[PATCH] ARM: EXYNOS4: Enable double linefill in PL310 Prefetch Control Register

[Siarhei Siamashka]

On Wed, Sep 14, 2011 at 10:57 AM, Kyungmin Park <kmpark at infradead.org> wrote:
> On Wed, Sep 14, 2011 at 4:43 PM, Siarhei Siamashka
> <siarhei.siamashka at gmail.com> wrote:
>> On Wed, Sep 14, 2011 at 9:08 AM, Kyungmin Park <kmpark at infradead.org> wrote:
>>> Hi Siarhei,
>>>
>>> Interesting feature, and it's not samsung soc issue, so add the arm
>>> mailing list.
>>> It checked and the see the read performance improvement from 868MiB/s
>>> to 981MiB/s with lmbench.
>>
>> Maybe lmbench does not try very hard to get the best out of the
>> hardware? On my origenboard, I'm getting ~1.15GB/s performance for the
>> standard LDM/STM based memcpy from libc-ports, which is ~2.3GB/s
>> memory bandwidth if both reads and writes are accounted separately.
>>
>>> It's helpful to test other SoC., e.g., OMAP4, STE and so on.
>>
>> The current (?) state of the support for this feature in OMAP4 is
>> explained here by Richard Woodruff:
>>    http://groups.google.com/group/pandaboard/msg/dfd2d2e1336d435b
>>
>>> BTW, why do you set the 27-bit? In my PL310 Spec., it's reserved bit
>>> and should be zero (SBZ).
>>
>> This PL310 thing seems to have been renamed to "CoreLink Level 2 Cache
>> Controller L2C-310" in later revisions, and its Prefetch Control
>> Register is described here:
>>    http://infocenter.arm.com/help/topic/com.arm.doc.ddi0246f/CHDHIECI.html
> Thanks for link. it has 27-bit description. but does it correct bit
> description for exynos4 PL310?
> I mean I received the PL310 TRM with exynos4 chip used. there's no
> 27-bit description. it's just reserved bit.
> Can it enable the 27-bit at exynos4210? or can be used for exynos4212 or later?

That's a good point. I think it is exynos4210 that is used in
origenboard. And according to the value in Cache ID Register
(0x4100c4c5), it has r3p0 revision of L2C-310. Which means that the
Prefetch Control Register is actually described at:
    http://infocenter.arm.com/help/topic/com.arm.doc.ddi0246d/CHDHIECI.html
And bit 27 is indeed reserved. However flipping it seems to have some
measurable impact on performance (unless I screwed up the benchmarks),
so maybe it does something but is undocumented? In any case, I agree
that it's better not to mess up with this bit.

By the way, does anybody have L2C-310 errata list? Is double linefill
actually safe to use in r3p0?

-- 
Best regards,
Siarhei Siamashka