dmaengine: qcom: bam_dma: Fix completed descriptors count

commit f6034225442c4a87906d36e975fd9e99a8f95487 upstream.

One space is left unused in circular FIFO to differentiate
'full' and 'empty' cases. So take that in to account while
counting for the descriptors completed.

Fixes the issue reported here,
	https://lkml.org/lkml/2019/6/18/669

Cc: stable@vger.kernel.org
Reported-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Tested-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/dma/qcom/bam_dma.c

@@ -808,6 +808,9 @@ static u32 process_channel_irqs(struct bam_device *bdev)
 		/* Number of bytes available to read */
 		avail = CIRC_CNT(offset, bchan->head, MAX_DESCRIPTORS + 1);
 
+		if (offset < bchan->head)
+			avail--;
+
 		list_for_each_entry_safe(async_desc, tmp,
 					 &bchan->desc_list, desc_node) {
 			/* Not enough data to read */