On 01/28/2016 12:29 PM, John Ogness wrote:
> When retrieving the residue value, the SRC/DST fields of the
> active PaRAM are read to determine the current position of
> the DMA engine. However, the AM335x Technical Reference Manual
> states:
>
> 11.3.3.6 Parameter Set Updates
>
> After the TR is read from the PaRAM (and is in the process
> of being submitted to the EDMA3TC), the following fields are
> updated as needed: ... SRC DST
>
> This means SRC/DST is incremented even though the DMA transfer
> may not have started yet or is in progress. Thus if the reader
> of the residue accesses the DMA buffer too quickly, the CPU is
> misinformed about the data that has been successfully processed.
>
> The CCSTAT.ACTV register is a boolean that is set if any TR is
> being processed by either the EMDA3CC or EDMA3TC. By polling
> this register it is possible to ensure that the residue value
> returned is valid for immediate processing. However, since the
> DMA engine may be active, polling may never hit a moment where
> no TR is being processed. To handle this, the SRC/DST is also
> polled to see if it changes. And as a last resort, a max loop
> count for the busy waiting exists to avoid an infinite loop.
>
> Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Acked-by: Peter Ujfalusi <peter.ujfalusi@xxxxxx>
> ---
> v1-v2 changes
> . rebased for next-20151016
> . added multiple exit conditions for busy wait loop
>
> v2-v3 changes
> . rebased for next-20160121
> . reduced max loops from 10000 to 1000
> . loop countdown instead of count
> . change debug print function and message
> . fine-tune comments
>
> v3-v4 changes
> . rebased for next-20160128
> . moved register bit definition
>
> drivers/dma/edma.c | 41 ++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 40 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/dma/edma.c b/drivers/dma/edma.c
> index d92d655..e3d7fcb 100644
> --- a/drivers/dma/edma.c
> +++ b/drivers/dma/edma.c
> @@ -113,6 +113,9 @@
> #define GET_NUM_REGN(x) ((x & 0x300000) >> 20) /* bits 20-21 */
> #define CHMAP_EXIST BIT(24)
>
> +/* CCSTAT register */
> +#define EDMA_CCSTAT_ACTV BIT(4)
> +
> /*
> * Max of 20 segments per channel to conserve PaRAM slots
> * Also note that MAX_NR_SG should be atleast the no.of periods
> @@ -1680,9 +1683,20 @@ static void edma_issue_pending(struct dma_chan *chan)
> spin_unlock_irqrestore(&echan->vchan.lock, flags);
> }
>
> +/*
> + * This limit exists to avoid a possible infinite loop when waiting for proof
> + * that a particular transfer is completed. This limit can be hit if there
> + * are large bursts to/from slow devices or the CPU is never able to catch
> + * the DMA hardware idle. On an AM335x transfering 48 bytes from the UART
> + * RX-FIFO, as many as 55 loops have been seen.
> + */
> +#define EDMA_MAX_TR_WAIT_LOOPS 1000
> +
> static u32 edma_residue(struct edma_desc *edesc)
> {
> bool dst = edesc->direction == DMA_DEV_TO_MEM;
> + int loop_count = EDMA_MAX_TR_WAIT_LOOPS;
> + struct edma_chan *echan = edesc->echan;
> struct edma_pset *pset = edesc->pset;
> dma_addr_t done, pos;
> int i;
> @@ -1691,7 +1705,32 @@ static u32 edma_residue(struct edma_desc *edesc)
> * We always read the dst/src position from the first RamPar
> * pset. That's the one which is active now.
> */
> - pos = edma_get_position(edesc->echan->ecc, edesc->echan->slot[0], dst);
> + pos = edma_get_position(echan->ecc, echan->slot[0], dst);
> +
> + /*
> + * "pos" may represent a transfer request that is still being
> + * processed by the EDMACC or EDMATC. We will busy wait until
> + * any one of the situations occurs:
> + * 1. the DMA hardware is idle
> + * 2. a new transfer request is setup
> + * 3. we hit the loop limit
> + */
> + while (edma_read(echan->ecc, EDMA_CCSTAT) & EDMA_CCSTAT_ACTV) {
> + /* check if a new transfer request is setup */
> + if (edma_get_position(echan->ecc,
> + echan->slot[0], dst) != pos) {
> + break;
> + }
> +
> + if (!--loop_count) {
> + dev_dbg_ratelimited(echan->vchan.chan.device->dev,
> + "%s: timeout waiting for PaRAM update\n",
> + __func__);
> + break;
> + }
> +
> + cpu_relax();
> + }
>
> /*
> * Cyclic is simple. Just subtract pset[0].addr from pos.
>
--
Péter
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