[U-Boot] cyclone5 reboot: warm or cold reset?

[Simon Goldschmidt]

Hi all,

I added some of those I think worth discussing this.

I was chasing a reboot problem on one of our cyclone5 boards today. That 
board boots from a "n25q256a" qspi flash. Up to now, U-Boot configures 
the boot ROM to just jump to OCRAM when rebooting and hope that the SPL 
is still there and fully working (nothing's overwritten).

For a long running production board, this is of course crap, as a 
pointer running wild could always overwrite the SRAM. But there was not 
much we could do about it as long as U-Boot and/or Linux were putting 
out 32 Mbyte chip into 4-byte mode: it's not accessible by the Boot ROM 
unless in 3-byte mode. And while Linux "reboot" could reset the chip 
into 3-byte mode, "reboot -f" and U-Boot "reset" can't.

Now with U-Boot and Linux having everything in place to leave the 
spi-nor in 3-byte mode (as required for the Boot ROM) using 4-byte 
opcodes, I thought rebooting from Linux would now work with loading the 
SPL from qspi (by disabling the magic that tells the Boot ROM to jump to 
OCRAM).

However, I found the Boot ROM still cannot load the SPL because it tries 
to load at 100 MHz (while on cold boot, it loads with 6.25 MHz).

However, when triggering cold reset instead of warm reset in rstmgr ctrl 
register [1], it works fine (and qspi clock is 6.25 MHz).

Now the question is: why do we trigger warm reset instead of cold reset? 
Isn't cold reset more stable, e.g. when rebooting watchdog-like? Linux 
also uses warm reset by default, but I'm tempted to send a patch for 
both U-Boot and Linux to use cold reset by default.

Any insight on the consequences of using cold reset instead of warm 
reset would be really appreciated!

Regards,
Simon

[1] 
https://www.intel.com/content/www/us/en/programmable/hps/cyclone-v/hps.html#topic/sfo1410067764332.html