linux-kernel - Re: [PATCH v2] iio: allow userspace to flush the hwfifo with non-blocking reads

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Date:	Sun, 21 Jun 2015 14:52:01 +0100
From:	Jonathan Cameron <jic23@...nel.org>
To:	Lars-Peter Clausen <lars@...afoo.de>,
	Octavian Purdila <octavian.purdila@...el.com>
CC:	Hartmut Knaack <knaack.h@....de>,
	Peter Meerwald <pmeerw@...erw.net>, linux-iio@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] iio: allow userspace to flush the hwfifo with non-blocking
 reads

On 15/06/15 12:44, Lars-Peter Clausen wrote:
> On 06/14/2015 04:33 PM, Jonathan Cameron wrote:
>> On 05/06/15 13:56, Octavian Purdila wrote:
>>> This patch changes the semantics of non-blocking reads so that a
>>> hardware fifo flush is triggered if the available data in the device
>>> buffer is less then the requested size.
>>>
>>> This allows userspace to accurately generate hardware fifo flushes, by
>>> doing a non-blocking read with a size greater then the sum of the
>>> device buffer and hardware fifo size.
>>>
>>> Signed-off-by: Octavian Purdila <octavian.purdila@...el.com>
>> Hi Octavian,
>>
>> I'm personally happy with this approach, but would like Lars to have
>> a chance to take a look as he's been getting his hands a lot dirtier in
>> this area than I have recently!
> 
> Looks good.
> 
> Reviewed-by: Lars-Peter Clausen <lars@...afoo.de>
cool,

Applied to the togreg branch of iio.git - initially pushed out as testing
etc.

Thanks

Jonathan
> 
>>
>> This seems a logical bit of API to me even without the desire to use it
>> to force a flush.  If we want whatever data is available now,
>> we want that data now, not when the fifo gets around to giving it to us!
>>
>> Feel free to poke me if this sits here uncommented upon for a few more
>> weeks!
>>
>> Jonathan
>>> ---
>>>
>>> This is the second iteration of the patch that allows userspace to
>>> accurately generate flush events. The first RFC patch set was
>>> discussed here:
>>>
>>> https://lkml.org/lkml/2015/4/29/202
>>>
>>>
>>>   drivers/iio/industrialio-buffer.c | 18 +++++++++---------
>>>   1 file changed, 9 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
>>> index df919f4..24085db 100644
>>> --- a/drivers/iio/industrialio-buffer.c
>>> +++ b/drivers/iio/industrialio-buffer.c
>>> @@ -71,8 +71,9 @@ static bool iio_buffer_ready(struct iio_dev *indio_dev, struct iio_buffer *buf,
>>>
>>>       if (avail >= to_wait) {
>>>           /* force a flush for non-blocking reads */
>>> -        if (!to_wait && !avail && to_flush)
>>> -            iio_buffer_flush_hwfifo(indio_dev, buf, to_flush);
>>> +        if (!to_wait && avail < to_flush)
>>> +            iio_buffer_flush_hwfifo(indio_dev, buf,
>>> +                        to_flush - avail);
>>>           return true;
>>>       }
>>>
>>> @@ -100,8 +101,7 @@ ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user *buf,
>>>       struct iio_dev *indio_dev = filp->private_data;
>>>       struct iio_buffer *rb = indio_dev->buffer;
>>>       size_t datum_size;
>>> -    size_t to_wait = 0;
>>> -    size_t to_read;
>>> +    size_t to_wait;
>>>       int ret;
>>>
>>>       if (!indio_dev->info)
>>> @@ -119,14 +119,14 @@ ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user *buf,
>>>       if (!datum_size)
>>>           return 0;
>>>
>>> -    to_read = min_t(size_t, n / datum_size, rb->watermark);
>>> -
>>> -    if (!(filp->f_flags & O_NONBLOCK))
>>> -        to_wait = to_read;
>>> +    if (filp->f_flags & O_NONBLOCK)
>>> +        to_wait = 0;
>>> +    else
>>> +        to_wait = min_t(size_t, n / datum_size, rb->watermark);
>>>
>>>       do {
>>>           ret = wait_event_interruptible(rb->pollq,
>>> -            iio_buffer_ready(indio_dev, rb, to_wait, to_read));
>>> +              iio_buffer_ready(indio_dev, rb, to_wait, n / datum_size));
>>>           if (ret)
>>>               return ret;
>>>
>>>
>>
> 
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