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

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Message-ID: <1d227152-2770-3e93-f78c-71e7a3b4fe76@arm.com>
Date:   Fri, 5 Oct 2018 14:42:32 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Matthias Brugger <mbrugger@...e.com>, matthias.bgg@...nel.org,
        catalin.marinas@....com, will.deacon@....com, tglx@...utronix.de,
        jason@...edaemon.net, robert.richter@...ium.com
Cc:     suzuki.poulose@....com, shankerd@...eaurora.org,
        xiexiuqi@...wei.com, Dave.Martin@....com, matthias.bgg@...il.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] irqchip/gic-v3-its: Add early memory allocation errata

On 05/10/18 13:33, Matthias Brugger wrote:
> 
> 
> 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@...nel.org wrote:
>>>> From: Matthias Brugger <mbrugger@...e.com>
>>>>
>>>> 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@...e.com>
>>>> ---
>>>>    arch/arm64/Kconfig               | 12 ++++++++
>>>>    arch/arm64/include/asm/cpucaps.h |  3 +-
>>>>    arch/arm64/kernel/cpu_errata.c   | 33 +++++++++++++++++++++
>>>>    drivers/irqchip/irq-gic-v3-its.c | 50 ++++++++++++++++++++------------
>>>>    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
>>>>            If unsure, say Y.
>>>>    +config CAVIUM_ALLOC_ITS_TABLE_EARLY
>>>> +    bool "Cavium Thunderx: Allocate the its table early"
>>>> +    default y
>>>> +    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 :)

Why is that a problem? What impact does this have on your favourite distro?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...