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.desroc...@atmel.com>
Fixes: e1f7c9eee707 ("dmaengine: at_xdmac: creation of the atmel
eXtended DMA Controller driver")
Cc: sta...@vger.kernel.org #v4.1 and later
---
drivers/dma/at_xdmac.c | 54 ++++++++++++++++++++++++++++++--------------------
1 file changed, 32 insertions(+), 22 deletions(-)
diff --git a/drivers/dma/at_xdmac.c b/drivers/dma/at_xdmac.c
index ba9b0b7..b02494e 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)) {
--
2.5.0
