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

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Date:   Mon, 16 Mar 2020 21:43:22 +0100
From:   Marco Felsch <m.felsch@...gutronix.de>
To:     Stefan Lengfeld <contact@...fanchrist.eu>
Cc:     Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        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
Subject: Re: [PATCH] i2c: at91: support atomic write xfer

On 20-03-16 18:20, Stefan Lengfeld wrote:
> 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.

Yep because we need to handle the runtime_pm stuff by our-self. So for
the imx case we need to handle the clk en-/disable stuff. Runtime pm is
using a workqueue which can't be used in that late case.

Regards,
  Marco

> > > ...
> > > > +	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
> 

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