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
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
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