Subject: Re: [PATCH] arm64: mm: make CONFIG_ZONE_DMA configurable without EXPERT

From: Catalin Marinas <[email-masked]>
Date: Tue, 24 Mar 2020 17:41:35 +0000

On Tue, Mar 10, 2020 at 08:48:46PM +0800, [email-masked] wrote:
> From: Peng Fan <[email-masked]>
> 
> commit 1a8e1cef7603 ("arm64: use both ZONE_DMA and ZONE_DMA32")
> enables both ZONE_DMA and ZONE_DMA32. The lower 1GB memory
> will be occupied by ZONE_DMA, this will cause CMA allocation
> fail on some platforms, because CMA area could not across
> different type of memory zones.
> 
> Make CONFIG_ZONE_DMA configurable without EXPERT option
> could let people build non debug kernel image with
> CONFIG_ZONE_DMA disabled.

While I see why you need to toggle this feature, I'd rather try to
figure out whether there is a better solution that does not break the
single kernel image aim (i.e. the same config works for all supported
SoCs).

When we decided to go ahead with a static 1GB ZONE_DMA for Raspberry Pi
4, we thought that other platforms would be fine, ZONE_DMA32 allocations
fall back to ZONE_DMA. We missed the large CMA case.

I see a few potential options:

a) Ensure the CMA is contained within a single zone. How large is it in
   your case? Is it allocated by the kernel dynamically or a fixed start
   set by the boot loader?

b) Change the CMA allocator to allow spanning multiple zones (last time
   I looked it wasn't trivial since it relied on some per-zone lock).

c) Make ZONE_DMA dynamic on arm64 and only enable it if RPi4.

-- 
Catalin

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