Re: [edk2-devel] [PATCH 25/35] OvmfPkg/VideoDxe: document EFI_EDID_OVERRIDE_PROTOCOL.GetEdid() call

["Laszlo Ersek" <lersek@redhat.com>]

On 09/26/19 14:43, Laszlo Ersek wrote:
> On 09/23/19 17:59, Philippe Mathieu-Daudé wrote:
>> Hi Laszlo,
>>
>> On 9/17/19 9:49 PM, Laszlo Ersek wrote:
>>> According to the UEFI spec -- and to the edk2 header
>>> "MdePkg/Include/Protocol/EdidOverride.h" too --,
>>> EFI_EDID_OVERRIDE_PROTOCOL_GET_EDID takes an (EFI_HANDLE*), and not an
>>> EFI_HANDLE, as second parameter ("ChildHandle").
>>>
>>> This is probably [*] a bug in the UEFI spec. Given that this CSM module
>>> (VideoDxe) had been used for a long time on physical platforms before it
>>> was moved to OvmfPkg, keep the current "ChildHandle" argument, just cast
>>> it explicitly.
>>>
>>> [*] The edk2 tree contains no other GetEdid() call, and also no GetEdid()
>>>     implementation.
>>>
>>>     The edk2-platforms tree contains two GetEdid() calls, at commit
>>>     022c212167e0, in files
>>>     - "Drivers/DisplayLink/DisplayLinkPkg/DisplayLinkGop/Edid.c",
>>>     - "Drivers/OptionRomPkg/CirrusLogic5430Dxe/Edid.c".
>>>
>>>     From these, the first passes an (EFI_HANDLE*) as "ChildHandle", the
>>>     second passes an EFI_HANDLE. It's difficult to draw a conclusion. :/
>>>
>>> No functional changes.
>>>
>>> (I've also requested a non-normative (informative) clarification for the
>>> UEFI spec: <https://mantis.uefi.org/mantis/view.php?id=2018>, in the
>>> direction that matches this patch.)
>>
>> (EFI_HANDLE*) makes sense to me, but I'd rather wait for the spec
>> clarification before Acking this patch, I don't want we silent a bug
>> with this cast.
> 
> Right, there's been some movement in Mantis#2018.
> 
> It looks like the spec is wrong, but all [*] consumers and producers of
> GetEdid(), investigated thus far, have simply ignored the mistake in the
> spec, and done the right thing in practice.
> 
> So there seems to be a chance for the spec to be fixed. That would be
> followed by fixing the GetEdid() prototype in edk2. And then this patch
> would be dropped.
> 
> [*] the only exception found thus far is the call site in
> edk2-platform's "DisplayLinkPkg", mentioned above in the commit message.
> However, that is a very recent code addition (commit 9df63499ea01,
> 2019-09-09), and it might not reflect "historical" usage, but an attempt
> to write brand new code, conforming to the *letter* of the spec. So in
> case the spec gets fixed, the DisplayLinkPkg code could be fixed in
> tandem, perhaps.

See new thread started here:

http://mid.mail-archive.com/985de369-7880-b6cc-46e7-5a2edca6582b@redhat.com
https://edk2.groups.io/g/devel/message/48487

Thanks!
Laszlo

>>> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>>> Cc: David Woodhouse <dwmw2@infradead.org>
>>> Cc: Jordan Justen <jordan.l.justen@intel.com>
>>> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
>>> ---
>>>
>>> Notes:
>>>     build-tested only
>>>
>>>  OvmfPkg/Csm/BiosThunk/VideoDxe/BiosVideo.c | 6 +++++-
>>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/OvmfPkg/Csm/BiosThunk/VideoDxe/BiosVideo.c b/OvmfPkg/Csm/BiosThunk/VideoDxe/BiosVideo.c
>>> index 0640656dba14..995136adee27 100644
>>> --- a/OvmfPkg/Csm/BiosThunk/VideoDxe/BiosVideo.c
>>> +++ b/OvmfPkg/Csm/BiosThunk/VideoDxe/BiosVideo.c
>>> @@ -1402,9 +1402,13 @@ BiosVideoCheckForVbe (
>>>        goto Done;
>>>      }
>>>
>>> +    //
>>> +    // Cast "ChildHandle" to (EFI_HANDLE*) in order to work around the spec bug
>>> +    // in UEFI v2.8, reported as Mantis#2018.
>>> +    //
>>>      Status = EdidOverride->GetEdid (
>>>                               EdidOverride,
>>> -                             BiosVideoPrivate->Handle,
>>> +                             (EFI_HANDLE *) BiosVideoPrivate->Handle,
>>>                               &EdidAttributes,
>>>                               &EdidOverrideDataSize,
>>>                               (UINT8 **) &EdidOverrideDataBlock
>>>
>>
>>
>>
> 
> 
> 
>