dmaengine: tegra: avoid overflow of byte tracking

BugLink: https://bugs.launchpad.net/bugs/1838116
[ Upstream commit e486df39305864604b7e25f2a95d51039517ac57 ]

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@gmail.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/drivers/dma/tegra20-apb-dma.c b/drivers/dma/tegra20-apb-dma.c
index b9d75a54c896a5d3d7d141d571c395acef2962ff..7db2766b5fe9e322994b6f5201ec966b671f6994 100644 (file)
--- a/drivers/dma/tegra20-apb-dma.c
+++ b/drivers/dma/tegra20-apb-dma.c
@@ -635,7 +635,10 @@ 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)