linux-kernel - Re: [PATCH v4 2/3] i2c: iproc: Add Broadcom iProc I2C Driver

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Date:	Sat, 17 Jan 2015 21:18:49 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Ray Jui <rjui@...adcom.com>
Cc:	Wolfram Sang <wsa@...-dreams.de>, Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Grant Likely <grant.likely@...aro.org>,
	Christian Daudt <bcm@...thebug.org>,
	Matt Porter <mporter@...aro.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Scott Branden <sbranden@...adcom.com>,
	linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	bcm-kernel-feedback-list@...adcom.com, devicetree@...r.kernel.org
Subject: Re: [PATCH v4 2/3] i2c: iproc: Add Broadcom iProc I2C Driver

Hello,

On Sat, Jan 17, 2015 at 11:58:33AM -0800, Ray Jui wrote:
> On 1/17/2015 8:01 AM, Uwe Kleine-König wrote:
> > On Fri, Jan 16, 2015 at 02:09:28PM -0800, Ray Jui wrote:
> >> On 1/15/2015 12:41 AM, Uwe Kleine-König wrote:
> >>> On Wed, Jan 14, 2015 at 02:23:32PM -0800, Ray Jui wrote:
> >>>> +	 */
> >>>> +	val = 1 << M_CMD_START_BUSY_SHIFT;
> >>>> +	if (msg->flags & I2C_M_RD) {
> >>>> +		val |= (M_CMD_PROTOCOL_BLK_RD << M_CMD_PROTOCOL_SHIFT) |
> >>>> +		       (msg->len << M_CMD_RD_CNT_SHIFT);
> >>>> +	} else {
> >>>> +		val |= (M_CMD_PROTOCOL_BLK_WR << M_CMD_PROTOCOL_SHIFT);
> >>>> +	}
> >>>> +	writel(val, iproc_i2c->base + M_CMD_OFFSET);
> >>>> +
> >>>> +	time_left = wait_for_completion_timeout(&iproc_i2c->done, time_left);
> >>>
> >>> When the interrupt fires here after the complete timed out and before
> >>> you disable the irq you still throw the result away.
> >> Yes, but then this comes down to the fact that if it has reached the
> >> point that is determined to be a timeout condition in the driver, one
> >> should really treat it as timeout error. In a normal condition,
> >> time_left should never reach zero.
> > I don't agree here. I'm not sure there is a real technical reason,
> > though. But still if you're in a "success after timeout already over"
> > situation it's IMHO better to interpret it as success, not timeout.
> > 
> The thing is, the interrupt should never fire after
> wait_for_completion_timeout returns zero here. If it does, then the
> issue is really that the timeout value set in the driver is probably not
> long enough. I just checked other I2C drivers. I think the way how
> timeout is handled here is consistent with other I2C drivers.
In the presence of Clock stretching there is no (theorethical) upper
limit for the time needed to transfer a given message, is there? So
(theoretically) you can never be sure not to interrupt an ongoing
transfer.

And other drivers doing the same is only an excuse to start similar, but
not to not improve :-)

> >>>> +static int bcm_iproc_i2c_remove(struct platform_device *pdev)
> >>>> +{
> >>>> +	struct bcm_iproc_i2c_dev *iproc_i2c = platform_get_drvdata(pdev);
> >>>> +
> >>>> +	i2c_del_adapter(&iproc_i2c->adapter);
> >>> You need to free the irq before i2c_del_adapter.
> >>>
> >> Yes. Thanks. Change back to use devm_request_irq, and use disable_irq
> >> here before removing the adapter.
> > The more lightweight approach is to set your device's irq-enable
> > register to zero and call synchronize_irq. (For a shared irq calling
> > disable_irq is even wrong here.)
> > 
> The fact that IRQF_SHARED flag is not set indicates this is a dedicated
> IRQ line, so I thought using disable_irq here makes sense. But if both
> you and Wolfram think masking all I2C interrupts at the block level +
> synchronize_irq is a better approach, I can change to that. Thanks!
I don't care much. Using synchronize_irq is the more universal approach
and so more likely correct for someone copying from your driver.

Best regards
Uwe

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