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

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Message-ID: <20150117211017.GD22880@pengutronix.de>
Date:	Sat, 17 Jan 2015 22:10:17 +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 12:51:50PM -0800, Ray Jui wrote:
> On 1/17/2015 12:18 PM, Uwe Kleine-König wrote:
> > 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.
> > 
> Yes. No theoretical upper limit in the case when clock is stretched by
> the slave. But how would adding an additional interrupt completion check
> below help? I assume you want the the check to be like the following?
> 
> 	time_left = wait_for_completion_timeout(&iproc_i2c->done, time_left);
> 
> 	/* disable all interrupts */
> 	writel(0, iproc_i2c->base + IE_OFFSET);
> 
> 	if (!time_left && !completion_done()) {
> 		dev_err(iproc_i2c->device, "transaction timed out\n");
> 
> 		/* flush FIFOs */
> 		val = (1 << M_FIFO_RX_FLUSH_SHIFT) |
> 		      (1 << M_FIFO_TX_FLUSH_SHIFT);
> 		writel(val, iproc_i2c->base + M_FIFO_CTRL_OFFSET);
> 		return -ETIMEDOUT;
> 	}
No, I want:

	time_left = wait_for_completion_timeout(&iproc_i2c->done, time_left);

	if (!transfer_was_complete) {
		handle_error();
		...

	}

	handle_successful_transfer();

and time_left == 0 is not a reliable indicator that the transfer failed.

Best regards
Uwe

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