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    Subject[PATCH AUTOSEL 5.0 252/262] dmaengine: tegra: avoid overflow of byte tracking
    Date
    From: Ben Dooks <ben.dooks@codethink.co.uk>

    [ 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>
    ---
    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)
    --
    2.19.1
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