[PATCH 3/8] spi: davinci: limit the transfer size if DMA enabled

[Peter Ujfalusi]

On 03/22/2017 01:35 PM, Sekhar Nori wrote:
> On Wednesday 22 March 2017 04:41 PM, Frode Isaksen wrote:
> I guess this means that most IPs continue to get more missed sync events
> even after the first sync event is generated at end of MAX_NR_SG PaRAMs.
> And may be even that first sync event miss is a problem because the IP
> does not pause and ends up shifting data in the shift register anyway.
>
> If this is so broken, should edma driver not refuse to handle more than
> MAX_NR_SG for all transfers but mem-to-mem transfers?

DM355, OMAP-L138, etc have only 128 paRAM slots. If we lift the restriction on 
the number of paRAM slots a channel can take, then DMA will most likely going 
to stop working: If any peripherals request 128 long SG transfer (and MMC 
easily does that alone) then we will have no available paRAM slot for other 
clients.

It might be possible to start the paRAM slot update for the next batch already 
when we have 1 or 2 slots to be processed, but that is going to be tricky and 
not even reliable.

Note: audio also sets limit on the number of periods to avoid this 
reconfiguration of paRAM slots.

-- 
Péter