On 04/17/2014 07:47 PM, Joel Fernandes wrote:
> 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
I'm sorry to have messed up Kyle's name, its Kyle Castille ;-)
thanks,
-Joel
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