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
