Subject: Re: [PATCH v3] powerpc32: memset: only use dcbz once cache is enabled

From: Christophe LEROY <[email-masked]>
Date: Mon, 14 Sep 2015 17:44:31 +0200

Le 14/09/2015 17:20, Scott Wood a écrit :
> On Mon, 2015-09-14 at 08:21 +0200, Christophe Leroy wrote:
>> memset() uses instruction dcbz to speed up clearing by not wasting time
>> loading cache line with data that will be overwritten.
>> Some platform like mpc52xx do no have cache active at startup and
>> can therefore not use memset(). Allthough no part of the code
>> explicitly uses memset(), GCC may makes calls to it.
>>
>> This patch modifies memset() such that at startup, memset()
>> unconditionally jumps to simple_memset() which doesn't use
>> the dcbz instruction.
>>
>> Once the initial MMU is set up, in machine_init() we patch memset()
>> by replacing this inconditional jump by a NOP
>>
>> Signed-off-by: Christophe Leroy <[email-masked]>
>> ---
>> This patch goes on to of [v3] powerpc32: memcpy: only use dcbz once cache
>> is enabled
>>
>> Changes in v2:
>>     was part of [v2] powerpc32: memcpy/memset: only use dcbz once cache is
>> enabled
>> changes in v3:
>>    Not using anymore feature-fixups
>>    Handling of memcpy() and memset() split in two patches
>>    
>>   arch/powerpc/kernel/setup_32.c |  1 +
>>   arch/powerpc/lib/copy_32.S     | 15 +++++++++++++++
>>   2 files changed, 16 insertions(+)
>>
>> diff --git a/arch/powerpc/kernel/setup_32.c b/arch/powerpc/kernel/setup_32.c
>> index 362495f..345ec3a 100644
>> --- a/arch/powerpc/kernel/setup_32.c
>> +++ b/arch/powerpc/kernel/setup_32.c
>> @@ -124,6 +124,7 @@ notrace void __init machine_init(u64 dt_ptr)
>>        udbg_early_init();
>>   
>>        patch_instruction((unsigned int *)&memcpy, PPC_INST_NOP);
>> +     patch_instruction((unsigned int *)&memset, PPC_INST_NOP);
>>   
>>        /* Do some early initialization based on the flat device tree */
>>        early_init_devtree(__va(dt_ptr));
>> diff --git a/arch/powerpc/lib/copy_32.S b/arch/powerpc/lib/copy_32.S
>> index da5847d..68a59d4 100644
>> --- a/arch/powerpc/lib/copy_32.S
>> +++ b/arch/powerpc/lib/copy_32.S
>> @@ -73,8 +73,13 @@ CACHELINE_MASK = (L1_CACHE_BYTES-1)
>>    * Use dcbz on the complete cache lines in the destination
>>    * to set them to zero.  This requires that the destination
>>    * area is cacheable.  -- paulus
>> + *
>> + * During early init, cache might not be active yet, so dcbz cannot be
>> used.
>> + * We therefore jump to simple_memset which doesn't use dcbz. This jump is
>> + * replaced by a nop once cache is active. This is done in machine_init()
>>    */
>>   _GLOBAL(memset)
>> +     b       simple_memset
>>        rlwimi  r4,r4,8,16,23
>>        rlwimi  r4,r4,16,0,15
>>   
>> @@ -122,6 +127,16 @@ _GLOBAL(memset)
>>        bdnz    8b
>>        blr
>>   
>> +/* Simple version of memset used during early boot until cache is enabled
>> */
>> +simple_memset:
>> +     cmplwi  cr0,r5,0
>> +     addi    r6,r3,-1
>> +     beqlr
>> +     mtctr   r5
>> +1:   stbu    r4,1(r6)
>> +     bdnz    1b
>> +     blr
> Instead couldn't you use the generic memset at label 2: and patch the "bne
> 2f"?

Yes I could but it means adding a global symbol there at the "bne 2f". I 
thought it was what Michael didn't like in my v1 of memcpy().
What name could I give to that symbol ? Something like 
memcpy_nocache_patch: ?
What would be the best ? Having b 2f, and replacing it with bne 2f ? Or 
have b 2f just above and replace it by nop once cache is up ?

Christophe