Re: [PATCH] irqchip/gic-v3-its: Add early memory allocation errata

[Matthias Brugger]

On 05/10/2018 12:55, Marc Zyngier wrote:
> Hi Matthias,
> 
> On 04/10/18 23:11, Matthias Brugger wrote:
>> Friendly reminder, if anyone has any comment on the patch :)
>>
>> On 9/12/18 11:52 AM, matthias.bgg@xxxxxxxxxx wrote:
>>> From: Matthias Brugger <mbrugger@xxxxxxxx>
>>>
>>> Some hardware does not implement two-level page tables so that
>>> the amount of contigious memory needed by the baser is bigger
>>> then the zone order. This is a known problem on Cavium Thunderx
>>> with 4K page size.
>>>
>>> We fix this by adding an errata which allocates the memory early
>>> in the boot cycle, using the memblock allocator.
>>>
>>> Signed-off-by: Matthias Brugger <mbrugger@xxxxxxxx>
>>> ---
>>> Â arch/arm64/KconfigÂÂÂÂÂÂÂÂÂÂÂÂÂÂ | 12 ++++++++
>>> Â arch/arm64/include/asm/cpucaps.h |Â 3 +-
>>> Â arch/arm64/kernel/cpu_errata.cÂÂ | 33 +++++++++++++++++++++
>>> Â drivers/irqchip/irq-gic-v3-its.c | 50 ++++++++++++++++++++------------
>>> Â 4 files changed, 79 insertions(+), 19 deletions(-)
> 
> My only comment would be to state how much I dislike both the HW and the
> patch... ;-) The idea that we have some erratum that depends on the page size
> doesn't feel good at all.
> 

Well ugly HW needs ugly patches ;-)

>>>
>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>> index 1b1a0e95c751..dfd9fe08f0b2 100644
>>> --- a/arch/arm64/Kconfig
>>> +++ b/arch/arm64/Kconfig
>>> @@ -597,6 +597,18 @@ config QCOM_FALKOR_ERRATUM_E1041
>>> Â ÂÂÂÂÂÂÂ If unsure, say Y.
>>> Â +config CAVIUM_ALLOC_ITS_TABLE_EARLY
>>> +ÂÂÂ bool "Cavium Thunderx: Allocate the its table early"
>>> +ÂÂÂ default y
>>> +ÂÂÂ depends on ARM64_4K_PAGES && FORCE_MAX_ZONEORDER < 13
> 
> Here's a though: Why don't we ensure that FORCE_MAX_ZONEORDER is such as we
> could always allocate the same amount of memory, no matter what the page size
> is? That, or bump FORCE_MAX_ZONEORDER to 13 if the kernel includes support for TX1.
> 

Bumping FORCE_MAX_ZONEORDER when TX1 is supported was proposed here:
https://patchwork.kernel.org/patch/6322281/

To bring in some more history, the CMA approach ended with this discussion:
https://patchwork.kernel.org/patch/9888041/

> Any of this of course requires buy-in from the arm64 maintainers, as this is
> quite a departure from the way things work so far.
> 

With my distribution head on, I would prefer a solution that does not change
FORCE_MAX_ZONEORDER. That's how I came to the idea providing a third solution to
the same problem :)

Regards,
Matthias