Re: visibility pf PcdSet

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 20 March 2017 at 16:06, Michael Zimmermann <sigmaepsilon92@gmail.com> wrote:
> Do I understand correctly, that a PcdSet on a 'PcdsPatchableInModule'
> is only visible to the current module?(SEC, driver, application, ...)

Yes.

> Because I've tested this and a PcdSet on
> gArmTokenSpaceGuid.PcdSystemMemoryBase inisde PrePi is not visible
> inside a DXE_DRIVER - which means that for everyone else the value is
> still 0x0.
>

Indeed. The relocatable PrePi needs a patchable PCD because it assigns
the value really early, in assembly code. But PrePi is a bit peculiar
as well, and I am pretty use SetPcd() on dynamic PCDs would not work
there either.

> If this is the case and I don't have some platform bug, then there is
> probably a bug in ArmVirtPkg's HighMemDxe where this Pcd is used in a
> DXE_DRIVER:
> https://github.com/tianocore/edk2/blob/76874be3d411bf8daac051718e20932e0bf97d70/ArmVirtPkg/HighMemDxe/HighMemDxe.c#L70
> It doesn't cause anything bad but it would show 'Failed to add System
> RAM @ START - END (Access Denied)' after calling AddMemorySpace for
> memory which has already been registered.
>

Ah yes, well spotted. To be honest, PrePi is a bit of a hack, and I
actually recommend against it for new ports. However, for a self
relocating image (which I think you need for your application), PrePi
is really the only way to go.

So the best way to pass information from PrePi to DXE is using HOBs.
Actually, it might be best for you to clone HighmemDxe if you need it,
and use a HOB instead.