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

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Message-ID: <20170112174902.j52foglkdouyz36n@pengutronix.de>
Date:   Thu, 12 Jan 2017 18:49:02 +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

On Thu, Jan 12, 2017 at 02:47:42PM +0100, M'boumba Cedric Madianga wrote:
> 2017-01-12 13:03 GMT+01:00 Uwe Kleine-König <u.kleine-koenig@...gutronix.de>:
> > 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.
> 
> Ok I understand your use case but I always think that the harware manages it.
> If I take the above example, the I2C SMBus block read transaction will
> be as below:
> S Addr Wr [A] Comm [A]
>            S Addr Rd [A] [Count] A [Data1] A [Data2] NA P
> 
> The first message is a single byte-transmission so there is no problem.
> 
> The second message is a N-byte reception with N = 3
> 
> When the I2C controller has finished to send the device address (S
> Addr Rd), the ADDR flag is set and an interrupt is raised.
> In the routine that handles ADDR event, we set ACK bit in order to
> generate ACK pulse as soon as a data byte is received in the shift
> register and then we clear the ADDR flag.
> Please note that the SCL line is stretched low until ADDR flag is cleared.
> So, as far I understand, the device could not sent any data as long as
> the SCL line is stretched low. Right ?
> 
> Then, as soon as the SCL line is high, the device could send the first
> data byte (Count).
> When this byte is received in the shift register, an ACK is
> automatically generated as defined during adress match phase and the
> data byte is pushed in DR (data register).
> Then, an interrupt is raised as RXNE (RX not empty) flag is set.
> In the routine that handles RXNE event, as N=3, we just clear all
> buffer interrupts in order to avoid another system preemption due to
> RXNE event but we does not read the data in DR.

In my example I want to receive a block of length 1, so only two bytes
are read, a 1 (the length) and the data byte (0xab in my example). I
think that as soon as you read the 1 it's already to late to schedule
the NA after the next byte?

Best regards
Uwe

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