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>

2 files changed, 109 insertions, 22 deletions

diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
index c2d5440aa226..a24b2e005eb3 100644
--- a/drivers/iio/industrialio-buffer.c
+++ b/drivers/iio/industrialio-buffer.c
@@ -37,11 +37,28 @@ static bool iio_buffer_is_active(struct iio_buffer *buf)
         return !list_empty(&buf->buffer_list);
 }
 
-static bool iio_buffer_data_available(struct iio_buffer *buf)
+static size_t iio_buffer_data_available(struct iio_buffer *buf)
 {
         return buf->access->data_available(buf);
 }
 
+static bool iio_buffer_ready(struct iio_dev *indio_dev, struct iio_buffer *buf,
+                             size_t to_wait)
+{
+        /* wakeup if the device was unregistered */
+        if (!indio_dev->info)
+                return true;
+
+        /* drain the buffer if it was disabled */
+        if (!iio_buffer_is_active(buf))
+                to_wait = min_t(size_t, to_wait, 1);
+
+        if (iio_buffer_data_available(buf) >= to_wait)
+                return true;
+
+        return false;
+}
+
 /**
  * iio_buffer_read_first_n_outer() - chrdev read for buffer access
  *
@@ -53,6 +70,8 @@ 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;
         int ret;
 
         if (!indio_dev->info)
@@ -61,19 +80,26 @@ ssize_t iio_buffer_read_first_n_outer(struct file *filp, char __user *buf,
         if (!rb || !rb->access->read_first_n)
                 return -EINVAL;
 
+        datum_size = rb->bytes_per_datum;
+
+        /*
+         * If datum_size is 0 there will never be anything to read from the
+         * buffer, so signal end of file now.
+         */
+        if (!datum_size)
+                return 0;
+
+        if (!(filp->f_flags & O_NONBLOCK))
+                to_wait = min_t(size_t, n / datum_size, rb->watermark);
+
         do {
-                if (!iio_buffer_data_available(rb)) {
-                        if (filp->f_flags & O_NONBLOCK)
-                                return -EAGAIN;
+                ret = wait_event_interruptible(rb->pollq,
+                                      iio_buffer_ready(indio_dev, rb, to_wait));
+                if (ret)
+                        return ret;
 
-                        ret = wait_event_interruptible(rb->pollq,
-                                        iio_buffer_data_available(rb) ||
-                                        indio_dev->info == NULL);
-                        if (ret)
-                                return ret;
-                        if (indio_dev->info == NULL)
-                                return -ENODEV;
-                }
+                if (!indio_dev->info)
+                        return -ENODEV;
 
                 ret = rb->access->read_first_n(rb, n, buf);
                 if (ret == 0 && (filp->f_flags & O_NONBLOCK))
@@ -96,9 +122,8 @@ unsigned int iio_buffer_poll(struct file *filp,
                 return -ENODEV;
 
         poll_wait(filp, &rb->pollq, wait);
-        if (iio_buffer_data_available(rb))
+        if (iio_buffer_ready(indio_dev, rb, rb->watermark))
                 return POLLIN | POLLRDNORM;
-        /* need a way of knowing if there may be enough data... */
         return 0;
 }
 
@@ -123,6 +148,7 @@ void iio_buffer_init(struct iio_buffer *buffer)
         INIT_LIST_HEAD(&buffer->buffer_list);
         init_waitqueue_head(&buffer->pollq);
         kref_init(&buffer->ref);
+        buffer->watermark = 1;
 }
 EXPORT_SYMBOL(iio_buffer_init);
 
@@ -416,6 +442,11 @@ static ssize_t iio_buffer_write_length(struct device *dev,
                 buffer->access->set_length(buffer, val);
                 ret = 0;
         }
+        if (ret)
+                goto out;
+        if (buffer->length && buffer->length < buffer->watermark)
+                buffer->watermark = buffer->length;
+out:
         mutex_unlock(&indio_dev->mlock);
 
         return ret ? ret : len;
@@ -472,6 +503,7 @@ static void iio_buffer_activate(struct iio_dev *indio_dev,
 static void iio_buffer_deactivate(struct iio_buffer *buffer)
 {
         list_del_init(&buffer->buffer_list);
+        wake_up_interruptible(&buffer->pollq);
         iio_buffer_put(buffer);
 }
 
@@ -754,16 +786,64 @@ done:
 
 static const char * const iio_scan_elements_group_name = "scan_elements";
 
