Re: [PATCH 1/2] ARM: Remove redundant ARCH_SPARSEMEM_DEFAULT setting

From: Florian Fainelli <f.fainelli@gmail.com>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Doug Berger <opendmb@gmail.com>, Arnd Bergmann <arnd@arndb.de>,
	sboyd@kernel.org, Kevin Cernekee <cernekee@gmail.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Gregory Fong <gregory.0xf0@gmail.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/2] ARM: Remove redundant ARCH_SPARSEMEM_DEFAULT setting
Date: Thu, 7 May 2020 10:25:16 -0700	[thread overview]
Message-ID: <3e7ee3ba-fae9-0ec3-fe1f-c076fc28aba9@gmail.com> (raw)
In-Reply-To: <20200507103039.GR1551@shell.armlinux.org.uk>

On 5/7/2020 3:30 AM, Russell King - ARM Linux admin wrote:
> On Wed, May 06, 2020 at 04:50:08PM -0700, Florian Fainelli wrote:
>> From: Kevin Cernekee <cernekee@gmail.com>
>>
>> If ARCH_SPARSEMEM_ENABLE=y and ARCH_{FLATMEM,DISCONTIGMEM}_ENABLE=n,
>> then the logic in mm/Kconfig already makes CONFIG_SPARSEMEM the only
>> choice.  This is true for all of the existing ARM users of
>> ARCH_SPARSEMEM_ENABLE.
>>
>> Forcing ARCH_SPARSEMEM_DEFAULT=y if ARCH_SPARSEMEM_ENABLE=y prevents
>> us from ever defaulting to FLATMEM, so we should remove this setting.
> 
> No explanation why that is desirable.

This was explained in the original submission out of which only the
first patch landed upstream:

https://lkml.org/lkml/2015/6/4/756

> 
>> -config ARCH_SPARSEMEM_DEFAULT
>> -	def_bool ARCH_SPARSEMEM_ENABLE
>> -
> 
> What this basically says is ARCH_SPARSEMEM_ENABLE=ARCH_SPARSEMEM_DEFAULT,
> which is exactly what we want for the non-multiplatform boards that
> select ARCH_SPARSEMEM_ENABLE - we _want_ them to default to sparsemem
> because that is what the platform requires.
> 
> For example, with RiscPC, which selects ARCH_SPARSEMEM_ENABLE, we have
> four banks of memory at 0x10000000, 0x14000000, 0x18000000 and
> 0x1c000000.  These correspond with the two memory slots - the first two
> for the first slot, and the second two for the second slot.  Each slot
> has two banks.  The size of each memory bank depends on the size of the
> module.
> 
> Flatmem is completely unable to work with this setup if all banks are
> populated, and the first bank does not contain enough memory to allocate
> the struct page array.  So, sparsemem is the only option there.
> 
> Hence, for these platforms, we want sparsemem and only sparsemem, not
> flatmem.
> 
> So, this patch which makes it possible to select flatmem is completely
> out of the question for these platforms.

OK.
-- 
Florian

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