dmaengine: at_xdmac: fix residue corruption

An unexpected value of CUBC can lead to a corrupted residue. A more
complex sequence is needed to detect an inaccurate value for NCA or CUBC.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Fixes: e1f7c9eee707 ("dmaengine: at_xdmac: creation of the atmel
eXtended DMA Controller driver")
Cc: stable@vger.kernel.org #v4.1 and later
Reviewed-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

1 files changed, 32 insertions, 22 deletions

diff --git a/drivers/dma/at_xdmac.c b/drivers/dma/at_xdmac.c
index ba9b0b73fa47..b02494e115fe 100644
--- a/drivers/dma/at_xdmac.c
+++ b/drivers/dma/at_xdmac.c
@@ -1400,6 +1400,7 @@ at_xdmac_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
         u32                     cur_nda, check_nda, cur_ubc, mask, value;
         u8                      dwidth = 0;
         unsigned long           flags;
+        bool                    initd;
 
         ret = dma_cookie_status(chan, cookie, txstate);
         if (ret == DMA_COMPLETE)
@@ -1435,34 +1436,43 @@ at_xdmac_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
         }
 
         /*
-         * When processing the residue, we need to read two registers but we
-         * can't do it in an atomic way. AT_XDMAC_CNDA is used to find where
-         * we stand in the descriptor list and AT_XDMAC_CUBC is used
-         * to know how many data are remaining for the current descriptor.
-         * Since the dma channel is not paused to not loose data, between the
-         * AT_XDMAC_CNDA and AT_XDMAC_CUBC read, we may have change of
-         * descriptor.
-         * For that reason, after reading AT_XDMAC_CUBC, we check if we are
-         * still using the same descriptor by reading a second time
-         * AT_XDMAC_CNDA. If AT_XDMAC_CNDA has changed, it means we have to
-         * read again AT_XDMAC_CUBC.
+         * The easiest way to compute the residue should be to pause the DMA
+         * but doing this can lead to miss some data as some devices don't
+         * have FIFO.
+         * We need to read several registers because:
+         * - DMA is running therefore a descriptor change is possible while
+         * reading these registers
+         * - When the block transfer is done, the value of the CUBC register
+         * is set to its initial value until the fetch of the next descriptor.
+         * This value will corrupt the residue calculation so we have to skip
+         * it.
+         *
+         * INITD --------                    ------------
+         *              |____________________|
+         *       _______________________  _______________
+         * NDA       @desc2             \/   @desc3
+         *       _______________________/\_______________
+         *       __________  ___________  _______________
+         * CUBC       0    \/ MAX desc1 \/  MAX desc2
+         *       __________/\___________/\_______________
+         *
+         * Since descriptors are aligned on 64 bits, we can assume that
+         * the update of NDA and CUBC is atomic.
          * Memory barriers are used to ensure the read order of the registers.
-         * A max number of retries is set because unlikely it can never ends if
-         * we are transferring a lot of data with small buffers.
+         * A max number of retries is set because unlikely it could never ends.
          */
-        cur_nda = at_xdmac_chan_read(atchan, AT_XDMAC_CNDA) & 0xfffffffc;
-        rmb();
-        cur_ubc = at_xdmac_chan_read(atchan, AT_XDMAC_CUBC);
         for (retry = 0; retry < AT_XDMAC_RESIDUE_MAX_RETRIES; retry++) {
-                rmb();
                 check_nda = at_xdmac_chan_read(atchan, AT_XDMAC_CNDA) & 0xfffffffc;
-
-                if (likely(cur_nda == check_nda))
-                        break;
-
-                cur_nda = check_nda;
+                rmb();
+                initd = !!(at_xdmac_chan_read(atchan, AT_XDMAC_CC) & AT_XDMAC_CC_INITD);
                 rmb();
                 cur_ubc = at_xdmac_chan_read(atchan, AT_XDMAC_CUBC);
+                rmb();
+                cur_nda = at_xdmac_chan_read(atchan, AT_XDMAC_CNDA) & 0xfffffffc;
+                rmb();
+
+                if ((check_nda == cur_nda) && initd)
+                        break;
         }
 
         if (unlikely(retry >= AT_XDMAC_RESIDUE_MAX_RETRIES)) {