linux-kernel - Re: [PATCH] i2c: at91: support atomic write xfer

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Date:   Mon, 16 Mar 2020 18:20:03 +0100
From:   Stefan Lengfeld <contact@...fanchrist.eu>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     Dmitry Osipenko <digetx@...il.com>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        Marco Felsch <m.felsch@...gutronix.de>
Subject: Re: [PATCH] i2c: at91: support atomic write xfer

Hi Michał,

On Mon, Mar 16, 2020 at 03:42:21PM +0100, Michał Mirosław wrote:
> On Sun, Mar 15, 2020 at 11:46:33PM +0300, Dmitry Osipenko wrote:
> > 15.03.2020 21:27, Michał Mirosław пишет:
> > > Implement basic support for atomic write - enough to get a simple
> > > write to PMIC on shutdown. Only for chips having ALT_CMD register,
> > > eg. SAMA5D2.
> > > 
> > > Signed-off-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> > > ---
> > 
> > Hello Michał,
> > 
> > ...
> > > +	ret = pm_runtime_get_sync(dev->dev);
> > > +	if (ret < 0)
> > > +		goto out;
> > 
> > Runtime PM can't be used while interrupts are disabled, unless
> > pm_runtime_irq_safe() is used and driver's RPM callback is IRQ-safe.
> 
> I didn't get any warnings from lockdep and friends, but I'll double
> check if this is by luck.

You can have a look at the I2C atomic patch for the imx-driver. See

   https://patchwork.ozlabs.org/patch/1225802/

In that patch Marco Felsch is using clk_enable() and clk_disable() calls.

> 
> > ...
> > > +	timeout = jiffies + (2 + msg->len) * HZ/1000;
> > > +	for (;;) {
> > > +		stat = at91_twi_read(dev, AT91_TWI_SR);
> > > +		if (stat & AT91_TWI_TXCOMP)
> > > +			break;
> > > +		if (time_after(jiffies, timeout)) {
> > > +			ret = -ETIMEDOUT;
> > > +			goto out;
> > > +		}
> > > +		udelay(100);
> > > +	}
> > 
> > Jiffies can't be used with the disabled interrupts because jiffies are
> > updated by timer's interrupt.
> > 
> > Either ktime() API or iterator-based loop should be used.
> 
> Thanks for the pointers. In my use-case power is cut from the CPU at this
> point so it didn't matter that the loop was infinite.

Here again you can have a look at Marco Felsch's patch. He used the
function readb_poll_timeout_atomic(). So the loop can potentially
replaced by a single line.

Kind regards,
Stefan