Re: [PATCH V2 5/5] DO NOT MERGE: iommu: disable list appending in dma-iommu

[Christoph Hellwig]

On Tue, Sep 08, 2020 at 06:36:19AM +0100, Christoph Hellwig wrote:
> On Mon, Sep 07, 2020 at 09:18:50PM +0100, Tom Murphy wrote:
> > Yeah we talked about passing an attr to map_sg to disable merging at
> > the following microconfernce:
> > https://linuxplumbersconf.org/event/7/contributions/846/
> > As far as I can remember everyone seemed happy with that solution. I
> > won't be working on this though as I don't have any more time to
> > dedicate to this. It seems Lu Baolu will take over this.
> 
> I'm absolutely again passing a flag.  Tha just invites further
> abuse.  We need a PCI ID based quirk or something else that can't
> be as easily abused.

Also, I looked at i915 and there are just three dma_map_sg callers.
The two dmabuf related ones are fixed by Marek in his series, leaving
just the one in i915_gem_gtt_prepare_pages, which does indeed look
very fishy.  But if that one is so hard to fix it can just be replaced
by an open coded for_each_sg loop that contains manual dma_map_page
calls.