dmaengine: at_xdmac: fix residue computation

commit 25c5e9626ca4d40928dc9c44f009ce2ed0a739e7 upstream.

When computing the residue we need two pieces of information: the current
descriptor and the remaining data of the current descriptor. To get
that information, we need to read consecutively two registers but we
can't do it in an atomic way. For that reason, we have to check manually
that current descriptor has not changed.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Suggested-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Reported-by: David Engraf <david.engraf@sysgo.com>
Tested-by: David Engraf <david.engraf@sysgo.com>
Fixes: e1f7c9eee707 ("dmaengine: at_xdmac: creation of the atmel
eXtended DMA Controller driver")
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/dma/at_xdmac.c b/drivers/dma/at_xdmac.c
index fa00f3a186da140694dbc4529f8327668f434d2c..02f9aa4ebe05fef65fbdf5d5a55d1213b296541e 100644 (file)
--- a/drivers/dma/at_xdmac.c
+++ b/drivers/dma/at_xdmac.c
@@ -176,6 +176,7 @@
 #define AT_XDMAC_MAX_CHAN      0x20
 #define AT_XDMAC_MAX_CSIZE     16      /* 16 data */
 #define AT_XDMAC_MAX_DWIDTH    8       /* 64 bits */
+#define AT_XDMAC_RESIDUE_MAX_RETRIES   5
 
 #define AT_XDMAC_DMA_BUSWIDTHS\
        (BIT(DMA_SLAVE_BUSWIDTH_UNDEFINED) |\
@@ -1383,8 +1384,8 @@ at_xdmac_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
        struct at_xdmac_desc    *desc, *_desc;
        struct list_head        *descs_list;
        enum dma_status         ret;
-       int                     residue;
-       u32                     cur_nda, mask, value;
+       int                     residue, retry;
+       u32                     cur_nda, check_nda, cur_ubc, mask, value;
        u8                      dwidth = 0;
        unsigned long           flags;
 
@@ -1421,7 +1422,42 @@ at_xdmac_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
                        cpu_relax();
        }
 
+       /*
+        * 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.
+        * 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.
+        */
        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();
+               cur_ubc = at_xdmac_chan_read(atchan, AT_XDMAC_CUBC);
+       }
+
+       if (unlikely(retry >= AT_XDMAC_RESIDUE_MAX_RETRIES)) {
+               ret = DMA_ERROR;
+               goto spin_unlock;
+       }
+
        /*
         * Remove size of all microblocks already transferred and the current
         * one. Then add the remaining size to transfer of the current
@@ -1434,7 +1470,7 @@ at_xdmac_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
                if ((desc->lld.mbr_nda & 0xfffffffc) == cur_nda)
                        break;
        }
-       residue += at_xdmac_chan_read(atchan, AT_XDMAC_CUBC) << dwidth;
+       residue += cur_ubc << dwidth;
 
        dma_set_residue(txstate, residue);