I spent the evening polishing a root filesystem for the ts7250. The flash is quite fat (128Mb) so I gave up on some of the
buggy busybox applets (udhcpc in particular) in
favour of their real counterparts from the Debian distro. This
approach put dropbear back onto its branch, and as the board can
reliably connect to the WiFi router I can now SSH into it almost
always after boot. It displays the time now, and synchronises with ntp, and the built-in real-time clock works too, albeit with some
hefty drift.
Unfortunately the system still does not schedule my program very satisfactorily: any perturbation in CPU load results in flicker, and it struggles to play an mp3 without skipping. This is with a low-latency Linux kernel (2.6.32.3). I get the impression that later kernels in that series are easy to get going on the ts7250, so I might try one, but apparently there will not be a full RT patch for 2.6.32. Bleeding edge it may have to be.
Part of the reason is that my program naively uses the kernel
scheduler for all delays, not just the larger ones. Thus when there is
contention for the CPU the system overhead spikes, taking roughly as
much time as user code. The sirq (presumably clock
interrupt) load is circa 10%. I can feel some busy waiting coming on.