On Tue, May 20, 2025 at 02:06:09PM +0100, Robin Murphy wrote:
> On 2025-05-20 12:31 pm, Will Deacon wrote:
> > On Thu, May 15, 2025 at 07:48:39AM -0700, Rob Clark wrote:
> > > On Thu, May 15, 2025 at 7:33 AM Will Deacon <w...@kernel.org> wrote:
> > > > On Wed, May 14, 2025 at 10:53:19AM -0700, Rob Clark wrote:
> > > > > diff --git a/include/linux/io-pgtable.h b/include/linux/io-pgtable.h
> > > > > index bba2a51c87d2..639b8f4fb87d 100644
> > > > > --- a/include/linux/io-pgtable.h
> > > > > +++ b/include/linux/io-pgtable.h
> > > > > @@ -88,6 +88,13 @@ struct io_pgtable_cfg {
> > > > > *
> > > > > * IO_PGTABLE_QUIRK_ARM_HD: Enables dirty tracking in stage 1
> > > > > pagetable.
> > > > > * IO_PGTABLE_QUIRK_ARM_S2FWB: Use the FWB format for the
> > > > > MemAttrs bits
> > > > > + *
> > > > > + * IO_PGTABLE_QUIRK_NO_WARN_ON: Do not WARN_ON() on conflicting
> > > > > + * mappings, but silently return -EEXISTS. Normally an
> > > > > attempt
> > > > > + * to map over an existing mapping would indicate some
> > > > > sort of
> > > > > + * kernel bug, which would justify the WARN_ON(). But for
> > > > > GPU
> > > > > + * drivers, this could be under control of userspace.
> > > > > Which
> > > > > + * deserves an error return, but not to spam dmesg.
> > > > > */
> > > > > #define IO_PGTABLE_QUIRK_ARM_NS BIT(0)
> > > > > #define IO_PGTABLE_QUIRK_NO_PERMS BIT(1)
> > > > > @@ -97,6 +104,7 @@ struct io_pgtable_cfg {
> > > > > #define IO_PGTABLE_QUIRK_ARM_OUTER_WBWA BIT(6)
> > > > > #define IO_PGTABLE_QUIRK_ARM_HD BIT(7)
> > > > > #define IO_PGTABLE_QUIRK_ARM_S2FWB BIT(8)
> > > > > + #define IO_PGTABLE_QUIRK_NO_WARN_ON BIT(9)
> > > >
> > > > This feels a bit fragile to me:
> > > > * IOMMU-API users of io-pgtable shouldn't be passing this quirk
> > > > but might end up doing so to paper over driver bugs.
> > > >
> > > > * Low-level users of io-pgtable who expose page-table operations to
> > > > userspace need to pass the quirk, but might well not bother because
> > > > well-behaved userspace doesn't trigger the warning.
> > > >
> > > > So overall, it's all a bit unsatisfactory. Is there a way we could have
> > > > the warnings only when invoked via the IOMMU API?
> > >
> > > iommu drivers _not_ setting this flag seems like a good way to achieve
> > > that ;-)
> > >
> > > The alternative is to move the warns to the iommu driver... but they
> > > could just as easily remove the WARN_ON()s as they could set the
> > > NO_WARN_ON quirk, so :shrug:?
> >
> > Bah, I also don't have a good idea to improve this, so I guess I'll take
> > what you have for now.
>
> Hmm, just a nit on reflection, how about fixing up the name to just
> IO_PGTABLE_QUIRK_NO_WARN? Given that it's already quite long, and we have a
> well-established DMA_ATTR_NO_WARN with equivalent semantics over in the DMA
> API.
Sure, I'll do that now...
Will
