, it supports
> hw-calculation of CRC only (using bit SMBAUXSTS_CRCE). The chip spec
> states the following, therefore never set bit SMBHSTCNT_PEC_EN.
> 
> Chapter SMBus CRC Generation and Checking
> If the AAC bit is set in the Auxiliary Control register, the PCH
> automatically calculates and drives CRC at the end of the transmitted
> packet for write cycles, and will check the CRC for read cycles. It will
> not transmit the contents of the PEC register for CRC. The PEC bit must
> not be set in the Host Control register. If this bit is set, unspecified
> behavior will result.
> 
> This patch is based solely on the specification and compile-tested only,
> because I have no PEC-capable devices.
> 
> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
> Tested-by: Jean Delvare <jdelvare@suse.de>
> Signed-off-by: Wolfram Sang <wsa@kernel.org>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>  drivers/i2c/busses/i2c-i801.c | 27 +++++++++++----------------
>  1 file changed, 11 insertions(+), 16 deletions(-)

This patch fixes a theoretical problem nobody has ever complained
about. I don't think it makes sense to backport it to stable kernel
branches.

-- 
Jean Delvare
SUSE L3 Support