Re: [PATCH] arm64: dts: renesas: r8a78000: Fix GIC-720AE View 1 Redistributor description

[Marek Vasut <marek.vasut@mailbox.org>]

On 5/14/26 9:04 AM, Marc Zyngier wrote:
> On Wed, 13 May 2026 23:30:08 +0100,
> Marek Vasut <marek.vasut+renesas@mailbox.org> wrote:
>>
>> The Renesas R-Car X5H (R8A78000) SoC contains Arm CoreLink GIC-720AE
>> Generic Interrupt Controller with Multi View capability. Firmware has
>> access to configuration View 0, Linux kernel has access to View 1.
> 
> Huh. That's pretty unexpected. The usual wisdom is to give the APs
> view 0 so that it looks like a "normal" machine, rather than only a
> partition of the system (which is what view != 0 indicates).
> 
> I guess there is some additional fun going on there, such as other
> CPUs getting a portion of the GIC for themselves, and firmware
> preventing whatever is running on the APs to interact with them...

This is my understanding as well, partitioning of the system is an 
intended use case. On the development hardware I have access to, no 
partitioning is applied, all cores are available to Linux.

>> The Arm CoreLink GIC-720AE Generic Interrupt Controller Technical
>> Reference Manual, currently latest r2p1 [1], chapter "Programmers model
>> for GIC-720AE", subchapter "Redistributor registers for control and
>> physical LPIs summary", part "GICR_TYPER, Redistributor Type Register"
>> clarifies register "GICR_TYPER" bit 4 "Last" behavior in Multi View
>> setup as follows:
>>
>> "
>> Last
>> Last Redistributor:
>>
>> 0 ... This Redistributor is not the last Redistributor on the chip.
>> 1 ... This Redistributor is the last Redistributor on the chip.
>>        When GICD_CFGID.VIEW == 1, for views 1, 2, or 3 this bit
>>        always returns 1.
>> "
>>
>> On this SoC, GICD_CFGID.VIEW is 1 and the Linux kernel has access to
>> View 1, therefore Linux kernel GICv3 driver will interpret register
>> "GICR_TYPER" bit 4 "Last" = 1 in the first Redistributor in continuous
>> Redistributor page as that first Redistributor being the one and only
>> Redistributor and will stop processing the continuous Redistributor
>> page further. This will prevent the other Redistributors from being
>> recognized by the system and used for other PEs.
>>
>> Because the hardware indicates that the continuous Redistributor page
>> is not continuous for View 1, 2, or 3, describe every Redistributor
>> separately in the DT. This makes all Redistributors for all cores
>> accessible in Linux.
>>
>> [1] https://developer.arm.com/documentation/102666/0201/Programmers-model-for-GIC-720AE/Redistributor-registers-for-control-and-physical-LPIs-summary/GICR-TYPER--Redistributor-Type-Register?lang=en
>>
> 
> I am amazed that you managed to find anything at all on this web site.
> 
> I would refrain from adding links to any ARM web sites in a commit
> message though. They tend to have a 10 minutes half-life period, and
> whole specs to disappear from public view when they are not flavour of
> the week anymore.
> 
> A link to the equivalent PDF has better chances to survive further
> creative^WAI slop driven reorg of the documentation:
> 
> https://documentation-service.arm.com/static/69ef3c1cd35efd294e335c43
> 
> but your best bet is to download it, archive it, and refer to it
> yourself.

Thank you for that. That is the one thing I did not manage to find on 
that website, a PDF of the GIC-720AE manual. I will include that link in 
a V2.

>> Fixes: 63500d12cf76 ("arm64: dts: renesas: Add R8A78000 SoC support")
>> Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
> 
> Acked-by: Marc Zyngier <maz@kernel.org>
> 
> 	M.
> 


-- 
Best regards,
Marek Vasut