linux-kernel - Re: [PATCH] i2c: cadence: Implement timeout

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Message-ID: <20181024105854.GU30658@n2100.armlinux.org.uk>
Date:   Wed, 24 Oct 2018 11:58:54 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     shubhrajyoti.datta@...il.com
Cc:     linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        Shubhrajyoti Datta <shubhrajyoti.datta@...inx.com>,
        michal.simek@...inx.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] i2c: cadence: Implement timeout

On Wed, Oct 24, 2018 at 04:20:03PM +0530, shubhrajyoti.datta@...il.com wrote:
> From: Shubhrajyoti Datta <shubhrajyoti.datta@...inx.com>
> 
> In some cases we are waiting in a loop. Replace the infinite wait with
> the  timeout.
> 
> Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@...inx.com>
> ---
>  drivers/i2c/busses/i2c-cadence.c | 30 ++++++++++++++++++++++++++----
>  1 file changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-cadence.c b/drivers/i2c/busses/i2c-cadence.c
> index b136057..9c38278 100644
> --- a/drivers/i2c/busses/i2c-cadence.c
> +++ b/drivers/i2c/busses/i2c-cadence.c
> @@ -209,6 +209,7 @@ static irqreturn_t cdns_i2c_isr(int irq, void *ptr)
>  	struct cdns_i2c *id = ptr;
>  	/* Signal completion only after everything is updated */
>  	int done_flag = 0;
> +	unsigned int timeout;
>  	irqreturn_t status = IRQ_NONE;
>  
>  	isr_status = cdns_i2c_readreg(CDNS_I2C_ISR_OFFSET);
> @@ -235,6 +236,7 @@ static irqreturn_t cdns_i2c_isr(int irq, void *ptr)
>  	    ((isr_status & CDNS_I2C_IXR_COMP) ||
>  	     (isr_status & CDNS_I2C_IXR_DATA))) {
>  		/* Read data if receive data valid is set */
> +		timeout = 1000;
>  		while (cdns_i2c_readreg(CDNS_I2C_SR_OFFSET) &
>  		       CDNS_I2C_SR_RXDV) {
>  			/*
> @@ -253,6 +255,16 @@ static irqreturn_t cdns_i2c_isr(int irq, void *ptr)
>  
>  			if (cdns_is_holdquirk(id, hold_quirk))
>  				break;
> +			timeout--;
> +			if (timeout)
> +				mdelay(1);
> +			else
> +				break;
> +		}
> +		if (!timeout) {
> +			id->err_status = -ETIMEDOUT;
> +			complete(&id->xfer_done);
> +			return IRQ_HANDLED;

Good kernel programming principle: Always check for the success
condition when exiting due to timeout rather than the fact that we
timed out.

Also, is this _really_ a loop that needs a timeout condition?  Looking
at the original code, it looks like the purpose of the loop is to read
more than one byte, and you are introducing a 1ms delay between the
read of each byte.

-- 
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