Message-ID: <4CE1E2CA.30001@codeaurora.org>
Date: Mon, 15 Nov 2010 17:47:54 -0800
From: Stepan Moskovchenko <stepanm@codeaurora.org>
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To: Daniel Walker <dwalker@codeaurora.org>
CC: davidb@codeaurora.org, bryanh@codeaurora.org,
        linux-arm-msm@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 09/14] msm: iommu: Kconfig option for cacheable page 
 tables
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On 11/15/2010 10:20 AM, Daniel Walker wrote:
> On Sun, 2010-11-14 at 18:56 -0800, Stepan Moskovchenko wrote:
>>> On Fri, 2010-11-12 at 19:29 -0800, Stepan Moskovchenko wrote:
>>>> +config IOMMU_PGTABLES_L2
>>>> +	depends on ARCH_MSM8X60
>>>> +	depends on MMU
>>>> +	depends on CPU_DCACHE_DISABLE=n
>>>> +	depends on SMP
>>>> +	bool "Cacheable IOMMU page tables"
>>>> +	default y
>>>> +	help
>>>> +	  Allows the IOMMU page tables to be brought into the L2 cache. This
>>>> +	  improves the TLB miss latency at the expense of potential pollution
>>>> +	  of the L2 cache. This option has been shown to improve multimedia
>>>> +	  performance in some cases.
>>>> +
>>>> +	  If unsure, say Y here.
>>> Why would someone want this off?
>>>
>>> The other thing is that you usually want this included with the code
>>> that uses the option.
>> The code that uses it had gone in during a previous patch series, but I
>> didn't want to meddle in the Kconfig just yet, especially since the option
>> only improves performance and does not add new functionality at a high
>> level. This patch should be the last of what is needed for this feature.
>>
>> You would want to turn this off if you wanted more deterministic behavior
>> from the multimedia subsystem, such as when trying to run benchmarks for
>> the worst-case behavior in terms of memory latency and TLB misses. You
>> might also want to turn it off if you are debugging memory problems that
>> you suspect might be related to the cache maintenance code, in which case
>> turning this off would give an idea as to whether that is the problem.
>> Similarly, this needs to be off (and will be, due to the dependencies) if
>> certain required things (like the Dcache, MMU, etc) have been disabled.
>> Finally, if you are doing things with the MMSS that you know will not
>> result in many TLB misses, (or if you know that you can tolerate high miss
>> latency) you may as well turn this off to avoid the (small) bit of cache
>> pollution.
> It sounds like you don't really want it off unless your a developer (or
> you turn one of the dependencies) .. I think this might be better as a
> hidden option just cause of the developer centric nature of it.
>
> Daniel

Alright. Fixed in v2.

Steve

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