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

[Will Deacon]

On Mon, Feb 13, 2017 at 12:04:18PM +0100, Geert Uytterhoeven 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 
> ---
> Is this useful?

I assume you're proposing it because you see an improvement? :)

> Should there instead be different defaults in Kconfig, depending on
> enabled platform support?

I don't object to updating defconfig as a quick hack, but the right solution
is probably to make the core Kconfig default value overridable by the
architecture.

Will

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

[Geert Uytterhoeven]

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 
---
Is this useful?

Should there instead be different defaults in Kconfig, depending on
enabled platform support?
---
 arch/arm64/configs/defconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/configs/defconfig b/arch/arm64/configs/defconfig
index 7735cfba01c8e650..7af449f3f41435bc 100644
--- a/arch/arm64/configs/defconfig
+++ b/arch/arm64/configs/defconfig
@@ -144,6 +144,7 @@ CONFIG_UEVENT_HELPER_PATH="/sbin/hotplug"
 CONFIG_DEVTMPFS=y
 CONFIG_DEVTMPFS_MOUNT=y
 CONFIG_DMA_CMA=y
+CONFIG_CMA_ALIGNMENT=9
 CONFIG_MTD=y
 CONFIG_MTD_BLOCK=y
 CONFIG_MTD_M25P80=y
-- 
1.9.1