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

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Date:   Wed, 11 Jan 2017 16:42:00 +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 Wed, Jan 11, 2017 at 03:20:41PM +0100, M'boumba Cedric Madianga wrote:
> >
> >> +              */
> >> +             reg = i2c_dev->base + STM32F4_I2C_CR1;
> >> +             stm32f4_i2c_clr_bits(reg, STM32F4_I2C_CR1_ACK);
> >> +             stm32f4_i2c_set_bits(reg, STM32F4_I2C_CR1_POS);
> >
> > You could get rid of this, when caching the value of CR1. Would save two
> > register reads here. This doesn't work for all registers, but it should
> > be possible to apply for most of them, maybe enough to get rid of the
> > clr_bits and set_bits function.
> 
> I agree at many places I could save registers read by not using
> clr_bits and set_bits function when the registers in question has been
> already read.
> But it is not enough to get rid of the clr_bits and set_bits function.
> For example when calling stm32f4_i2c_terminate_xfer(), the CR1
> register is never read before so set_bits function is useful.

I didn't double check the manual, but I would expect that CR1 isn't
modified by hardware. So you can cache the result in the driver data
structure and do the necessary modifications with that one.

Best regards
Uwe

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