[ipxe-devel] [PATCH 1/1] efi: provide possibility to disable cpu_nap

[Heinrich Schuchardt]

On 07/13/2017 01:35 AM, Michael Brown wrote:
> On 12/07/17 19:50, Heinrich Schuchardt wrote:
>> On 07/12/2017 08:35 PM, Michael Brown wrote:
>>> On 12/07/17 19:15, Heinrich Schuchardt wrote:
>>>> Nope. Interrupts are not supported by U-Boot. To get them running every
>>>> architecture implementation would have to be changed.
>>>>
>>>> U-Boot simply uses a call inside the waiting loop to check if new data
>>>> is available.
>>>>
>>>> E.g. in UEFI WaitForEvent it will check if new data becomes
>>>> available on
>>>> the serial console or on the network interface inside a loop. In
>>>> CheckEvent it will do the same. If data becomes available it will fire
>>>> the UEFI event.
>>>
>>> OK.  What facility does U-Boot provide to allow the CPU to sleep (i.e.
>>> to allow an idling program to avoid running with 100% CPU usage)?
>>
>> None. U-Boot loads grub or the OS kernel within a few seconds. So why
>> bother about reducing CPU load of the only core that is running?
>>
>> E.g. on the Davinci boards the waiting routine udelay is implemented
>> using
>>
>> while (get_ticks() < endtime)
>>    ;
> 
> iPXE similarly doesn't bother about reducing CPU load while downloading
> the OS since it will generally be finished in under a second, but we do
> sleep the CPU when we have nothing to do for long periods of time (e.g.
> when waiting for user input, or in the middle of an explicit "sleep"
> command).
> 
> You could add code in efiarm_nap.c and/or efix86_nap.c to detect whether
> or not it is safe to sleep the CPU.  The attached (completely untested)
> patch demonstrates this idea: modify the cpu_nap() implementation in
> efix86_nap.c to execute "hlt" only if the platform firmware has enabled
> interrupts.
> 
> Michael

Why do we need cpu_nap in the keyboard loop at all when we are in an
UEFI context. Shouldn't UEFI WaitForEvent be used to care about the idle
CPU?

Best regards

Heinrich