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 <tglx@xxxxxxxxxxxxx>
> ---
> 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
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