Re: [PATCH] arm: use SPARSMEM_STATIC when SPARSEMEM is enabled (Was: [PATCH 1/2] ARM: Remove redundant ARCH_SPARSEMEM_DEFAULT setting)

From: Florian Fainelli <f.fainelli@gmail.com>
To: Mike Rapoport <rppt@linux.ibm.com>,
	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>,
	Gregory Fong <gregory.0xf0@gmail.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm: use SPARSMEM_STATIC when SPARSEMEM is enabled (Was: [PATCH 1/2] ARM: Remove redundant ARCH_SPARSEMEM_DEFAULT setting)
Date: Fri, 8 May 2020 13:20:05 -0700	[thread overview]
Message-ID: <235b21a0-f0c9-24d9-a78e-a54010aa87ef@gmail.com> (raw)
In-Reply-To: <20200507200859.GF683243@linux.ibm.com>


On 5/7/2020 1:08 PM, Mike Rapoport wrote:
> On Thu, May 07, 2020 at 11:30:39AM +0100, 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.
>>
>>> -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.
> 
> Out of curiosity I've run 
> 
> 	make ARCH=arm rpc_defconfig
> 	grep SPARSEMEM .config
> 
> and I was surprised to find out that it has
> 
> 	CONFIG_SPARSEMEM_EXTREME=y
> 
> Which would waste several kilibytes of memory for nothing.
> Here is the fix:
> 
> From 7097c114226b5b1b2fc6bb605bf0d7eae601cc7f Mon Sep 17 00:00:00 2001
> From: Mike Rapoport <rppt@linux.ibm.com>
> Date: Thu, 7 May 2020 22:39:12 +0300
> Subject: [PATCH] arm: use SPARSMEM_STATIC when SPARSEMEM is enabled
> 
> The commit 3e347261a80b5 ("[PATCH] sparsemem extreme implementation")
> made SPARSMEM_EXTREME the default option for configurations that enable
> SPARSEMEM.
> 
> For ARM systems with handful of memory banks SPARSEMEM_EXTREME is an
> overkill.
> 
> Ensure that SPARSMEM_STATIC is enabled in the configurations that use
> SPARSEMEM.
> 
> Fixes: 3e347261a80b5 ("[PATCH] sparsemem extreme implementation")
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>

Acked-by: Florian Fainelli <f.fainelli@gmail.com>
-- 
Florian

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