Re: [PATCH 18/21] ARM: drop SMP support for ARM11MPCore

Le 30/03/2023 à 12:03, Arnd Bergmann a écrit :
> On Thu, Mar 30, 2023, at 09:48, Neil Armstrong wrote:
>> On 27/03/2023 14:13, Arnd Bergmann wrote:
>>> From: Arnd Bergmann <[email protected]>
>>>
>>> The cache management operations for noncoherent DMA on ARMv6 work
>>> in two different ways:
>>>
>>>    * When CONFIG_DMA_CACHE_RWFO is set, speculative prefetches on in-flight
>>>      DMA buffers lead to data corruption when the prefetched data is written
>>>      back on top of data from the device.
>>>
>>>    * When CONFIG_DMA_CACHE_RWFO is disabled, a cache flush on one CPU
>>>      is not seen by the other core(s), leading to inconsistent contents
>>>      accross the system.
>>>
>>> As a consequence, neither configuration is actually safe to use in a
>>> general-purpose kernel that is used on both MPCore systems and ARM1176
>>> with prefetching enabled.
>>>
>>> We could add further workarounds to make the behavior more dynamic based
>>> on the system, but realistically, there are close to zero remaining
>>> users on any ARM11MPCore anyway, and nobody seems too interested in it,
>>> compared to the more popular ARM1176 used in BMC2835 and AST2500.
>>>
>>> The Oxnas platform has some minimal support in OpenWRT, but most of the
>>> drivers and dts files never made it into the mainline kernel, while the
>>> Arm Versatile/Realview platform mainly serves as a reference system but
>>> is not necessary to be kept working once all other ARM11MPCore are gone.
>>
>> Acked-by: Neil Armstrong <[email protected]>
>>
>> It's sad but it's the reality, there's no chance full OXNAS support will
>> ever come upstream and no real work has been done for years.
>>
>> I think OXNAS support can be programmed for removal for next release,
>> it would need significant work to rework current support to make it acceptable
>> before trying to upstream missing bits anyway.
> 
> Ok, thanks for your reply!
> 
> To clarify, do you think we should plan for removal after the next
> stable release (6.3, removed in 6.4), or after the next LTS
> release (probably 6.6, removed in 6.7)? As far as I understand,
> the next OpenWRT release (23.x) will be based on linux-5.15,
> and the one after that (24.x) would likely still use 6.1, unless
> they skip an LTS kernel.

I think it's ok to remove it ASAP, or at least before the next LTS,
not having SMP makes the platform barely usable so the earliest is the best.

Neil

> 
>       Arnd