iio: add watermark logic to iio read and poll

Currently the IIO buffer blocking read only wait until at least one
data element is available.
This patch makes the reader sleep until enough data is collected before
returning to userspace. This should limit the read() calls count when
trying to get data in batches.

Co-author: Yannick Bedhomme <yannick.bedhomme@mobile-devices.fr>
Signed-off-by: Josselin Costanzi <josselin.costanzi@mobile-devices.fr>
[rebased and remove buffer timeout]
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-iio b/Documentation/ABI/testing/sysfs-bus-iio
index 580b3f85437b..0b6f0abf3370 100644
--- a/Documentation/ABI/testing/sysfs-bus-iio
+++ b/Documentation/ABI/testing/sysfs-bus-iio
@@ -1280,3 +1280,18 @@ Contact:	linux-iio@vger.kernel.org
 Description:
                 Specifies number of seconds in which we compute the steps
                 that occur in order to decide if the consumer is making steps.
+
+What:           /sys/bus/iio/devices/iio:deviceX/buffer/watermark
+KernelVersion:  4.2
+Contact:        linux-iio@vger.kernel.org
+Description:
+                A single positive integer specifying the maximum number of scan
+                elements to wait for.
+                Poll will block until the watermark is reached.
+                Blocking read will wait until the minimum between the requested
+                read amount or the low water mark is available.
+                Non-blocking read will retrieve the available samples from the
+                buffer even if there are less samples then watermark level. This
+                allows the application to block on poll with a timeout and read
+                the available samples after the timeout expires and thus have a
+                maximum delay guarantee.