Firmware builds reprogram the SDRAM controller twice. system_init()
sets burst 8, T_RP = T_RCD = 1 and the refresh, with CAS 3 since
a6538abd16 - CAS 2 on HM60X/HM801. And set_sdram_timing(), on every CPU
frequency change, rewrites the mode: CAS 2 whenever the AHB runs at
100 MHz or below, which is every clock this code sets.
On a generic rk2705 the firmware hangs in system_init() on those writes;
with them skipped it boots. Doing the same writes from IRAM, so that
nothing touches the SDRAM while the controller reprograms the chip,
hangs the same way: it is the settings themselves. The board's SDRAM
is an Elpida EDS1216AATA-75 (16 MB), a 133 MHz part at CAS 3 whose
minimum clock period at CAS 2 is 10 ns - exactly the 100 MHz it runs
at here, with no margin, next to minimal T_RP/T_RCD.
On a Samsung YP-CP3 the firmware boots but corrupts memory at random
once the clock first changes: data aborts on valid addresses in
unrelated code, undefined instruction exceptions on valid instructions,
glitches in the boot logo, crashes on USB plug and unplug. A memory
test over 15 MB passes with the boot's setup (CAS 3, burst 1,
T_RP = T_RCD = 2) and with system_init()'s values alike - it never
changes the clock - and skipping only the system_init() writes is not
enough, as set_sdram_timing() still selects CAS 2. With both removed
the YP-CP3 runs, passes a USB mass storage stress test and survives
USB unplug.
The boot ROM's and the bootloaders' setup works on every rk27xx target
seen, and nothing before Rockbox changes it (the NAND bootloader's
stage 1 and rk27load's s1 only probe the organisation). The gain
claimed for the tweak was a slight improvement in memory throughput.
So remove it for every rk27xx target, the HM60X/HM801 CAS 2 included,
and have set_sdram_timing() adjust only the refresh to the bus clock.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Change-Id: Icd063367783b0ae6eb80bad67d908269c07e704f
17db971b66