Re: [PATCH] sparc: use asm-generic/scatterlist.h

From: FUJITA Tomonori
Date: Tue Mar 02 2010 - 09:06:49 EST

On Tue, 02 Mar 2010 05:55:34 -0800 (PST)
David Miller <davem@xxxxxxxxxxxxx> wrote:

> From: Arnd Bergmann <arnd@xxxxxxxx>
> Date: Tue, 2 Mar 2010 14:54:11 +0100
> > Broken in the sense that arch/powerpc/Kconfig describes CONFIG_IOMMU_VMERGE:
> >
> > Cause IO segments sent to a device for DMA to be merged virtually
> > by the IOMMU when they happen to have been allocated contiguously.
> > This doesn't add pressure to the IOMMU allocator. However, some
> > drivers don't support getting large merged segments coming back
> > from *_map_sg().
> >
> > Most drivers don't have this problem; it is safe to say Y here.
> >
> > I don't know if this comment still applies to any drivers in the mainline
> > kernel, but it's possible.
> That really has to be out of date these days.

Yeah, I think so.

I added dma_get_max_seg_size() several years ago so that device drives
can tell IOMMU about the maximum segment length that they can
handle. And the default limit (64K) should work for everyone, I think.

I guess that the comment was written when IOMMU was able to merge as
many segments as possible with ignoring the device limitation.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at