Re: [PATCH 1/3] MIPS: Introduce WAR_4KC_LLSC config option

[Jiaxun Yang]

> 2023年5月22日 23:03，Jonas Gorski <jonas.gorski@xxxxxxxxx> 写道：
> 
> On Mon, 22 May 2023 at 22:38, Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx> wrote:
>> 
>> 
>> 
>>> 2023年5月22日 19:40，Jonas Gorski <jonas.gorski@xxxxxxxxx> 写道：
>>> 
>>> Hi,
>>> 
>>> On Fri, 19 May 2023 at 18:49, Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx> wrote:
>>>> 
>>>> WAR_4KC_LLSC is used to control workaround of 4KC LLSC issue
>>>> that affects 4Kc up to version 0.9.
>>>> 
>>>> Early ath25 chips are known to be affected.
>>>> 
>>>> Signed-off-by: Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>
>>>> ---
>>>> arch/mips/Kconfig                                        | 6 ++++++
>>>> arch/mips/include/asm/cpu.h                              | 1 +
>>>> arch/mips/include/asm/mach-ath25/cpu-feature-overrides.h | 2 +-
>>>> arch/mips/kernel/cpu-probe.c                             | 7 +++++++
>>>> 4 files changed, 15 insertions(+), 1 deletion(-)
>>>> 
>>>> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
>>>> index 30e90a2d53f4..354d033364ad 100644
>>>> --- a/arch/mips/Kconfig
>>>> +++ b/arch/mips/Kconfig
>>>> @@ -230,6 +230,7 @@ config ATH25
>>>>       select SYS_SUPPORTS_BIG_ENDIAN
>>>>       select SYS_SUPPORTS_32BIT_KERNEL
>>>>       select SYS_HAS_EARLY_PRINTK
>>>> +       select WAR_4KC_LLSC if !SOC_AR5312
>>> 
>>> Shouldn't this be "if SOC_AR5312"?
>> 
>> Ah sorry, I misread the original code.
>> 
>>> 
>>> Though since you are adding runtime detection/correction below, I
>>> wonder if this would be really needed as an extra symbol, and rather
>>> use the later introduced (CPU_MAY_HAVE_LLSC) directly.
>> 
>> I bet it’s better to have a symbol just for tracking errata. So we can easily know
>> if SoC is affected by a errata and have some extra documentation.
>> 
>>> 
>>> Or rather have select "CPU_HAS_LLSC if !SOC_AR5312" in that case.
>>> 
>>>>       help
>>>>         Support for Atheros AR231x and Atheros AR531x based boards
>>>> 
>>>> @@ -2544,6 +2545,11 @@ config WAR_ICACHE_REFILLS
>>>> config WAR_R10000_LLSC
>>>>       bool
>>>> 
>>>> +# On 4Kc up to version 0.9 (PRID_REV < 1) there is a bug that may cause llsc
>>>> +# sequences to deadlock.
>>>> +config WAR_4KC_LLSC
>>>> +       bool
>>>> +
>>>> # 34K core erratum: "Problems Executing the TLBR Instruction"
>>>> config WAR_MIPS34K_MISSED_ITLB
>>>>       bool
>>>> diff --git a/arch/mips/include/asm/cpu.h b/arch/mips/include/asm/cpu.h
>>>> index ecb9854cb432..84bb1931a8b4 100644
>>>> --- a/arch/mips/include/asm/cpu.h
>>>> +++ b/arch/mips/include/asm/cpu.h
>>>> @@ -247,6 +247,7 @@
>>>> #define PRID_REV_VR4122                        0x0070
>>>> #define PRID_REV_VR4181A               0x0070  /* Same as VR4122 */
>>>> #define PRID_REV_VR4130                        0x0080
>>>> +#define PRID_REV_4KC_V1_0              0x0001
>>>> #define PRID_REV_34K_V1_0_2            0x0022
>>>> #define PRID_REV_LOONGSON1B            0x0020
>>>> #define PRID_REV_LOONGSON1C            0x0020  /* Same as Loongson-1B */
>>>> diff --git a/arch/mips/include/asm/mach-ath25/cpu-feature-overrides.h b/arch/mips/include/asm/mach-ath25/cpu-feature-overrides.h
>>>> index ec3604c44ef2..5df292b1ff04 100644
>>>> --- a/arch/mips/include/asm/mach-ath25/cpu-feature-overrides.h
>>>> +++ b/arch/mips/include/asm/mach-ath25/cpu-feature-overrides.h
>>>> @@ -24,7 +24,7 @@
>>>> #define cpu_has_counter                        1
>>>> #define cpu_has_ejtag                  1
>>>> 
>>>> -#if !defined(CONFIG_SOC_AR5312)
>>>> +#if !defined(WAR_4KC_LLSC)
>>>> #  define cpu_has_llsc                 1
>>> 
>>> since the #else path defines cpu_has_llsc as 0, it means that kernels
>>> targeting both SoCs would force llsc to be unavailable (not introduced
>>> by you).
>> 
>> I’m a little bit confused.
>> The logic seems very clear to me: If a SoC is not affected by WAR_4KC_LLSC,
>> then wire  cpu_has_llsc to 1, else wire it to 0.
> 
> ATH25 allows you building for multiple SoCs at the same time, and if
> you do so, you don't know in advance on which SoC you boot. So you
> need to have third path here where cpu_has_llsc isn't wired to
> anything.

Thanks for pointing out the missing piece, I thought ATH25 can only be
built for a single SoC :-)

> 
> This is wrong in the current code already, so should be fixed there.
> 

[...]

> AFAICT the core issue is if the platform hardcodes cpu_has_llsc to 1.
> 
> So the error/warning this should be then something like this
> 
> if ((c->processor_id & PRID_REV_MASK) < PRID_REV_4KC_V1_0) {
>   c->options &= ~MIPS_CPU_LLSC;
>   if (cpu_has_llsc) { // <- should now be false, unless the platform
> defines it as 1
>      pr_err("CPU has LLSC erratum, but cpu_has_llsc is force enabled!\n");
>   }
> 
> because clearing MIPS_CPU_LLSC does nothing if cpu_has_llsc is
> #defined as 1, regardless if it selected WAR_4K_LLSC or not.
> 
> (also your error print is missing a \n at the end)

Ah, thanks.
I’m planning to replace this pr_err with WARN_TAINT_ONCE.

Thanks
- Jiaxun

> 
> Regards,
> Jonas