From mboxrd@z Thu Jan  1 00:00:00 1970
From: Benjamin Herrenschmidt <benh-XVmvHMARGAS8U2dJNN8I7kB+6BGkLq7r@public.gmane.org>
Subject: Re: [PATCH 01/20] kernel/dma/direct: take DMA offset into account
	in dma_direct_supported
Date: Thu, 23 Aug 2018 15:24:48 +1000
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To: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
Cc: Fenghua Yu <fenghua.yu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>, Tony Luck <tony.luck-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>, linux-ia64-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Konrad Rzeszutek Wilk <konrad.wilk-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>, Michael Ellerman <mpe-Gsx/Oe8HsFggBc27wqDAHg@public.gmane.org>, linuxppc-dev-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Paul Mackerras <paulus-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org>, Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>
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On Thu, 2018-08-23 at 07:24 +0200, Christoph Hellwig wrote:
> > Well, iommus can have bypass regions, which we also use for
> > performance, so we do at dma_set_mask() time "swap" the ops around, and
> > in that case, we do want to check the mask against the actual top of
> > memory...
> 
> That is a bit of a powerpc special case (we also had one other arch
> doing that, but it got removed in the great purge, can't rember which
> one right now).  Everyone else has one set of ops, and they just switch
> to the direct mapping inside the iommu ops.

We more or less do that too in some of ours these days bcs of the whole
coherent_mask vs mask where a given device might need either depending
on the type of mapping.

Ben.