linux-kernel - Re: [PATCH v1 1/1] net: stmmac: dwmac-tegra: Read iommu stream id from device tree

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Message-ID: <20241118084400.35f4697a.parker@finest.io>
Date: Mon, 18 Nov 2024 08:44:00 -0500
From: Parker Newman <parker@...est.io>
To: Andrew Lunn <andrew@...n.ch>
Cc: Paolo Abeni <pabeni@...hat.com>, Alexandre Torgue
 <alexandre.torgue@...s.st.com>, Jose Abreu <joabreu@...opsys.com>, Andrew
 Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Maxime
 Coquelin <mcoquelin.stm32@...il.com>, Thierry Reding
 <thierry.reding@...il.com>, Jonathan Hunter <jonathanh@...dia.com>,
 netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
 linux-arm-kernel@...ts.infradead.org, linux-tegra@...r.kernel.org,
 linux-kernel@...r.kernel.org, Parker Newman <pnewman@...necttech.com>
Subject: Re: [PATCH v1 1/1] net: stmmac: dwmac-tegra: Read iommu stream id
 from device tree

On Sat, 16 Nov 2024 20:22:53 +0100
Andrew Lunn <andrew@...n.ch> wrote:

> On Fri, Nov 15, 2024 at 01:59:40PM -0500, Parker Newman wrote:
> > On Fri, 15 Nov 2024 18:17:07 +0100
> > Paolo Abeni <pabeni@...hat.com> wrote:
> >
> > > On 11/15/24 17:31, Parker Newman wrote:
> > > > From: Parker Newman <pnewman@...necttech.com>
> > > >
> > > > Read the iommu stream id from device tree rather than hard coding to mgbe0.
> > > > Fixes kernel panics when using mgbe controllers other than mgbe0.
> > >
> > > It's better to include the full Oops backtrace, possibly decoded.
> > >
> >
> > Will do, there are many different ones but I can add the most common.
> >
> > > > Tested with Orin AGX 64GB module on Connect Tech Forge carrier board.
> > >
> > > Since this looks like a fix, you should include a suitable 'Fixes' tag
> > > here, and specify the 'net' target tree in the subj prefix.
> > >
> >
> > Sorry I missed the "net" tag.
> >
> > The bug has existed since dwmac-tegra.c was added. I can add a Fixes tag but
> > in the past I was told they aren't needed in that situation?
> >
> > > > @@ -241,6 +243,12 @@ static int tegra_mgbe_probe(struct platform_device *pdev)
> > > >  	if (IS_ERR(mgbe->xpcs))
> > > >  		return PTR_ERR(mgbe->xpcs);
> > > >
> > > > +	/* get controller's stream id from iommu property in device tree */
> > > > +	if (!tegra_dev_iommu_get_stream_id(mgbe->dev, &mgbe->iommu_sid)) {
> > > > +		dev_err(mgbe->dev, "failed to get iommu stream id\n");
> > > > +		return -EINVAL;
> > > > +	}
> > >
> > > I *think* it would be better to fallback (possibly with a warning or
> > > notice) to the previous default value when the device tree property is
> > > not available, to avoid regressions.
> > >
> >
> > I debated this as well... In theory the iommu must be setup for the
> > mgbe controller to work anyways. Doing it this way means the worst case is
> > probe() fails and you lose an ethernet port.
>
> New DT properties are always optional. Take the example of a board
> only using a single controller. It should happily work. It probably
> does not have this property because it is not needed. Your change is
> likely to cause a regression on such a board.
>
> Also, is a binding patch needed?
>
> 	Andrew

This is not a new dt property, the "iommus" property is an existing property
that is parsed by the Nvidia implementation of the arm-smmu driver.

Here is a snippet from the device tree:

smmu_niso0: iommu@...00000 {
        compatible = "nvidia,tegra234-smmu", "nvidia,smmu-500";
...
}

/* MGBE0 */
ethernet@...0000 {
	compatible = "nvidia,tegra234-mgbe";
...
	iommus = <&smmu_niso0 TEGRA234_SID_MGBE>;
...
}

/* MGBE1 */
ethernet@...0000 {
	compatible = "nvidia,tegra234-mgbe";
...
	iommus = <&smmu_niso0 TEGRA234_SID_MGBE_VF1>;
...
}

The 2nd field of the iommus propert is the "Stream ID" which arm-smmu stores
in the device's struct iommu_fwspec *fwspec. This is what the existing
tegra_dev_iommu_get_stream_id() function uses to get the SID.

If the iommus property is missing completely from the MGBE's device tree node it
causes secure read/write errors which spam the kernel log and can cause crashes.

I can add the fallback in V2 with a warning if that is preferred.

Thanks,
Parker