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Date:   Wed, 07 Sep 2022 13:14:54 +0100
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From:   Marc Zyngier <maz@kernel.org>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Jason Gunthorpe <jgg@nvidia.com>,
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Subject: Re: [RFC PATCH v3 4/7] bus/cdx: add cdx-MSI domain with gic-its domain as parent
In-Reply-To: <4ca6383e-bd21-59bf-cc4e-cf3313164957@arm.com>
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        <4ca6383e-bd21-59bf-cc4e-cf3313164957@arm.com>
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On Wed, 07 Sep 2022 12:33:12 +0100,
Robin Murphy <robin.murphy@arm.com> wrote:
> 
> On 2022-09-07 12:17, Marc Zyngier wrote:
> > On Tue, 06 Sep 2022 18:19:06 +0100,
> > Jason Gunthorpe <jgg@nvidia.com> wrote:
> >> 
> >> On Tue, Sep 06, 2022 at 07:17:58PM +0530, Nipun Gupta wrote:
> >> 
> >>> +static void cdx_msi_write_msg(struct irq_data *irq_data,
> >>> +			      struct msi_msg *msg)
> >>> +{
> >>> +	/*
> >>> +	 * Do nothing as CDX devices have these pre-populated
> >>> +	 * in the hardware itself.
> >>> +	 */
> >>> +}
> >> 
> >> Huh?
> >> 
> >> There is no way it can be pre-populated, the addr/data pair,
> >> especially on ARM, is completely under SW control.
> > 
> > There is nothing in the GIC spec that says that.
> > 
> >> There is some commonly used IOVA base in Linux for the ITS page, but
> >> no HW should hardwire that.
> > 
> > That's not strictly true. It really depends on how this block is
> > integrated, and there is a number of existing blocks that know *in HW*
> > how to signal an LPI.
> > 
> > See, as the canonical example, how the mbigen driver doesn't need to
> > know about the address of GITS_TRANSLATER.
> > 
> > Yes, this messes with translation (the access is downstream of the
> > SMMU) if you relied on it to have some isolation, and it has a "black
> > hole" effect as nobody can have an IOVA that overlaps with the
> > physical address of the GITS_TRANSLATER register.
> > 
> > But is it illegal as per the architecture? No. It's just stupid.
> 
> If that were the case, then we'd also need a platform quirk so the
> SMMU driver knows about it. Yuck.

Yup. As I said, this is stupid.

> But even then, are you suggesting there is some way to convince the
> ITS driver to allocate a specific predetermined EventID when a driver
> requests an MSI? Asking for a friend...

Of course not. Whoever did that has decided to hardcode the Linux
behaviour into the HW, because it is well known that SW behaviour
never changes. Nononono.

I am >this< tempted to sneak a change into the allocation scheme to
start at 5 or 13 (alternatively), and to map LPIs top-down. That
should get people thinking.

Cheers,

	M.

-- 
Without deviation from the norm, progress is not possible.