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

[Nicolas Saenz Julienne]

Hi Peng,

On Wed, 2020-03-25 at 12:30 +0000, Peng Fan wrote:
> > > > 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).

I like this as a solution, ultimately why should CMA be linked to a specific
zone. Also, crossing the ZONE_DMA/ZONE_DMA32 boundary shouldn't be an issue
since we already default to ZONE_DMA32 as the default area for CMA.

That said, easier said than done.

> > > > 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://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.ke
> > rnel.org%2Flinux-arm-kernel%2F20190820145821.27214-5-nsaenzjulienne%
> > 40suse.de%2F&data=02%7C01%7Cpeng.fan%40nxp.com%7C6403ddf37
> > 89b452ae5ee08d7d0a5a659%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0
> > %7C0%7C637207282191026738&sdata=t2cZ9HTCcRuaL9RO4kD%2BzN
> > 2n4VqM%2F66zYNZIOComCVs%3D&reserved=0
> > 
> > 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.
> 
> Thanks for the information. I'll check the patchset and work out something
> proper to fix the issue I met.

Just an FYI, I'm follwing the conversation, and will try to look into as soon
as I get some free time. In the meantime if you want me to test/review anything
on the Raspberry Pi, I'll be happy to.

Regards,
Nicolas

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