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>

diff --git a/drivers/dma/at_xdmac.c b/drivers/dma/at_xdmac.c
index ba9b0b73fa479a7afae7c8934e4fbdd09925ebb9..b02494e115fe2df1611c700fa75a46cd39a87398 100644 (file)
--- 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)) {