ar71xx SPI speed

[Patrick Kelsey]

Hi,

I've been doing some SPI-related work on an AR7161-based board (MikroTik
Routerboard RB450G, CPU @ 680 MHz, DDR @ 340 MHz, AHB @ 170 MHz), and I've
noticed, via both software cyclecount and logic analyzer traces, that the
SPI bus clock tops out in the neighborhood of 7 MHz or so.  I can get a
little more performance out of it if I manually unroll/debranch the loop in
ar71xx_spi_txrx, but not terribly much.  The (closed source) boot loader
for this board manages something in the neighborhood of 8.5 MHz (perhaps
due to not going through a bus abstraction layer in its code).  8MHz-ish
does seem a bit lethargic for an SoC with otherwise fast moving parts, but
I don't have any technical documentation on which to base a meaningful
expectation.

I'm wondering if anyone has had any experience with this SPI controller
that either corroborates or contradicts this apparent speed limit.

Thanks,
Patrick