Re: [edk2-platforms][PATCH v2 2/3] Platform/RPi: Add PlatformPcdLib to set the Genet MAC address

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

On Fri, 31 Jan 2020 at 18:48, Pete Batard <pete@akeo.ie> wrote:
>
> Hi Ard,
>
> On 2020.01.31 17:05, Ard Biesheuvel wrote:
> > On Fri, 24 Jan 2020 at 12:54, Pete Batard <pete@akeo.ie> wrote:
> >>
> >> The Genet driver stub used by the Raspberry Pi 4 platform is
> >> designed to set the MAC address according to a PCD.
> >>
> >> To be able to set that PCD at runtime, by using the Raspberry
> >> Pi firmware interface, that has a dedicated call to retrieve
> >> the MAC address, and satisfy driver dependencies in a generic
> >> manner, we create a new PlatformPcdLib that can be referenced
> >> by the Genet driver, to set the MAC PCD before use there.
> >>
> >> While it is currently only tailored around MAC PCD population
> >> for Genet, we do expect this PCD library to be extended in the
> >> future, to provide additional PCD facilities for other drivers.
> >>
> >> Signed-off-by: Pete Batard <pete@akeo.ie>
> >> ---
> >>   Platform/RaspberryPi/Library/PlatformPcdLib/PlatformPcdLib.c   | 51 ++++++++++++++++++++
> >>   Platform/RaspberryPi/Library/PlatformPcdLib/PlatformPcdLib.inf | 43 +++++++++++++++++
> >>   2 files changed, 94 insertions(+)
> >>
> >> diff --git a/Platform/RaspberryPi/Library/PlatformPcdLib/PlatformPcdLib.c b/Platform/RaspberryPi/Library/PlatformPcdLib/PlatformPcdLib.c
> >> new file mode 100644
> >> index 000000000000..792073a903e9
> >> --- /dev/null
> >> +++ b/Platform/RaspberryPi/Library/PlatformPcdLib/PlatformPcdLib.c
> >> @@ -0,0 +1,51 @@
> >> +/** @file
> >> + *
> >> + *  Copyright (c) 2020, Pete Batard <pete@akeo.ie>
> >> + *
> >> + *  SPDX-License-Identifier: BSD-2-Clause-Patent
> >> + *
> >> + **/
> >> +
> >> +#include <Library/DebugLib.h>
> >> +#include <Library/PcdLib.h>
> >> +#include <Library/PrintLib.h>
> >> +#include <Library/UefiBootServicesTableLib.h>
> >> +#include <Library/UefiLib.h>
> >> +#include <Library/UefiRuntimeServicesTableLib.h>
> >> +#include <Protocol/RpiFirmware.h>
> >> +
> >> +EFI_STATUS
> >> +EFIAPI
> >> +PlatformPcdLibConstructor (
> >> +  IN EFI_HANDLE ImageHandle,
> >> +  IN EFI_SYSTEM_TABLE *SystemTable
> >> +  )
> >> +{
> >> +
> >> +  //
> >> +  // If Genet is in use and a MAC address PCD has not been set, do it here.
> >> +  //
> >> +#if (FixedPcdGet64 (PcdBcmGenetRegistersAddress) != 0)
> >
> > I still don't see why we need this conditional. This is a fixed PCD,
> > so it must be set in the .DSC. If you are going to set it in the .DSC,
> > why include this driver in the first place?
>
> Let's consider this:
>
> 1. We expand on PlatformPcdLib to do more than set the MAC Address PCD
> for the Pi 4. For instance, we use it to set some other PCD, that is
> specific to the Pi 3, or for an upcoming Pi. In this usage, we would be
> using PlatformPcdLib with a .DSC where PcdBcmGenetRegistersAddress is
> not set and therefore defaults to 0.
>

I don't think a single PlatformPcdLib is realistic, since you'd need
to link it into different drivers via NULL resolution, all of which
would inherited the conjunction of all the DEPEX clauses, potentially
causing circular dependencies.

I'd much rather have either a named library class in BcmGenet.dec
scope that the platform can implement to provide the MAC, or one that
can be used via NULL resolution (in cases where it is imaginable that
the driver does not need the MAC to be provided at all)

> 2. Since the whole PcdBcmGenetMacAddress setup section would not apply
> then, we prefer ensuring that it cannot interfere or create issues, by
> dropping it altogether at build time. Granted, the
>    if (PcdGet64 (PcdBcmGenetMacAddress) == 0)
> condition could also be seen as enough of a guard, but I don't see much
> of a reason not to go further, so that further alteration of the code
> does not risk impacting the Pi 3 or other Pi platforms with future
> developments.
>
> Does that make sense to you? Or do you see this as too far fetched a
> scenario?
>

No, I don't think this is something we should care about.

> I'm basically trying to make this whole library easier to maintain in
> the long run, by adding some pre-emptive provisions to drop code that
> may not apply (even if, in the current scenario, this library is only
> used for the Pi 4 platform, where PcdBcmGenetRegistersAddress is always
> set).
>

I don't see a single library that sets all kinds of unrelated PCDs as
something highly useful tbh.