--- Comment #6 from Oleg Endo <olegendo at gcc dot gnu.org> ---
(In reply to Rich Felker from comment #5)
> Of course, fancy memcpy in general is only a win beyond a certain size. For
> DMA I did not mean I want to use DMA for any size beyond gcc's proposed
> function-call threshold. Rather, the vdso-provided function would choose
> what to do appropriately for the hardware. But on J2 (nommu, no special
> kernel mode) I suspect DMA could be a win at sizes as low as 256 bytes, with
> spin-to-completion and a lock shared between user (vdso) and kernel rather
> than using a syscall (not sure this is justified, though). Using a syscall
> with sleep-during-dma would have a significantly larger threshold before
> it's worthwhile.
I see. Anyway, I agree that something like attachment 39642 is useful.
> Regarding how I measured kernel performance increase, I was just looking at
> boot timing with printk timestamps enabled. The main time consumer is
> unpacking initramfs.
Ah right, that sounds like copying memory around. Do you happen to have any
other runtime measurements?