+static ssize_t iio_buffer_show_watermark(struct device *dev,
+                                         struct device_attribute *attr,
+                                         char *buf)
+{
+        struct iio_dev *indio_dev = dev_to_iio_dev(dev);
+        struct iio_buffer *buffer = indio_dev->buffer;
+
+        return sprintf(buf, "%u\n", buffer->watermark);
+}
+
+static ssize_t iio_buffer_store_watermark(struct device *dev,
+                                          struct device_attribute *attr,
+                                          const char *buf,
+                                          size_t len)
+{
+        struct iio_dev *indio_dev = dev_to_iio_dev(dev);
+        struct iio_buffer *buffer = indio_dev->buffer;
+        unsigned int val;
+        int ret;
+
+        ret = kstrtouint(buf, 10, &val);
+        if (ret)
+                return ret;
+        if (!val)
+                return -EINVAL;
+
+        mutex_lock(&indio_dev->mlock);
+
+        if (val > buffer->length) {
+                ret = -EINVAL;
+                goto out;
+        }
+
+        if (iio_buffer_is_active(indio_dev->buffer)) {
+                ret = -EBUSY;
+                goto out;
+        }
+
+        buffer->watermark = val;
+out:
+        mutex_unlock(&indio_dev->mlock);
+
+        return ret ? ret : len;
+}
+
 static DEVICE_ATTR(length, S_IRUGO | S_IWUSR, iio_buffer_read_length,
                    iio_buffer_write_length);
 static struct device_attribute dev_attr_length_ro = __ATTR(length,
         S_IRUGO, iio_buffer_read_length, NULL);
 static DEVICE_ATTR(enable, S_IRUGO | S_IWUSR,
                    iio_buffer_show_enable, iio_buffer_store_enable);
+static DEVICE_ATTR(watermark, S_IRUGO | S_IWUSR,
+                   iio_buffer_show_watermark, iio_buffer_store_watermark);
 
 static struct attribute *iio_buffer_attrs[] = {
         &dev_attr_length.attr,
         &dev_attr_enable.attr,
+        &dev_attr_watermark.attr,
 };
 
 int iio_buffer_alloc_sysfs_and_mask(struct iio_dev *indio_dev)
@@ -944,8 +1024,18 @@ static const void *iio_demux(struct iio_buffer *buffer,
 static int iio_push_to_buffer(struct iio_buffer *buffer, const void *data)
 {
         const void *dataout = iio_demux(buffer, data);
+        int ret;
+
+        ret = buffer->access->store_to(buffer, dataout);
+        if (ret)
+                return ret;
 
-        return buffer->access->store_to(buffer, dataout);
+        /*
+         * We can't just test for watermark to decide if we wake the poll queue
+         * because read may request less samples than the watermark.
+         */
+        wake_up_interruptible_poll(&buffer->pollq, POLLIN | POLLRDNORM);
+        return 0;
 }
 
 static void iio_buffer_demux_free(struct iio_buffer *buffer)
diff --git a/drivers/iio/kfifo_buf.c b/drivers/iio/kfifo_buf.c
index b2beea01c49b..847ca561afe0 100644
--- a/drivers/iio/kfifo_buf.c
+++ b/drivers/iio/kfifo_buf.c
@@ -83,9 +83,6 @@ static int iio_store_to_kfifo(struct iio_buffer *r,
         ret = kfifo_in(&kf->kf, data, 1);
         if (ret != 1)
                 return -EBUSY;
-
-        wake_up_interruptible_poll(&r->pollq, POLLIN | POLLRDNORM);
-
         return 0;
 }
 
@@ -109,16 +106,16 @@ static int iio_read_first_n_kfifo(struct iio_buffer *r,
         return copied;
 }
 
-static bool iio_kfifo_buf_data_available(struct iio_buffer *r)
+static size_t iio_kfifo_buf_data_available(struct iio_buffer *r)
 {
         struct iio_kfifo *kf = iio_to_kfifo(r);
-        bool empty;
+        size_t samples;
 
         mutex_lock(&kf->user_lock);
-        empty = kfifo_is_empty(&kf->kf);
+        samples = kfifo_len(&kf->kf);
         mutex_unlock(&kf->user_lock);
 
-        return !empty;
+        return samples;
 }
 
 static void iio_kfifo_buffer_release(struct iio_buffer *buffer)