Re: [PATCH staging][BaseToolsOpt 0/4] Enable multiple driver combination

[Laszlo Ersek <lersek@redhat.com>]

Hi Liming,

(CC Jordan)

On 06/08/17 08:55, Liming Gao wrote:
> Combine more drivers into the single one can reduce the image size and 
> compile link time. This patch adds this support in BaseTools.
> 
> Liming Gao (4):
>   BaseTools: Merge multiple drivers into one for size and link
>     performance
>   OvmfPkg: Update QemuVideo and VirtioGpuDxe to use NULL as
>     DriverBindingHandle
>   OvmfPkg: Combine QemuVideoDxe and VirtioGpuDxe to one driver
>   Update Readme.MD to include multiple driver combination.
> 
>  BaseTools/Source/Python/AutoGen/GenC.py | 24 +++++++++++++++++++-----
>  OvmfPkg/OvmfPkgIa32X64.dsc              |  6 ++++--
>  OvmfPkg/OvmfPkgIa32X64.fdf              |  2 +-
>  OvmfPkg/QemuVideoDxe/Driver.c           |  2 +-
>  OvmfPkg/VirtioGpuDxe/DriverBinding.c    |  2 +-
>  Readme.MD                               |  1 +
>  6 files changed, 27 insertions(+), 10 deletions(-)
> 

I don't have a lot of hands-on practice with staging branches, but I
believe that ultimately such changes are meant to be merged into the
edk2 master branch. Is that right?

If that's the case, then I don't think we should do this in OvmfPkg (on
the master branch). Instead, I think example usage should be shown in:

- the commit message (it's already there, so that's great),

- in the DSC specification.

If you absolutely need an in-tree platform to use this BaseTools
feature, and you think OvmfPkg is better for this purpose than, say,
EmulatorPkg or Nt32Pkg, then:

(1) Please make this dependent on a new build flag (-D), which should
default to FALSE.

(2) Please make the same change to all three DSC files under OvmfPkg.

(3) Please introduce a new FeaturePCD which corresponds to the new build
flag, and controls the handle value in the entry points of the affected
drivers.

In particular I'm asking for (3) because the UEFI Driver Writer's Guide
(Version 1.01, 03/08/2012) says:

  6.1.4 Device drivers with one driver binding protocol

  [...] The driver entry point is responsible for installing the Driver
  Binding Protocol onto the driver’s image handle. [...]

  6.1.5 Device drivers with multiple driver binding protocols

  [...] The first instance of EFI_DRIVER_BINDING_PROTOCOL is installed
  onto the driver’s image handle, and the additional instances of the
  Driver Binding Protocol are installed onto newly created driver
  binding handles. [...]

So, in order to follow these recommendations,
- when the drivers are not combined, each driver should stick with its
non-NULL image handle,
- when the drivers are combined, one driver should stick with its image
handle, and the rest should use NULL (based on the feature PCD)


Regarding the BaseTools feature itself, I think it should be restricted
to UEFI_DRIVER modules (maybe it is already restricted, but then the
documentation should say it). I'm suggesting that becasue UEFI_DRIVERs
are supposed to have identical DEPEXes. With DXE_DRIVER modules for
example, their DEPEXes could be different, and I couldn't say how those
should be combined.

Thanks,
Laszlo