Re: [PATCH 03/11] MIPS: support RAM beyond 32-bit

[Gregory CLEMENT <gregory.clement@bootlin.com>]

Hello Jiaxun,

> 在2023年10月4日十月 下午5:10，Gregory CLEMENT写道：
>> From: Vladimir Kondratiev <vladimir.kondratiev@intel.com>
>>
>> Support platforms where RAM is mapped beyond 32-bit.
>>
>> The kernel parameter ddr32_alias allows to setup the alias to point
>> outside the first 4 GB of memory.
>
> Are you trying to fix the problem that if kernel text is loaded in
> XKPHYS there is no way to to set EBASE to that region?

Yes that exactly we try to fix.

>
> The common practice for other 64bit MIPS system is to load kernel
> in KSEG0 and add low 4G mirror with rest of the high memory to buddy
> system. By doing this Kernel still have access to all memory beyond
> 32 bit, the only draw back is Kernel's text and data can't be relocted
> beyond 32-bit.
>
> Loading kernel into KSEG0 (i.e. with KBUILD_SYM32) have significant benefit
> on performance, so I think you shouldn't try to load kernel into XKPHYS
> without a good reason, but it might be helpful to add a BUG_ON at
> CPS driver to handle such situation.

I guess that being in KSEG0 allows to use shorter pointer.  But in our
case the RAM is physically connected beyond 32bits, so it is not
accessible in KSEG0.

>
> Btw: Is your target hardware publicly available? Folks at CIP United
> are looking for EyeQ5 boards for a while, they are supporting MIPS R6
> support at various projects.

We use evaluation boards and I don't know if they are publicly
available.

Gregory

>
> Thanks
> Jiaxun
>
>>
>> Signed-off-by: Vladimir Kondratiev <vladimir.kondratiev@intel.com>
>> Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
>> ---
>>  arch/mips/kernel/smp-cps.c | 12 +++++++++++-
>>  1 file changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/mips/kernel/smp-cps.c b/arch/mips/kernel/smp-cps.c
>> index 47e76722a306..fcfb19487612 100644
>> --- a/arch/mips/kernel/smp-cps.c
>> +++ b/arch/mips/kernel/smp-cps.c
>> @@ -34,6 +34,16 @@ static unsigned __init core_vpe_count(unsigned int 
>> cluster, unsigned core)
>>  	return min(smp_max_threads, mips_cps_numvps(cluster, core));
>>  }
>> 
>> +static int ddr32_alias;
>> +
>> +static int __init ddr32_alias_setup(char *str)
>> +{
>> +	get_option(&str, &ddr32_alias);
>> +
>> +	return 0;
>> +}
>> +early_param("ddr32_alias", ddr32_alias_setup);
>> +
>>  /**
>>   * plat_core_entry - query reset vector for NMI/reset
>>   *
>> @@ -52,7 +62,7 @@ static u32 plat_core_entry(void)
>>  {
>>  #if defined(CONFIG_USE_XKPHYS)
>>  	return (UNCAC_ADDR(mips_cps_core_entry) & 0xffffffff)
>> -			| CM_GCR_Cx_RESET_BASE_MODE;
>> +			| ddr32_alias | CM_GCR_Cx_RESET_BASE_MODE;
>>  #else
>>  	return CKSEG1ADDR((unsigned long)mips_cps_core_entry);
>>  #endif
>> -- 
>> 2.40.1
>
> -- 
> - Jiaxun

-- 
Gregory Clement, Bootlin
Embedded Linux and Kernel engineering
http://bootlin.com