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

From: Mike Rapoport <rppt@linux.ibm.com>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Doug Berger <opendmb@gmail.com>, Arnd Bergmann <arnd@arndb.de>,
	Stephen Boyd <sboyd@kernel.org>,
	Kevin Cernekee <cernekee@gmail.com>,
	Florian Fainelli <f.fainelli@gmail.com>,
	Gregory Fong <gregory.0xf0@gmail.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 2/2] ARM: Allow either FLATMEM or SPARSEMEM on the multiplatform build
Date: Tue, 19 May 2020 18:27:14 +0300	[thread overview]
Message-ID: <20200519152714.GF1059226@linux.ibm.com> (raw)
In-Reply-To: <20200519150452.GH1551@shell.armlinux.org.uk>

On Tue, May 19, 2020 at 04:04:52PM +0100, Russell King - ARM Linux admin wrote:
> On Tue, May 19, 2020 at 05:43:06PM +0300, Mike Rapoport wrote:
> > On Tue, May 19, 2020 at 09:59:48AM +0200, Arnd Bergmann wrote:
> > > On Mon, May 18, 2020 at 9:45 PM Mike Rapoport <rppt@linux.ibm.com> wrote:
> > > > On Mon, May 18, 2020 at 08:58:36AM -0700, Florian Fainelli wrote:
> > > > >
> > > > > Mike, do you want to make a formal submission to Russell's patch
> > > > > tracker? If so, feel free to add:
> > > >
> > > > I actually hoped to hear from people what do they think about switching
> > > > over to SPARSEMEM for the multiplatform builds. I think at least v7 CPUs
> > > > it would make sense.
> > > 
> > > It seems that at least s5p/exynos and clps711x (armv4) used to default to
> > > sparsemem, and that got lost in the multiplatform conversion.
> > > 
> > > I also see discontiguous memory ranges in multiple broadcom chips,
> > > on TI dm8168, ecx-2000 and imx.
> > > 
> > > > Russel, Arnd, can you comment please?
> > > 
> > > I see no problem with giving users the choice for all multiplatform
> > > builds. No idea on what the default should be really, i.e. whether
> > > only v7 configurations should make it the default, or rather none of
> > > them or all of them.
> > 
> > Me neither :)
> > 
> > I'd say let's move forward with letting user select the memory model for
> > all multiplaform builds and keep flatmem the default.
> > 
> > Enabling sparsemem (with SPARSEMEM_STATIC) for defconfig build increases
> > the kernel nearly by 8k and most probably will make things slower.
> 
> I think when selecting the default, you need to consider whether
> platforms can boot with that default configuration or not.
> 
> 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.

> -- 
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> FTTC for 0.8m (est. 1762m) line in suburbia: sync at 13.1Mbps down 424kbps up

-- 
Sincerely yours,
Mike.

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