[U-Boot] [PATCH 11/21] i386: Generic system restart support

[Graeme Russ]

On Tue, Mar 8, 2011 at 9:57 AM, Moffett, Kyle D
<Kyle.D.Moffett at boeing.com> wrote:
> On Mar 07, 2011, at 17:26, Graeme Russ wrote:
>> On Tue, Mar 8, 2011 at 9:06 AM, Moffett, Kyle D <Kyle.D.Moffett at boeing.com> wrote:
>>> On Mar 07, 2011, at 16:54, Graeme Russ wrote:
>>>> This part does not make much sense - If the CPU is in 'a bad state' then
>>>> it will probably be lights out anyway. As I understand it, an emergency
>>>> restart is a restart not initiated by the user (divide by zero, unhandled
>>>> exception etc), in which case i386 will make use of it
>>>
>>> I was considering unhandled exceptions, etc. to be "a bad state" :-D.
>>>
>>> Maybe I didn't explain it well enough in the patch summary, but basically the default for "__arch_emergency_restart()" is to just call "__arch_restart()".  Since the i386 "__arch_restart()" function should work fine even when U-Boot is in trouble, that architecture does not need to override the default.
>>>
>>> Hopefully I am making sense now?  Should I reword it from "when the CPU is in a bad state" to "when U-Boot is in trouble", or is there something else that would be easier to understand?
>>
>> I understand what you are doing now, thanks.
>>
>> I think you can scrap this part of the description and I will have i386
>> start using __arch_emergency_restart() for 'Internal U-Boot Errors' such
>> as divide by zero, unhandled exception, general protection faults etc
>>
>> I don't particularly like the 'emergency' naming - It's like if we don't
>> do it things will blow up :) I think 'automatic' might be a closer term
>
> The name "emergency_restart()" was borrowed from the Linux kernel; the kernel shuts down disks and network interfaces prior to a regular restart but not during an emergency restart.  The best analogy is to the "EMERG" log level (KERN_EMERG inside the Linux kernel).

OK, makes sense to stick with emergency then

>
> Furthermore, you should *not* directly call __arch_emergency_restart(), that is simply the architecture hook you provide to the generic code.  Your exception handlers should call "emergency_restart()" to allow board-specific code to override the reboot, for example by triggering an onboard watchdog to reset other hardware.
>
> EG:
>
> => some_i386_trap_handler()
>   => emergency_restart()
>       => __board_emergency_restart()
>       => __arch_emergency_restart()
>
> => do_reset()  [The new, generic version]
>   => system_restart()
>      => __board_restart()
>      => __arch_restart()
>

OK

> The __{board,arch}_restart(), etc are just predefined weak functions that can be overridden by the implementation.
>
> For example, my HWW-1U-1A board file effectively does this:
>
> int __board_restart(void)
> {
>        while (poll_external_software()) {
>                if (ctrlc())
>                        return -1;
>        }
>        return 0;
> }
>
> void __board_emergency_restart(void)
> {
>        while (poll_external_software_uninterruptible())
>                ;
> }
>
> During a normal restart, I allow the polling to be interrupted with Ctrl-C, but during an emergency restart I don't.  In both cases, my function just *returns* to allow the default MPC85xx code to actually perform the hardware-level restart (by poking at the "reset request" bit in a CPU register).
>
>
>> Is there anywhere yet where the code paths for the emergency and non
>> emergency variants differ in the way they end up resetting the board?
>
> There are several U-Boot board ports whose do_reset() functions (now called "__board_restart()") just perform an unconditional jump to "_start" or a FLASH "soft-reset" address.  If the system has experienced an unexpected exception or other problem then that is not safe, and it would be better to just hang().
>

OK - All sounds good to me. I'll try run it all up on my board 'soon'

Regards,

Graeme