Re: [PATCH 1/2] iio: adc: ti_am335x_adc: Protect FIFO1 from concurrent access

[Jonathan Cameron <jic23@kernel.org>]

On 08/08/16 12:05, Vignesh R wrote:
> It is possible that two or more ADC channels can be simultaneously
> requested for raw samples, in which case there can be race in access to
> FIFO data resulting in loss of samples.
> If am335x_tsc_se_set_once() is called again from tiadc_read_raw(), when
> ADC is still acquired to sample one of the channels, the second process
> might be put into uninterruptible sleep state. Fix these issues, by
> protecting FIFO access and channel configurations with a mutex. Since
> tiadc_read_raw() might take anywhere between few microseconds to few
> milliseconds to finish execution (depending on averaging and delay
> values supplied via DT), its better to use mutex instead of spinlock.
> 
> Signed-off-by: Vignesh R <vigneshr@ti.com>
Hi,

Thanks for the patch.

As this is clearly a fix for a long standing issue, I'd like to send
it for stable inclusion.  Would you mind doing a bit of detective work
to added a Fixes tag to say which original patch introduced the issue?

I'm going to start pushing back on this in general as it's helpful
to the stable maintainers if we provide this info whenever possible.

Thanks,

Jonathan
> ---
>  drivers/iio/adc/ti_am335x_adc.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iio/adc/ti_am335x_adc.c b/drivers/iio/adc/ti_am335x_adc.c
> index 8a368756881b..bed9977a1863 100644
> --- a/drivers/iio/adc/ti_am335x_adc.c
> +++ b/drivers/iio/adc/ti_am335x_adc.c
> @@ -32,6 +32,7 @@
>  
>  struct tiadc_device {
>  	struct ti_tscadc_dev *mfd_tscadc;
> +	struct mutex fifo1_lock; /* to protect fifo access */
>  	int channels;
>  	u8 channel_line[8];
>  	u8 channel_step[8];
> @@ -359,6 +360,7 @@ static int tiadc_read_raw(struct iio_dev *indio_dev,
>  		int *val, int *val2, long mask)
>  {
>  	struct tiadc_device *adc_dev = iio_priv(indio_dev);
> +	int ret = IIO_VAL_INT;
>  	int i, map_val;
>  	unsigned int fifo1count, read, stepid;
>  	bool found = false;
> @@ -372,6 +374,7 @@ static int tiadc_read_raw(struct iio_dev *indio_dev,
>  	if (!step_en)
>  		return -EINVAL;
>  
> +	mutex_lock(&adc_dev->fifo1_lock);
>  	fifo1count = tiadc_readl(adc_dev, REG_FIFO1CNT);
>  	while (fifo1count--)
>  		tiadc_readl(adc_dev, REG_FIFO1);
> @@ -388,7 +391,8 @@ static int tiadc_read_raw(struct iio_dev *indio_dev,
>  
>  		if (time_after(jiffies, timeout)) {
>  			am335x_tsc_se_adc_done(adc_dev->mfd_tscadc);
> -			return -EAGAIN;
> +			ret = -EAGAIN;
> +			goto err_unlock;
>  		}
>  	}
>  	map_val = adc_dev->channel_step[chan->scan_index];
> @@ -414,8 +418,11 @@ static int tiadc_read_raw(struct iio_dev *indio_dev,
>  	am335x_tsc_se_adc_done(adc_dev->mfd_tscadc);
>  
>  	if (found == false)
> -		return -EBUSY;
> -	return IIO_VAL_INT;
> +		ret =  -EBUSY;
> +
> +err_unlock:
> +	mutex_unlock(&adc_dev->fifo1_lock);
> +	return ret;
>  }
>  
>  static const struct iio_info tiadc_info = {
> @@ -483,6 +490,7 @@ static int tiadc_probe(struct platform_device *pdev)
>  
>  	tiadc_step_config(indio_dev);
>  	tiadc_writel(adc_dev, REG_FIFO1THR, FIFO1_THRESHOLD);
> +	mutex_init(&adc_dev->fifo1_lock);
>  
>  	err = tiadc_channel_init(indio_dev, adc_dev->channels);
>  	if (err < 0)
>