Messages in this thread | | | Date | Wed, 25 Mar 2020 10:16:52 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH] arm64: mm: make CONFIG_ZONE_DMA configurable without EXPERT |
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On Wed, Mar 25, 2020 at 12:34:15AM +0000, Peng Fan wrote: > > On Tue, Mar 10, 2020 at 08:48:46PM +0800, peng.fan@nxp.com wrote: > > > From: Peng Fan <peng.fan@nxp.com> > > > > > > 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. > > This will break legacy dts with new version kernel. > > > How large is it in your case? > > It is 1GB > > > Is it allocated by the kernel dynamically or a fixed start set by > > the boot loader? > > We use alloc-ranges and size in kernel dts. > > But there is only 2GB DRAM in the board.
So I guess without changing the dts, option (a) doesn't really work.
> > 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. > > Option c seems a bit easier to me :) > > I will try to explore both, but if you have time to help, that would be > appreciated.
I don't have time but option (c) was already discussed and there are patches from Nicolas on the list:
https://lore.kernel.org/linux-arm-kernel/20190820145821.27214-5-nsaenzjulienne@suse.de/
The above series was checking whether the platform is RPi4 and limiting the ZONE_DMA size to 1GB (otherwise 4GB with ZONE_DMA32 empty). We ended up with a static 1GB for ZONE_DMA but we missed the fact that it may break existing platforms.
So I don't think it would be too hard to revive the above series (most of it was already merged).
-- Catalin
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