On Mi, 2020-02-12 at 08:50 +0000, Russell King - ARM Linux admin wrote:

On Tue, Feb 11, 2020 at 05:03:02PM -0800, Linus Torvalds wrote:

quoted

On Tue, Feb 11, 2020 at 4:47 PM Andrew Morton [off-list ref] wrote:

quoted

What's the situation with highmem on ARM?

Afaik it's exactly the same as highmem on x86 - only 32-bit ARM ever
needed it, and I was ranting at some people for repeating all the
mistakes Intel did.

But arm64 doesn't need it, and while 32-bit arm is obviosuly still
selling, I think that in many ways the switch-over to 64-bit has been
quicker on ARM than it was on x86. Partly because it happened later
(so all the 64-bit teething pains were dealt with), but largely
because everybody ended up actively discouraging 32-bit on the Android
side.

There were a couple of unfortunate early 32-bit arm server attempts,
but they were - predictably - complete garbage and nobody bought them.
They don't exist any more.

So at least my gut feel is that the arm people don't have any big
reason to push for maintaining HIGHMEM support either.

But I'm adding a couple of arm people and the arm list just in case
they have some input.

[ Obvious background for newly added people: we're talking about
making CONFIG_HIGHMEM a deprecated feature and saying that if you want
to run with lots of memory on a 32-bit kernel, you're doing legacy
stuff and can use a legacy kernel ]

Well, the recent 32-bit ARM systems generally have more than 1G
of memory, so make use of highmem as a rule.  You're probably
talking about crippling support for any 32-bit ARM system produced
in the last 8 to 10 years.

I know of quite a few embedded ARMv7 systems that will need to be
supported for at least 10 years from now, with RAM sizes between 1GB
and even the full 4GB (well 3.75GB due to MMIO needing some address
space). Deprecating highmem would severely cripple our ability to
support those platforms with new kernels.

Regards,
Lucas


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