linux-kernel - Re: [PATCH v3 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group

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Date:   Thu, 30 Jun 2022 08:47:40 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Robin Murphy <robin.murphy@....com>
CC:     Yong Wu <yong.wu@...iatek.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Baolu Lu <baolu.lu@...ux.intel.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "jordan@...micpenguin.net" <jordan@...micpenguin.net>,
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        <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH v3 1/5] iommu: Return -EMEDIUMTYPE for incompatible
 domain and device/group

On Thu, Jun 30, 2022 at 09:21:42AM +0100, Robin Murphy wrote:
> External email: Use caution opening links or attachments
> 
> 
> On 2022-06-29 20:47, Nicolin Chen wrote:
> > On Fri, Jun 24, 2022 at 03:19:43PM -0300, Jason Gunthorpe wrote:
> > > On Fri, Jun 24, 2022 at 06:35:49PM +0800, Yong Wu wrote:
> > > 
> > > > > > It's not used in VFIO context. "return 0" just satisfy the iommu
> > > > > > framework to go ahead. and yes, here we only allow the shared
> > > > > > "mapping-domain" (All the devices share a domain created
> > > > > > internally).
> > > 
> > > What part of the iommu framework is trying to attach a domain and
> > > wants to see success when the domain was not actually attached ?
> > > 
> > > > > What prevent this driver from being used in VFIO context?
> > > > 
> > > > Nothing prevent this. Just I didn't test.
> > > 
> > > This is why it is wrong to return success here.
> > 
> > Hi Yong, would you or someone you know be able to confirm whether
> > this "return 0" is still a must or not?
> 
> From memory, it is unfortunately required, due to this driver being in
> the rare position of having to support multiple devices in a single
> address space on 32-bit ARM. Since the old ARM DMA code doesn't
> understand groups, the driver sets up its own canonical
> dma_iommu_mapping to act like a default domain, but then has to politely
> say "yeah OK" to arm_setup_iommu_dma_ops() for each device so that they
> do all end up with the right DMA ops rather than dying in screaming
> failure (the ARM code's per-device mappings then get leaked, but we
> can't really do any better).
> 
> The whole mess disappears in the proper default domain conversion, but
> in the meantime, it's still safe to assume that nobody's doing VFIO with
> embedded display/video codec/etc. blocks that don't even have reset drivers.

Thanks for the input! I'll just respin it by dropping mtk_v1 diff.

Nic