efi: Set Variable Runtime implementation

[Shantur Rathore]

Hi Peter,

On Sat, Nov 25, 2023 at 6:19 AM Peter Robinson <pbrobinson at gmail.com> wrote:
>
> Hi Shantur,
>
> On Fri, Nov 24, 2023 at 11:55 PM Shantur Rathore <i at shantur.com> wrote:
> >
> > Hi Ilias,
> >
> > On Fri, Nov 24, 2023 at 10:50 PM Ilias Apalodimas
> > <ilias.apalodimas at linaro.org> wrote:
> > >
> > > Hi Shantur
> > >
> > > On Fri, 24 Nov 2023 at 18:51, Shantur Rathore <i at shantur.com> wrote:
> > > >
> > > > Hi Heinrich,
> > > >
> > > > I am trying to work out how to enable the SetVariableRT service in
> > > > U-Boot and came across your patch [1] which initially had the
> > > > SetVariable RT service enabled in EFI but in the final patch this was
> > > > removed.
> > > > I am hoping to implement it on top of the SPI Flash EFI store [2] to
> > > > be able to set Boot order and boot items from Linux the UEFI way.
> > > >
> > > > Can I pick your brain on why it was dropped in the patch?
> > > > Is there any limitation in SetVariableRT service ?
> > >
> > > I recently had a talk about it in Plumbers [0]. Generally speaking, RT
> > > + hardware owned by the kernel is a very weird combination since you
> > > can't guarantee exclusive access to the flash or the bus and you have
> > > to preserve a *lot* of code alive in u-boot.
> > >
> > > I'll respond to your v1 patchset and we can discuss details there as well.
> > >
> > > [0] https://lpc.events/event/17/contributions/1653/
> >
> > Thanks for the background and helping me understand the problem.
> > Makes me wonder how things work in the PC world.
> > U-boot being only ~1MB, can we not leave it all in memory and maybe
> > just disable SPI access to Linux.
>
> That's basically it, on x86 there's specific HW that's owned by
> firmware, I don't know the exact low level details of how that works.
>
> I think x86 devices generally use eSPI for this HW [1] but I don't
> know the low level details. The Arm SBSA (Server HW spec) and SBBR
> (Server Base Boot Requirements) specs that are key to ServerReady may
> go into some details too if you're curious.

Thanks,
I think the firmware is still accessible to PCs as one could update the firmware
in Windows so Windows has access to that device.

I had some try myself and found that setting a variable to memory backed storage
is doable with SetVariable call but we want to store it in any
non-volatile storage
things really don't look good.

To be able to write SetVariable to any device, the whole u-boot driver
model would need
to be kept in memory, might as well just keep the whole u-boot in
memory at this point, it's anyway small.
I don't have much knowledge on how to or pros and cons of doing this.

>
> BTW I plan to test your other patches on the Pinebook Pro.
>
> [1] https://www.totalphase.com/blog/2021/09/what-is-the-espi-protocol-and-how-does-it-improve-upon-lpc/

Thanks, that would be helpful.
I am particularly keen on the usb patch here
https://patchwork.ozlabs.org/project/uboot/patch/20231110141311.512334-1-i@shantur.com/

Kind regards,
Shantur