linux-kernel - Re: [PATCH 1/1] mfd: da9063: Support SMBus and I2C mode

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Date:   Thu, 28 Jan 2021 21:53:11 +0100
From:   Wolfram Sang <wsa@...nel.org>
To:     "Jonas Mark (BT-FIR/ENG1-Grb)" <Mark.Jonas@...bosch.com>
Cc:     Adam Thomson <Adam.Thomson.Opensource@...semi.com>,
        Support Opensource <Support.Opensource@...semi.com>,
        Lee Jones <lee.jones@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        Steve Twiss <stwiss.opensource@...semi.com>,
        "marek.vasut@...il.com" <marek.vasut@...il.com>,
        "RUAN Tingquan (BT-FIR/ENG1-Zhu)" <Tingquan.Ruan@...bosch.com>,
        "Streidl Hubert (BT-FIR/ENG1-Grb)" <Hubert.Streidl@...bosch.com>,
        "festevam@...il.com" <festevam@...il.com>,
        "o.rempel@...gutronix.de" <o.rempel@...gutronix.de>,
        "anson.huang@....com" <anson.huang@....com>
Subject: Re: [PATCH 1/1] mfd: da9063: Support SMBus and I2C mode

Hi,

> None of that seems to model the inability to perform atomic transactions
> within the SMBus timeout. This is either a bug of this specific driver
> or maybe the expressiveness of I2C_FUNC_* is not sufficient.
> 
> https://elixir.bootlin.com/linux/latest/source/include/uapi/linux/i2c.h#L88
> 
> What flag do you think we could check to find out whether the bus is
> able to obey the SMBus timeout or not?

While not perfect, you can reasonably assume that the bus cannot obey to
SMBus timings when I2C_FUNC_I2C is set. Because in the vast majority of
all cases with I2C_FUNC_I2C, SMBus commands are emulated which is prone
to the latency you described. You'd need a native SMBus controller to
avoid that which usually has not I2C_FUNC_I2C set.

Happy hacking,

   Wolfram


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