linux-kernel - Re: [PATCH v8 2/5] i2c: Add STM32F4 I2C driver

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Date:   Thu, 12 Jan 2017 13:03:09 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     M'boumba Cedric Madianga <cedric.madianga@...il.com>
Cc:     Wolfram Sang <wsa@...-dreams.de>, Rob Herring <robh+dt@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Patrice Chotard <patrice.chotard@...com>,
        Russell King <linux@...linux.org.uk>,
        linux-i2c@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 2/5] i2c: Add STM32F4 I2C driver

Hello Cedric,

On Thu, Jan 12, 2017 at 12:23:12PM +0100, M'boumba Cedric Madianga wrote:
> 2017-01-11 16:39 GMT+01:00 Uwe Kleine-König <u.kleine-koenig@...gutronix.de>:
> > On Wed, Jan 11, 2017 at 02:58:44PM +0100, M'boumba Cedric Madianga wrote:
> >> 2017-01-11 9:22 GMT+01:00 Uwe Kleine-König <u.kleine-koenig@...gutronix.de>:
> >> > This is surprising. I didn't recheck the manual, but that looks very
> >> > uncomfortable.
> >>
> >> I agree but this exactly the hardware way of working described in the
> >> reference manual.
> >
> > IMHO that's a hw bug. This makes it for example impossible to implement
> > SMBus block transfers (I think).
> 
> This is not correct.
> Setting STOP/START bit does not mean the the pulse will be sent right now.
> Here we have just to prepare the hardware for the 2 next pulse but the
> STOP/START/ACK pulse will be generated at the right time as required
> by I2C specification.
> So SMBus block transfer will be possible.

A block transfer consists of a byte that specifies the count of bytes
yet to come. So the device sends for example:

	0x01 0xab

So when you read the 1 in the first byte it's already too late to set
STOP to get it after the 2nd byte.

Not sure I got all the required details right, though.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |