Re: [Xenomai-help] I-pipe clock source change and MMC issues (AT91)

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

at91_enthus wrote:
> Hi.
> 
> In order to get better accuracy with Xenomai, I modified the I-pipe
> timebase (MCK divider) in arch/arm.mach-at91/at91_ipipe_time.c.
> Unfortunately, I cannot mount the rootfs on the MMC, since the MMC
> controller is no longer functioning. I tried to change TCx in kernel
> configuration to no avail.
> When I switch back to a timebase of 1 MHz, the MMC works fine.

The thing is that we are a bit tight on AT91. A 16 bits counter is used
for both the timer and the tsc emulation, and this tsc must be refreshed
at least once before it wraps. The problem is that since it is a 16 bits
counter, it wraps really fast, on my AT91SAM9263 it wraps in 20 ms, and
since the Linux default period is 10ms we are actually quite close to
the limit.

Anyway, trying to get a better accuracy than 1us is kind of useless on
AT91, since even reading this counter takes more than 1us. So, you are
not in fact improving anything. The 1MHz is even a bit overkill.

-- 
                                                                Gilles.