dma: tegra: avoid overflow of byte tracking

Quoted 53 lines · Ben Dooks

The dma_desc->bytes_transferred counter tracks the number of bytes moved by the DMA channel. This is then used to calculate the information
passed back in the in the tegra_dma_tx_status callback, which is usually
fine.

When the DMA channel is configured as continous, then the bytes_transferred
counter will increase over time and eventually overflow to become negative
so the residue count will become invalid and the ALSA sound-dma code will
report invalid hardware pointer values to the application. This results in
some users becoming confused about the playout position and putting audio
data in the wrong place.

To fix this issue, always ensure the bytes_transferred field is modulo the
size of the request. We only do this for the case of the cyclic transfer
done ISR as anyone attempting to move 2GiB of DMA data in one transfer is unlikely.

Note, we don't fix the issue that we should /never/ transfer a negative
number of bytes so we could make those fields unsigned.

Reviewed-by Dmitry Osipenko <[email protected]>

Signed-off-by Ben Dooks <[email protected]>

---
 drivers/dma/tegra20-apb-dma.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

drivers/dma/tegra20-apb-dma.c +4 -1

index 9a558e30c461..8219ab88a507 100644
@@ -636,7 +636,10 @@ static void 
 handle_cont_sngl_cycle_dma_done(struct tegra_dma_channel *tdc,
 
 	sgreq = list_first_entry(&tdc->pending_sg_req, typeof(*sgreq), 
 node);
 	dma_desc = sgreq->dma_desc;
-	dma_desc->bytes_transferred += sgreq->req_len;
+	/* if we dma for long enough the transfer count will wrap */
+	dma_desc->bytes_transferred =
+		(dma_desc->bytes_transferred + sgreq->req_len) %
+		dma_desc->bytes_requested;
 

drivers/dma/tegra20-apb-dma.c

Quoted 3 lines

drivers/dma/tegra20-apb-dma.c +0 -0

 	/* Callback need to be call */
 	if (!dma_desc->cb_count)

I also actually tested that audio playback breaks after the overflow and this patch fixes it.

Tested-by Dmitry Osipenko <[email protected]>