[PATCH v2] ARM: edma: fix residue race for cyclic

[Peter Ujfalusi]

On 10/16/2015 01:26 PM, John Ogness wrote:
> When retrieving the residue value for cyclic transfers, the
> SRC/DST fields of the active PaRAM are read. 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.

I'm not sure what this actually going to solve, except that we are going to
wait for the next PaRAM parameter update.
As you have already described the issue is that when you first submit the
transfer, the first PaRAM will be submitted to TC and at this point the
parameters will be updated to be prepared for the next update.
This means that after we initiate the DMA the SRC/DST will be updated to point
to the next batch of data. Reading SRC/DST before the second parameter update
will always give you this. But this is also true further in the line.
We never know exactly where the DMA is, we only know that the DMA is somewhere
in between 0x1234 - (0x1234 + data until next parameter update). At the next
parameter update you again can not be sure where the DMA is, since it is now
somewhere between (0x1234 + parameter update number of data) - till the next
update, and so on.
In the cyclic case we use AB-sync mode, in this case the parameter update is
going to happen after each period if I'm not mistaken.

To achieve the same thing (waiting for the next parameter update to happen)
you could just poll the PaRAM's CCNT in AB-sync to see if it changing and when
it did you return the SRC/DST address since those will be close enough at the
time.

But what happens if the period size if big and the position is asked just
right after the parameter update? We can not know this, so we spin a bit here
and give up and return whatever we had in SRC/DST.

Not sure how to deal with this, but for sure needs more thought...

-- 
Péter

> Signed-off-by: John Ogness <john.ogness at linutronix.de>
> ---
>  v1-v2 changes
>  . rebased for next-20151016
>  . added multiple exit conditions for busy wait loop
> 
>  drivers/dma/edma.c |   40 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
> diff --git a/drivers/dma/edma.c b/drivers/dma/edma.c
> index 7eefbf1..8d3b3ac 100644
> --- a/drivers/dma/edma.c
> +++ b/drivers/dma/edma.c
> @@ -24,6 +24,8 @@
>  #include <linux/platform_device.h>
>  #include <linux/slab.h>
>  #include <linux/spinlock.h>
> +#include <linux/ratelimit.h>
> +#include <linux/printk.h>
>  #include <linux/of.h>
>  #include <linux/of_dma.h>
>  #include <linux/of_irq.h>
> @@ -1785,11 +1787,24 @@ static void edma_issue_pending(struct dma_chan *chan)
>  	spin_unlock_irqrestore(&echan->vchan.lock, flags);
>  }
>  
> +#define EDMA_CCSTAT_ACTV (1 << 4)
> +
> +/*
> + * This limit exists to avoid a possible infinite loop when waiting
> + * for confirmation that a particular transfer is completed. However,
> + * large bursts to/from slow devices might actually require many
> + * loops (in which case busy waiting is bad anyway). 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 10000
> +
>  static u32 edma_residue(struct edma_desc *edesc)
>  {
>  	bool dst = edesc->direction == DMA_DEV_TO_MEM;
>  	struct edma_pset *pset = edesc->pset;
>  	dma_addr_t done, pos;
> +	int loop_count = 0;
>  	int i;
>  
>  	/*
> @@ -1799,6 +1814,31 @@ static u32 edma_residue(struct edma_desc *edesc)
>  	pos = edma_get_position(edesc->echan->ecc, edesc->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
> +	 * one of the situations occurs:
> +	 *   1. no transfer requests are active
> +	 *   2. a different transfer request is being processed
> +	 *   3. we hit the loop limit
> +	 */
> +	while (edma_read(edesc->echan->ecc, EDMA_CCSTAT) & EDMA_CCSTAT_ACTV) {
> +		/* check if a different transfer request is active */
> +		if (edma_get_position(edesc->echan->ecc,
> +				      edesc->echan->slot[0], dst) != pos) {
> +			break;
> +		}
> +
> +		loop_count++;
> +		if (loop_count == EDMA_MAX_TR_WAIT_LOOPS) {
> +			pr_debug_ratelimited("%s: possibly returning "
> +					     "invalid residue\n", __func__);
> +			break;
> +		}
> +
> +		cpu_relax();
> +	}
> +
> +	/*
>  	 * Cyclic is simple. Just subtract pset[0].addr from pos.
>  	 *
>  	 * We never update edesc->residue in the cyclic case, so we
>