Re: [PATCH] EmbeddedPkg/DtPlatformDxe: remove /chosen/stdout-path on GOP registration

[Laszlo Ersek <lersek@redhat.com>]

On 10/20/17 13:40, Ard Biesheuvel wrote:

> I think the best way forward is to implement a separate driver with
> its own HII form, that removes /chosen/stdout-path and uninstalls the
> SPCR table if either or both are present. I couldn't figure out how to
> uninstall a ACPI table though, so I may need to move the SPCR
> installation into that driver as well.

In general it's best to consider an ACPI table "owned" by the driver
that installs it in the first place. The

  EFI_ACPI_TABLE_PROTOCOL.InstallAcpiTable()

function (from the UEFI spec) outputs a TableKey parameter, and only
that parameter can be used as input to the

  EFI_ACPI_TABLE_PROTOCOL.UninstallAcpiTable()

function (also from the UEFI spec).


The PI spec defines

  EFI_ACPI_SDT_PROTOCOL

which has a member function called

  EFI_ACPI_SDT_PROTOCOL.GetAcpiTable()

Using this function, you can iterate over all the tables, investigate
each, and even get the TableKey for uninstallation. (See the function's
specification itself.)


So, if you are willing to "go PI", then independent uninstallation is
possible. But, IMO, the unclear ownership of any table can lead to a
mess down the road.

In edk2, EFI_ACPI_SDT_PROTOCOL is available from

  MdeModulePkg/Universal/Acpi/AcpiTableDxe

if

  gEfiMdeModulePkgTokenSpaceGuid.PcdInstallAcpiSdtProtocol

is TRUE.

For one example, OVMF set the PCD in commit 259d87146b07 ("OvmfPkg:
Modify FDF/DSC files for RamDiskDxe's adding NFIT report feature",
2016-04-19).

Thanks!
Laszlo