Re: [PATCH v6 1/2] i2c: imx: add DMA support for freescale i2c driver

["Lothar Waßmann" <LW@KARO-electronics.de>]

Hi,

fugang.duan@freescale.com wrote:
> From: Yuan Yao-B46683 Data: Thursday, August 07, 2014 4:05 PM
> >To: Duan Fugang-B38611; wsa@the-dreams.de; marex@denx.de
> >Cc: LW@KARO-electronics.de; mark.rutland@arm.com; shawn.guo@linaro.org;
> >linux-kernel@vger.kernel.org; linux-arm-kernel@lists.infradead.org; linux-
> >i2c@vger.kernel.org; Li Frank-B20596
> >Subject: RE: [PATCH v6 1/2] i2c: imx: add DMA support for freescale i2c
> >driver
> >
> >Hi Fugang,
> >
> >> >> >+	/* Waiting for Transfer complete. */
> >> >> >+	while (timeout--) {
> >> >> >+		temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2SR);
> >> >> >+		if (temp & I2SR_ICF)
> >> >> >+			break;
> >> >> >+		udelay(10);
> >> >> >+	}
> >> >> Whether there have better method like interrupt to avoid dead wait
> >> >> here until timeout ?
> >> >
> >> >Can you give me more suggestion? We have discussed it with our team,
> >> >It seems the short query wait is necessary.
> >> >
> >> At least, you can use schdule_timeout() instead of udelay() ?
> >
> >In fact, the waiting time normally is less than 10-50us, but the minimum
> >time interval for schdule_timeout() is 1 jiffies.
> >So maybe schdule_timeout() is not very necessary?
> >
> Oh, if the waiting time is 10 ~ 50us, you can use usleep_range(10, 50).
>
The loop is not meant to generate a certain delay, but to wait for some
HW flag to change within a certain time frame. So usleep_range() is
rather inadequate here!


But looking a little closer at the function, the timeout value seems to
be rather bogus to me. The loop counter 'timeout' is initialized from 
the constant IMX_I2C_DMA_TIMEOUT which is also used in
|wait_for_completion_interruptible_timeout(
|                               &i2c_imx->dma->cmd_complete,
|                               msecs_to_jiffies(IMX_I2C_DMA_TIMEOUT));
 				                 ^^^^^^^^^^^^^^^^^^^
as a number of milliseconds to wait for DMA completion.


Lothar Waßmann
-- 
___________________________________________________________

Ka-Ro electronics GmbH | Pascalstraße 22 | D - 52076 Aachen
Phone: +49 2408 1402-0 | Fax: +49 2408 1402-10
Geschäftsführer: Matthias Kaussen
Handelsregistereintrag: Amtsgericht Aachen, HRB 4996

www.karo-electronics.de | info@karo-electronics.de
___________________________________________________________