State of RFC PATCH Remove CONFIG_DCACHE_WORD_ACCESS

State of RFC PATCH Remove CONFIG_DCACHE_WORD_ACCESS

[Lukas Bulwahn]

Hi Joe, hi Ben,

While reviewing some kernel config, I came across
CONFIG_DCACHE_WORD_ACCESS and tried to understand its purpose.

Then, I discovered this RFC patch from 2014 that seems never to have
been integrated:

https://lore.kernel.org/all/1393964591.20435.58.camel@joe-AO722/
[RFC] Remove CONFIG_DCACHE_WORD_ACCESS

The discussion seemed to just not continue and the patch was just not
integrated by anyone.

In the meantime, the use of CONFIG_DCACHE_WORD_ACCESS has spread into
a few more files, but replacing it with
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS still seems feasible.

Are you aware of reasons that this patch from 2014 should not be integrated?

I would spend some time to move the integration of this patch further
if you consider that the patch is not completely wrong.


Best regards,

Lukas

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Re: State of RFC PATCH Remove CONFIG_DCACHE_WORD_ACCESS

[Christophe Leroy]

Le 12/09/2022 à 15:46, Lukas Bulwahn a écrit :
> Hi Joe, hi Ben,
> 
> While reviewing some kernel config, I came across
> CONFIG_DCACHE_WORD_ACCESS and tried to understand its purpose.
> 
> Then, I discovered this RFC patch from 2014 that seems never to have
> been integrated:
> 
> https://lore.kernel.org/all/1393964591.20435.58.camel@joe-AO722/
> [RFC] Remove CONFIG_DCACHE_WORD_ACCESS
> 
> The discussion seemed to just not continue and the patch was just not
> integrated by anyone.
> 
> In the meantime, the use of CONFIG_DCACHE_WORD_ACCESS has spread into
> a few more files, but replacing it with
> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS still seems feasible.
> 
> Are you aware of reasons that this patch from 2014 should not be integrated?
> 
> I would spend some time to move the integration of this patch further
> if you consider that the patch is not completely wrong.
> 

As far as I can see, for the time being this is not equivalent on powerpc:

select HAVE_EFFICIENT_UNALIGNED_ACCESS  if !(CPU_LITTLE_ENDIAN && 
POWER7_CPU)

select DCACHE_WORD_ACCESS               if PPC64 && CPU_LITTLE_ENDIAN

This will need to be investigated I guess.

In the meantime I'll try to see if it makes any difference for ppc32.

Christophe
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Re: State of RFC PATCH Remove CONFIG_DCACHE_WORD_ACCESS

[Christophe Leroy]

Le 12/09/2022 à 17:22, Christophe Leroy a écrit :
> 
> 
> Le 12/09/2022 à 15:46, Lukas Bulwahn a écrit :
>> Hi Joe, hi Ben,
>>
>> While reviewing some kernel config, I came across
>> CONFIG_DCACHE_WORD_ACCESS and tried to understand its purpose.
>>
>> Then, I discovered this RFC patch from 2014 that seems never to have
>> been integrated:
>>
>> https://lore.kernel.org/all/1393964591.20435.58.camel@joe-AO722/
>> [RFC] Remove CONFIG_DCACHE_WORD_ACCESS
>>
>> The discussion seemed to just not continue and the patch was just not
>> integrated by anyone.
>>
>> In the meantime, the use of CONFIG_DCACHE_WORD_ACCESS has spread into
>> a few more files, but replacing it with
>> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS still seems feasible.
>>
>> Are you aware of reasons that this patch from 2014 should not be 
>> integrated?
>>
>> I would spend some time to move the integration of this patch further
>> if you consider that the patch is not completely wrong.
>>
> 
> As far as I can see, for the time being this is not equivalent on powerpc:
> 
> select HAVE_EFFICIENT_UNALIGNED_ACCESS  if !(CPU_LITTLE_ENDIAN && 
> POWER7_CPU)
> 
> select DCACHE_WORD_ACCESS               if PPC64 && CPU_LITTLE_ENDIAN
> 
> This will need to be investigated I guess.
> 
> In the meantime I'll try to see if it makes any difference for ppc32.
> 

Selecting DCACHE_WORD_ACCESS on powerpc32 provides an improvement of 
approx 9% on a powerpc 8xx and about 2% on a powerpc 832x, using the 
benchmark test in commit a75c380c7129 ("powerpc: Enable 
DCACHE_WORD_ACCESS on ppc64le")

Christophe

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