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dmaengine: at_xdmac: fix residue corruption

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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: e1f7c9eee7 ("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>
This commit is contained in:
Ludovic Desroches 2016-05-12 16:54:09 +02:00 committed by Vinod Koul
parent 4a9723e8df
commit 53398f4888
1 changed files with 32 additions and 22 deletions
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54
drivers/dma/at_xdmac.c
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@ -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)) {