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

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Date:   Fri, 25 Sep 2020 10:11:01 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Christian Eggers <ceggers@...i.de>
Cc:     Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Oleksij Rempel <linux@...pel-privat.de>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-i2c@...r.kernel.org
Subject: Re: [PATCH 1/3] i2c: imx: Fix reset of I2SR_IAL flag

On Thu, Sep 17, 2020 at 04:13:50PM +0200, Christian Eggers wrote:
> Hello Uwe,
> 
> On Thursday, 17 September 2020, 16:02:35 CEST, Uwe Kleine-König wrote:
> > Hello,
> >
> > On Thu, Sep 17, 2020 at 02:20:27PM +0200, Christian Eggers wrote:
> > ...
> > >             /* check for arbitration lost */
> > >             if (temp & I2SR_IAL) {
> > >                     temp &= ~I2SR_IAL;
> > > +                   temp |= (i2c_imx->hwdata->i2sr_clr_opcode & I2SR_IAL);
> > >                     imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2SR);
> > >                     return -EAGAIN;
> > ...
> 
> > This looks strange. First the flag is cleared and then it is (in some
> > cases) set again.
> i.MX controllers require writing a 0 to clear these bits. Vybrid controllers
> need writing a 1 for the same.

Yes, I understood that.

> > If I2SR_IIF is set in temp you ack this irq without handling it. (Which
> > might happen if atomic is set and irqs are off?!)
> This patch is only about using the correct processor specific value for
> acknowledging an IRQ... But I think that returning EAGAIN (which aborts the
> transfer) should be handling enough. At the next transfer, the controller will
> be set back to master mode.

Either you didn't understand what I meant, or I don't understand why you
consider your patch right anyhow. So I try to explain in other and more
words.

IMHO the intention here (and also what happens on i.MX) is that exactly
the AL interrupt pending bit should be cleared and the IF irq is
supposed to be untouched.

Given there are only two irq flags in the I2SR register (which is called
IBSR on Vybrid) the status quo (i.e. without your patch) is:

  On i.MX IAL is cleared
  On Vybrid IIF (which is called IBIF there) is cleared.

With your patch we get:

  On i.MX IAL is cleared
  On Vybrid both IIF (aka IBIF) and IAL (aka IBAL) are cleared.

To get it right for both SoC types you have to do (e.g.):

	temp = ~i2c_imx->hwdata->i2sr_clr_opcode ^ I2SR_IAL;

(and in i2c_imx_isr() the same using I2SR_IIF instead of I2SR_IAL
because there currently IAL might be cleared by mistake on Vybrid).

I considered creating a patch, but as I don't have a Vybrid on my desk
and on i.MX there is no change, I let you do this.

Best regards
Uwe

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

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