Re: [PATCH 08/10] iommu/tegra: Use tegra_dev_iommu_get_stream_id() in the remaining places

From: Jason Gunthorpe <jgg@nvidia.com>
To: Thierry Reding <thierry.reding@gmail.com>
Cc: David Airlie <airlied@gmail.com>,
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Subject: Re: [PATCH 08/10] iommu/tegra: Use tegra_dev_iommu_get_stream_id() in the remaining places
Date: Wed, 29 Nov 2023 15:26:03 -0400	[thread overview]
Message-ID: <20231129192603.GA1387263@nvidia.com> (raw)
In-Reply-To: <ZWdlcboM4Xzs38NI@orome.fritz.box>

On Wed, Nov 29, 2023 at 05:23:13PM +0100, Thierry Reding wrote:
> > diff --git a/drivers/memory/tegra/tegra186.c b/drivers/memory/tegra/tegra186.c
> > index 533f85a4b2bdb7..3e4fbe94dd666e 100644
> > --- a/drivers/memory/tegra/tegra186.c
> > +++ b/drivers/memory/tegra/tegra186.c
> > @@ -111,21 +111,21 @@ static void tegra186_mc_client_sid_override(struct tegra_mc *mc,
> >  static int tegra186_mc_probe_device(struct tegra_mc *mc, struct device *dev)
> >  {
> >  #if IS_ENABLED(CONFIG_IOMMU_API)
> > -	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev);
> >  	struct of_phandle_args args;
> >  	unsigned int i, index = 0;
> > +	u32 sid;
> >  
> > +	WARN_ON(!tegra_dev_iommu_get_stream_id(dev, &sid));
> 
> I know the code previously didn't check for any errors, but we may want
> to do so now. If tegra_dev_iommu_get_stream_id() ever fails we may end
> up writing some undefined value into the override register.

My assumption was it never fails otherwise this probably already
doesn't work?

> I'm also unsure if WARN_ON() is appropriate here. I vaguely recall that
> ->probe_device() was called for all devices on the bus and not all of
> them may have been associated with the IOMMU. Not all of them may in
> fact access memory in the first place.

So you are thinkin that of_parse_phandle_with_args() is a NOP
sometimes so it will tolerate the failure?

Seems like the best thing to do is just continue to ignore it then?

> Perhaps I'm misremembering and the IOMMU core now takes care of only
> calling this when fwspec is indeed valid?

Can't advise, I have no idea what tegra_mc_ops is for :)

Jason