Re: [PATCH 2/2] ARM: Allow either FLATMEM or SPARSEMEM on the multiplatform build

[Florian Fainelli <f.fainelli@gmail.com>]

On 5/19/2020 1:42 PM, Arnd Bergmann wrote:
> On Tue, May 19, 2020 at 6:54 PM Russell King - ARM Linux admin
> <linux@armlinux.org.uk> wrote:
>> On Tue, May 19, 2020 at 05:32:52PM +0200, Arnd Bergmann wrote:
>>> On Tue, May 19, 2020 at 5:27 PM Mike Rapoport <rppt@linux.ibm.com> wrote:
>>>> On Tue, May 19, 2020 at 04:04:52PM +0100, Russell King - ARM Linux admin wrote:
>>>>> There are cases where the reason to use sparsemem is not an optional
>>>>> one but is out of necessity - they require the page array to be split
>>>>> up in order to boot successfully.
>>>>>
>>>>> With that in mind, flatmem becomes an "optimisation" over sparsemem.
>>>>
>>>> At the moment, there are three platforms that enable SPARSEMEM: ARCH_EP93XX,
>>>> ARCH_RPC and ARCH_SA1100. All the rest have FLATMEM implcitly selected.
>>>>
>>>> I do not intend to change that, I am only going add an ability to select
>>>> the memory model for ARCH_MULTIPLATFORM.
>>>>
>>>> I'll respin the series on the list before adding the patches to the
>>>> patch system.
>>>
>>> I think we'll make EP93xx part of multiplatform at some point, IIRC
>>> only the missing clock driver is stopping us at the moment, and I already
>>> discussed with Linus Walleij how that can be done.
>>>
>>> My guess is that ep93xx is one platform on which sparsemem is
>>> just an optimization to reduce the initial memory consumption, but
>>> we should verify that when we get there.
>>
>> When you have a platform where the memory is segmented into separate
>> blocks widely spaced, sparsemem is not an optimisation.  For example,
>> If you have four blocks spaced across 1GB, that requires about
>> 256Ki struct page's.  Assuming 32 byte struct page, that requires 8MiB
>> of contiguous memory.
> 
> I checked the datasheet: ep93xx has four chip-selects, each of them
> with their own 256MB memory area. Most boards we support use only
> the last chipselect, but those that have more than one would waste
> up to 2MB per additional memory chip, which is not nice but doesn't
> make it a hard requirement as long as the kernel can fit into the first one.
> 
> However, the chips has a second problem: depending on the
> specific memory configuration, only every other 2/4/8MB within
> that range may actually be used, so the mem_map[] array would
> double its size in practice and (worse) the kernel would likely
> not fit into the first section without sparsemem.
> 
> This means we should indeed treat it as a requirement for ep93xx.

That is already captured with the select ARCH_SPARSEMEM_ENABLE or the
ARCH_EP93XX entry.

Should we just make it possible for multi platform users to override the
memory model in their configuration file and later we just default
ARCH_MULTI_V7 to sparsemem?
-- 
Florian

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