linux-kernel - Re: [PATCH 4/7] clocksource: arch_timer: Add support for not-fw-configured timer on ARM64

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Date:   Mon, 15 Oct 2018 14:27:01 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-samsung-soc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Inki Dae <inki.dae@...sung.com>,
        Mark Rutland <Mark.Rutland@....com>
Subject: Re: [PATCH 4/7] clocksource: arch_timer: Add support for
 not-fw-configured timer on ARM64

On 15/10/2018 14:12, Marek Szyprowski wrote:
> Hi All,
> 
> On 2018-10-08 15:17, Marc Zyngier wrote:
>> + Mark Rutland
>>
>> Hi Marek,
>>
>> On 08/10/18 13:50, Marek Szyprowski wrote:
>>> Use common infrastructure for ARM Architected Timers erratum to enable
>>> support for systems with broken CPU firmware (timer registers not
>>> properly configured). This mode has been already availabled on ARM
>>> (32bits) architecture. This enables to run Linux kernel on ARM64 boards
>>> using physical architected timers instead of the virtual ones. Examples
>>> of such system with broken firmware are Samsung Exynos5433 SoC based
>>> TM2(e) boards, which is already deployed for years and updating firmware
>>> is not possible.
>>>
>>> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
>>> ---
>>>   drivers/clocksource/Kconfig          | 11 +++++++++++
>>>   drivers/clocksource/arm_arch_timer.c | 15 ++++++++++++---
>>>   2 files changed, 23 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
>>> index a11f4ba98b05..a30752579b03 100644
>>> --- a/drivers/clocksource/Kconfig
>>> +++ b/drivers/clocksource/Kconfig
>>> @@ -364,6 +364,17 @@ config ARM64_ERRATUM_858921
>>>         The workaround will be dynamically enabled when an affected
>>>         core is detected.
>>>   +config ARCH_TIMER_REGISTERS_NOT_FW_CONFIGURED
>>> +    bool "Workaround for arch timer registers not configured by
>>> firmware"
>>> +    default y
>>> +    select ARM_ARCH_TIMER_OOL_WORKAROUND
>>> +    depends on ARM_ARCH_TIMER && ARM64
>>> +    help
>>> +      This option enables a workaround for boards, on which arch timer
>>> +      registers are not properly configured by the board firmware.
>>> +      The workaround will be dynamically enabled when an affected
>>> +      board is detected.
>>> +
>>
>> I'm sorry, but I'm strongly pushing back on this.
>>
>> This horrible hack was accepted with the express condition that it
>> would be limited to ARMv7 platforms (on the ground that we never
>> really documented the arch timer boot requirements on that version of
>> the architecture), and would never proliferate on arm64. From day 1,
>> we established what the boot protocol was, and we mandated that either:
>>
>> - kernel is entered at EL2 on all CPUs
>> - cntvoff_el2 is zeroed on all CPUs
>>
>> and we've got most people to fix their firmware, or live with the
>> consequences. If these machines cannot receive a non-broken firmware,
>> what are the odds that they will receive a mainline kernel?
> 
> Well, I know that the firmware is broken, but I cannot do anything about it.
> On the other hand updating kernel is still possible and mainline runs fine
> on TM2(e) boards. I will send v2 without this patch then.

I second Marc's opinion.



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