Re: [PATCH for-6.2] hw/arm/virt_acpi_build: Generate DBG2 table

[Eric Auger]

Hi Samer, Ard,

On 8/10/21 3:54 PM, Ard Biesheuvel wrote:
> On Tue, 10 Aug 2021 at 15:11, Samer El-Haj-Mahmoud
> <Samer.El-Haj-Mahmoud@arm.com> wrote:
>>
>>
>>> -----Original Message-----
>>> From: Eric Auger <eric.auger@redhat.com>
>>> Sent: Tuesday, August 10, 2021 6:25 AM
>>> To: Ard Biesheuvel <ardb@kernel.org>
>>> Cc: eric.auger.pro@gmail.com; Michael S. Tsirkin <mst@redhat.com>; Igor
>>> Mammedov <imammedo@redhat.com>; Philippe Mathieu-Daudé
>>> <philmd@redhat.com>; Peter Maydell <peter.maydell@linaro.org>; Shannon
>>> Zhao <shannon.zhaosl@gmail.com>; qemu-arm <qemu-arm@nongnu.org>;
>>> qemu-devel@nongnu.org; Andrew Jones <drjones@redhat.com>;
>>> gshan@redhat.com; Samer El-Haj-Mahmoud <Samer.El-Haj-
>>> Mahmoud@arm.com>; Al Stone <ahs3@redhat.com>; jcm@redhat.com
>>> Subject: Re: [PATCH for-6.2] hw/arm/virt_acpi_build: Generate DBG2 table
>>>
>>> Hello Ard,
>>> On 8/10/21 11:36 AM, Ard Biesheuvel wrote:
>>>> On Tue, 10 Aug 2021 at 10:31, Eric Auger <eric.auger@redhat.com> wrote:
>>>>> ARM SBBR specification mandates DBG2 table (Debug Port Table 2).
>>>>> this latter allows to describe one or more debug ports.
>>>>>
>>>>> Generate an DBG2 table featuring a single debug port, the PL011.
>>>>>
>>>>> The DBG2 specification can be found at:
>>>>> https://docs.microsoft.com/en-us/windows-hardware/drivers/bringup/acpi-
>>> debug-port-table?redirectedfrom=MSDN
>>>> Have the legal issues around this table been resolved in the mean
>>>> time?
>>> I don't know exactly what they are. Adding Al and Jon in the loop they
>>> have more information about this.
>>> How did you resolve the issue for EDK2
>>> (DynamicTablesPkg/Library/Acpi/Arm/AcpiDbg2LibArm/Dbg2Generator.c)?
>>>>  Also, any clue why this table is mandatory to begin with? The
>>>> SBBR has been very trigger happy lately with making things mandatory
>>>> that aren't truly required from a functional perspective.
>>> It seems there are kernel FW test suites that check all mandated tables
>>> are available and they currently fail for ARM virt.
>>> Indeed from a function pov, I don't know much about its usage on ARM.
>>>
>>> Maybe the SBBR spec should not flag the DBG2 as mandatory and test
>>> suites shall be updated. I think this should be clarified at ARM then,
>>> all the more so if there are legal issues as its spec is owned by Microsoft?
>>>
>> DBG2 has been required in SBBR since SBBR ver 1.0 (published 2016, with the 
>> 0.9 draft since 2014)
>> https://developer.arm.com/documentation/den0044/b/?lang=en
>>
>> SBBR requires DBG2 because Windows requires it on all systems: 
>> https://docs.microsoft.com/en-us/windows-hardware/drivers/bringup/acpi-system-description-tables#debug-port-table-2-dbg2
>>  , and Windows is one of the key OSes targeted by SBBR.
>>
>> The DBG2 (and SPCR) spec license issue has been resolved since August 2015. 
>> Microsoft updated both specs with identical license language, giving patent 
>> rights for implementations under the Microsoft Community Promise, and the 
>> Open OWF 1.0. This Foundation.
OK thank you for confirming all the previously known DBG2 legal issues
were resolved.
>>
>> DBG2: 
>> https://docs.microsoft.com/en-us/windows-hardware/drivers/bringup/acpi-debug-port-table
>> SPCR: 
>> https://docs.microsoft.com/en-us/windows-hardware/drivers/serports/serial-port-console-redirection-table
>>
> Thanks Samer, for stating this on record here - and apologies for
> suggesting that this was another frivolous addition to a recent SBBR
> revision.
>
> As for the difference between the two: SPCR describes the serial
> console, which is an actual interactive console used for maintenance,
> which exists in addition to the full blown Windows GUI, which is
> always the primary interface.
https://bugzilla.redhat.com/show_bug.cgi?id=1661288#c56
seems to contradict the above statement. Do I understand correctly that
if the SPCR is exposed and if "console=ttyAMA0 console=tty0" is not set
in the kernel params, the serial becomes the primary console instead of
the tty0 (hence your choice of developping console preference DXE in
tianocore)?

Anyway I will respin this patch and use the build_append_int_noprefix() API.

Thanks

Eric
>
> DBG2 is used as a debug port, which is used for the kernel debugger,
> if I am not mistaken. So SPCR and DBG2 are complementary, and it does
> make sense to have both.
>