On 2018-11-14 12:12, Dmitry Osipenko wrote:
On 14.11.2018 13:13, Ben Dooks wrote:
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 <digetx@xxxxxxxxx>
Signed-off-by: Ben Dooks <ben.dooks@xxxxxxxxxxxxxxx>
---
drivers/dma/tegra20-apb-dma.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/drivers/dma/tegra20-apb-dma.c
b/drivers/dma/tegra20-apb-dma.c
index 9a558e30c461..8219ab88a507 100644
--- a/drivers/dma/tegra20-apb-dma.c
+++ b/drivers/dma/tegra20-apb-dma.c
@@ -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;
/* 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.
Thanks, I should have posted a link to a test-patch I had
a while ago.
Tested-by: Dmitry Osipenko <digetx@xxxxxxxxx>
