About LPAE supporting on EXYNOS5440

[subashrp@gmail.com (Subash Patel)]

On 06/11/2013 06:13 PM, Arnd Bergmann wrote:
> On Tuesday 11 June 2013 19:05:22 Kukjin Kim wrote:
>> Arnd Bergmann wrote:
>>>
>>> On Tuesday 11 June 2013 17:33:33 Kukjin Kim wrote:
>>>> Hi all,
>>>>
>>>> The exynos5440 can support LPAE and referece boards, SSDK5440 and SD5v1
>>> have
>>>> over 8GiB memory.
>>>>
>>>> Current exynos_defconfig selects every EXYNOS SoCs including exynos5440
>>> it
>>>> means supporting LPAE will be selected for other EXYNOS SoCs.
>>>>
>>>> I'm wondering if seleting LPAE causes some problem for multiplatform.
>>> Can we
>>>> support LPAE enabled and disabled platform together with single zImage?
>>>
>>> No, that isn't possible.
>>
>> Yeah, I know that.
>>
>>> I think we should enable all exynos support in
>>> the
>>> regular multi_v7_defconfig and leave LPAE turned off for that,
>>
>> But if so, problem will happen in the memory parsing without LPAE
>> enabling...so I'm wondering I should deselect exynos5440 in the defconfig
>> for other exynos SoCs...
>
> I think there is a bug in the memory parsing code and already two people
> have submitted independent fixes for that. It really should just work
> even if you have more memory that ends up getting ignored.
>
>>> but create
>>> another multi_lpae_defconfig that has all LPAE capable machines turned on.
>>> This is also what I expect the distros to do.
>>>
>> Yeah, it could be a solution for multiplatform at this moment. And if so,
>> probably I should deselect exynos5440 in the exynos_defconfig and need to
>> add another defconfig for exynos lpae stuff.
>
> I would definitely leave on exynos5440 support in defconfig. It's not a
> lot of extra code, and if you have a system with less than 4GB memory,
> you really don't want to enable LPAE because of the overhead.

Even if we have <= 4GB memory, and the system designers have placed the 
memory banks in addresses > 2^32 bytes, then we need LPAE support. I 
think we may have such systems in future. So memory capacity is not the 
only parameter to judge if we need to enable LPAE or not.

Regards,
Subash

>
>> BTW, do you know what SoCs can support LPAE now?
>
> Upstream I think we support exynos, vexpress (using A15, A7 and A12
> core tiles), omap5 (partially), mvebu (Armada XP), highbank (Midway)
> and some of the shmobile parts we added in 3.10.
>
> In mach-sunxi, there will soon be support for Allwinner-A20 and -A31,
> which are both Cortex-A7 based. MSM also has support for Cortex-A7
> but not upstream. A couple of new platforms are under discussion
> already that will also have LPAE, mostly server parts.
>
> 	Arnd
>
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