linux-kernel - Re: [4.1,013/123] MIPS: c-r4k: Fix cache flushing for MT cores

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Date:	Mon, 10 Aug 2015 11:36:34 -0700
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	<linux-kernel@...r.kernel.org>
CC:	<stable@...r.kernel.org>,
	Markos Chandras <markos.chandras@...tec.com>,
	<linux-mips@...ux-mips.org>, Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [4.1,013/123] MIPS: c-r4k: Fix cache flushing for MT cores

On 08/08/2015 03:08 PM, gregkh@...uxfoundation.org wrote:
> 4.1-stable review patch.  If anyone has any objections, please let me know.
>
>
Yes, I have objection. Please look into excepts from my mail exchange 
with Markos:

> On 06/25/2015 03:59 AM, Markos Chandras wrote:
>
>> @@ -51,9 +51,8 @@ static inline void r4k_on_each_cpu(void (*func) (void *info), void *info)
>>   {
>>   	preempt_disable();
>>   
>> -#ifndef CONFIG_MIPS_MT_SMP
>> -	smp_call_function(func, info, 1);
>> -#endif
>> +	if (config_enabled(CONFIG_SMP))
>> +		smp_call_function_many(&cpu_foreign_map, func, info, 1);
>>   	func(info);
>>   	preempt_enable();
>>   }
>
> You can NOT do this because r4k_on_each_cpu() is still used for 
> non-MIPS/IMG processors for SAFE INDEX cache flushes - 
> cpu_has_safe_index_cacheops (it is not safe in CM/CM2/CM3 environment).
>
>
> And a little explanation and history:
>
> The function r4k_on_each_cpu() can NOT be used simultaneously for 
> index cacheops and address cacheops because both have a different 
> rules in applying in other cores and that is different in inter-core 
> HW blocks of various vendors. CM propogates address cacheops from 
> core-to-core (no IPI calls are needed) but another vendors may do not 
> - this is indicated by CONFIG_MIPS_MT_SMP (and a dropped now 
> CONFIG_MIPS_MT_SMTC).
>
> Unfortunately, before 2.6.35.9 this function was used for index 
> cacheops too in any kernel and that is WRONG, at least for CM-based 
> systems.
> So, I splitted index and address cacheops and wrote a functions 
> r4k_indexop_on_each_cpu and put it in use in at least in dlm-2.6.35.9 
> and it finally made a way to dev-linux-mti-3.6. This is a famous patch 
> named:
>
>     MIPS: Cache flush functions are reworked.
>
>     This patch is a preparation for EVA support in kernel.
>
>     However, it also fixes a bug then index cacheop was not ran
>     on multiple CPUs with unsafe index cacheops (flush_cache_vmap,
>     flush_icache_range, flush_cache_range, __flush_cache_all).
>
>     Additionally, it optimizes a usage of index and address cacheops for
>     address range flushes depending from address range size.
>
>     Because of that reasons it is a separate patch from EVA support.
>
>     Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
>     Signed-off-by: Steven J. Hill <Steven.Hill@...tec.com>
>     (cherry picked from commit 6b05dd71da1136fbad0ce642790c4c99343f05e7)
>

(history is skipped)

Note: the replacement of

     if (config_enabled(CONFIG_SMP))

to
if (!mips_cm_present())

doesn't solve a problem - in CM-based environment the index cache ops 
MUST be delivered to other core via IPI.

- Leonid.

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