linux-kernel - Re: [PATCH v3 1/3] i2c: imx: Fix reset of I2SR

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Message-ID: <2080355.2eSLTprjVn@n95hx1g2>
Date:   Tue, 6 Oct 2020 14:30:55 +0200
From:   Christian Eggers <ceggers@...i.de>
To:     David Laight <David.Laight@...lab.com>
CC:     Oleksij Rempel <linux@...pel-privat.de>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        NXP Linux Team <linux-imx@....com>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>
Subject: Re: [PATCH v3 1/3] i2c: imx: Fix reset of I2SR_IAL flag

Hi David,

On Tuesday, 6 October 2020, 14:06:36 CEST, David Laight wrote:
> From: Christian Eggers
> > +static void i2c_imx_clear_irq(struct imx_i2c_struct *i2c_imx, unsigned
> > int bits) +{
> > +	unsigned int temp;
> > +
> > +	/*
> > +	 * i2sr_clr_opcode is the value to clear all interrupts.
> > +	 * Here we want to clear only <bits>, so we write
> > +	 * ~i2sr_clr_opcode with just <bits> toggled.
> > +	 */
> > +	temp = ~i2c_imx->hwdata->i2sr_clr_opcode ^ bits;
> > +	imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2SR);
> > +}
> 
> That looks either wrong or maybe just overcomplicated.
Yes, it looks so.

> Why isn't:
> 	imx_i2c_write_reg(bits, i2c_imx, IMX_I2C_I2SR);
> enough?
i.MX requires W1C and Vybrid requires W0C in order to clear status bits.

> More usually you just write back the read value of such
> 'write 1 to clear' status registers and then act on all
> the set bits.
This pattern has been suggested by Uwe Klein-Koenig. It works because write 
access to read-only register bits is ignored, independent whether 0 or 1 is 
written. W0C is quite unusual, but I didn't design the hardware... The pattern 
ensures that not accidentally more status bits are cleared than desired.

> That ensures you clear all interrupts that were pending.
I think that Uwe's intention was not clearing bits which are not handled at 
this place. Otherwise events may get lost.

> If you need to avoid writes of bits that aren't in the
> 'clear all interrupts' value then you just need:
> 	bits &= i2c_imx->hwdata->i2sr_clr_opcode;
> prior to the write.
I think this wouldn't fit the W0C case for Vybrid.

Best regards
Christian