Subject: [patch 5/6] edma: Make reading the position of active channels work

From: [email-masked] (Joel Fernandes)
Date: Thu, 17 Apr 2014 19:47:24 -0500

On 04/17/2014 09:40 AM, Thomas Gleixner wrote:
> Reading destination and source is pointless. In DEV_TO_MEM transfers
> we are only interested in the destination, in MEM_TO_DEV we care about
> the source. In MEM_TO_MEM it really does not matter which one you
> read.
> 
> Remove the extra pointer and select dest/source via a bool.
> 
> Reading the src/dst data from an active parameter set in the EDMA
> parameter RAM is not reliable, as there might be a concurrent update
> from the controller.
> 
> But experimentation showed, that a double readout with comparing the
> results and a limited loop works nicely. I've actually never found a
> case where the loop limit triggered, but we have it there for sanity
> reasons. In case it triggers we return -EBUSY and let the caller deal
> with it.
> 
> Remove the export of this function while at it. The only potential
> user is the dmaengine and that's always builtin.
> 
> Signed-off-by: Thomas Gleixner <[email-masked]>
> ---
>  arch/arm/common/edma.c             |   48 ++++++++++++++++++++++++++-----------
>  include/linux/platform_data/edma.h |    2 -
>  2 files changed, 35 insertions(+), 15 deletions(-)
> 
> Index: linux/arch/arm/common/edma.c
> ===================================================================
> --- linux.orig/arch/arm/common/edma.c
> +++ linux/arch/arm/common/edma.c
> @@ -994,29 +994,49 @@ void edma_set_dest(unsigned slot, dma_ad
>  EXPORT_SYMBOL(edma_set_dest);
>  
>  /**
> - * edma_get_position - returns the current transfer points
> + * edma_get_position - returns the current transfer point
>   * @slot: parameter RAM slot being examined
> - * @src: pointer to source port position
> - * @dst: pointer to destination port position
> + * @pos:  where to store the data
> + * @dst:  true selects the dest position, false the source
>   *
> - * Returns current source and destination addresses for a particular
> - * parameter RAM slot.  Its channel should not be active when this is called.
> + * Return 0 indicates a stable readout. -EBUSY indicates that the
> + * readout failed due to concurrent updates.
> + *
> + * Call this on active channels with care. For inactive channels this
> + * never fails.
>   */
> -void edma_get_position(unsigned slot, dma_addr_t *src, dma_addr_t *dst)
> +int edma_get_position(unsigned slot, dma_addr_t *pos, bool dst)
>  {
> -	struct edmacc_param temp;
> -	unsigned ctlr;
> +	u32 dat, ctlr, offs;
> +	int i;
>  
>  	ctlr = EDMA_CTLR(slot);
>  	slot = EDMA_CHAN_SLOT(slot);
>  
> -	edma_read_slot(EDMA_CTLR_CHAN(ctlr, slot), &temp);
> -	if (src != NULL)
> -		*src = temp.src;
> -	if (dst != NULL)
> -		*dst = temp.dst;
> +	if (slot >= edma_cc[ctlr]->num_slots)
> +		return -EINVAL;
> +
> +	offs = PARM_OFFSET(slot);
> +	offs += dst ? PARM_DST : PARM_SRC;
> +
> +	/*
> +	 * If the channel is active, we need to double read as we
> +	 * might see half updated data. We limit this to 5
> +	 * attempts. If that fails we return -EBUSY and let the caller
> +	 * deal with it.
> +	 */
> +	dat = edma_read(ctlr, offs);
> +	for (i = 0; i < 5; i++) {
> +		u32 tmp = edma_read(ctlr, offs);
> +
> +		if (tmp == dat) {
> +			*pos = dat;
> +			return 0;
> +		}
> +		dat = tmp;
> +	}
> +	return -EBUSY;
>  }
> -EXPORT_SYMBOL(edma_get_position);
>  
>  /**

The access is synchronized though and you shouldn't see unreliable
results, unless you _are_ seeing unreliable results in which case it
could be a silicon issue.

The original code also doesn't have the double read so I would just drop
that, unless you are seeing reliability issues.

Here's a thread that discusses it and the end conclusion is that it
should be fine (Kyle Kastile is one of the EDMA hardware designers).
http://e2e.ti.com/support/dsp/davinci_digital_media_processors/f/99/t/66011.aspx

thanks,
  -Joel