Re: [PATCH/RFC] arm64: defconfig: Enlarge CMA alignment to 2 MiB

[Geert Uytterhoeven]

Hi Robin,

On Wed, Feb 1, 2017 at 3:06 PM, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> On 01/02/17 13:45, Magnus Damm wrote:
>> On Sat, Jan 28, 2017 at 1:03 AM, Geert Uytterhoeven
>> <geert+renesas@xxxxxxxxx> wrote:
>>> Some IOMMUs (e.g. Renesas IPMMU/VMSA) support only page sizes of 4 KiB,
>>> 2 MiB, and 1 GiB.
>>>
>>> With the default setting of CONFIG_CMA_ALIGNMENT = 8, allocations larger
>>> than 1 MiB are aligned to a 1 MiB boundary only.  Hence a 2 MiB
>>> allocation may not be aligned, leading to a mapping of 512 4 KiB pages.
>>>
>>> Increase CONFIG_CMA_ALIGNMENT to allow mapping a 2 MiB buffer using a
>>> single PTE, decreasing memory usage and TLB pressure.
>>>
>>> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
>>> ---
>>> Is this useful?
>>>
>>> Should there instead be different defaults in Kconfig, depending on
>>> enabled platform support?
>>
>> I think there is a dependency on the kernel page size configuration as
>> well. In case of 16 KiB or 64 KiB page size configuration other large
>> page sizes may be required.
>
> I don't see the original patch (linux-arm-kernel doesn't look to have it
> archived either), but I think bumping the default up to 2MB to match our

These days linux-arm-kernel (infradead) tends to reject my patches :-(

linux-renesas-soc patchwork does have it:
https://patchwork.kernel.org/patch/9542173/

> normal section size sounds generally reasonable - users can still
> override it manually in their config, right? For 16KB and 64KB granules,
> it is at least the size covered by contiguous-hinted pages; the actual
> section sizes there are far too big to be practical for alignment
> purposes anyway (32MB and 512MB respectively).

Yes, it can be overridden by the user.

One slight annoyance is the dependency on PAGE_SHIFT: if you increase
ARM64_PAGE_SHIFT, you want to decrease CONFIG_CMA_ALIGNMENT.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds