[Design] PSU firmware update

[Lei YU]

> > 3 more quick notes:
> >
> > 1) PSs can be hot pluggable, so when a new one is detected, the code
> > update should run then too if the new PS needs one, assuming all other
> > conditions are met.
> >
> > 2) A single system may support multiple models of PS (will definitely
> > happen for us), so this design should be able to store multiple PS
> > images and send the correct image to the correct model.
> >
> > 3) You mentioned the combined image stuff before.  We should just check
> > the timeline for that support aligns with this one.
> >
> >
> Good point Matt on the PS install.  It would probably be a good idea to
> get the newly installed PS to the same image as the rest of the PS's in
> the system.

Yup, really good point.
This implies that BMC shall keep a local copy of the PSU image for future
updates.

> We do support PS's that don't provide control supply (standby voltage)
> when reset at the end of the update, while other PS's do.  Therefore for
> the former case, if only 1 PS has AC attached, we cannot update/reset
> that PS, so please let that be selectable by the user (eg vendor
> specific tool).

This is somehow complex, but if we could defer this to vendor specific tool,
that's OK.
However, if a system has multiple models of PS, I am not sure how the vendor
specific tool will be.
Should we defer that to vendor specfic tool, too?

>
> Also, please provide a way to know that the updates have finished.  As
> we don't want to update the PS's when the power is on (this is vendor
> specific as well), we also do not want to power the system on in the
> middle of an update.  For example, if after a BMC update the PS's are
> being updated, we want to hold off the next system power on until the PS
> updates have finished. Thanks.

This is already supported by the existing interface.


> >
> >
> >>
> >>> The reason I ask is because if we could get clear requirements, it
> >>> is possible
> >>> to simplify the design.
> >
> Would it be possible to support both methods?  The general use case
> being done during/after BMC code update, but also support the more
> manual method that could be used perhaps in the lab to test new psu
> images or in the field if there are problems with an existing image? Thanks.

This design doc will be updated to support both cases.