On 01/02/17 13:45, Magnus Damm wrote: > Hi Geert, > > 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 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). Robin. > > Thanks, > > / magnus > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel >
