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

[Parker Newman <parker@finest.io>]

On Wed, 4 Dec 2024 19:06:30 +0100
Thierry Reding <thierry.reding@gmail.com> wrote:

> On Wed, Dec 04, 2024 at 11:53:17AM -0500, Parker Newman wrote:
> > On Wed, 4 Dec 2024 17:23:53 +0100
> > Thierry Reding <thierry.reding@gmail.com> wrote:
> >
> > > On Tue, Nov 19, 2024 at 02:47:29PM -0500, Parker Newman wrote:
> > > > On Tue, 19 Nov 2024 20:18:00 +0100
> > > > Andrew Lunn <andrew@lunn.ch> wrote:
> > > >
> > > > > > I think there is some confusion here. I will try to summarize:
> > > > > > - Ihe iommu is supported by the Tegra SOC.
> > > > > > - The way the mgbe driver is written the iommu DT property is REQUIRED.
> > > > >
> > > > > If it is required, please also include a patch to
> > > > > nvidia,tegra234-mgbe.yaml and make iommus required.
> > > > >
> > > >
> > > > I will add this when I submit a v2 of the patch.
> > > >
> > > > > > - "iommus" is a SOC DT property and is defined in tegra234.dtsi.
> > > > > > - The mgbe device tree nodes in tegra234.dtsi DO have the iommus property.
> > > > > > - There are no device tree changes required to to make this patch work.
> > > > > > - This patch works fine with existing device trees.
> > > > > >
> > > > > > I will add the fallback however in case there is changes made to the iommu
> > > > > > subsystem in the future.
> > > > >
> > > > > I would suggest you make iommus a required property and run the tests
> > > > > over the existing .dts files.
> > > > >
> > > > > I looked at the history of tegra234.dtsi. The ethernet nodes were
> > > > > added in:
> > > > >
> > > > > 610cdf3186bc604961bf04851e300deefd318038
> > > > > Author: Thierry Reding <treding@nvidia.com>
> > > > > Date:   Thu Jul 7 09:48:15 2022 +0200
> > > > >
> > > > >     arm64: tegra: Add MGBE nodes on Tegra234
> > > > >
> > > > > and the iommus property is present. So the requires is safe.
> > > > >
> > > > > Please expand the commit message. It is clear from all the questions
> > > > > and backwards and forwards, it does not provide enough details.
> > > > >
> > > >
> > > > I will add more details when I submit V2.
> > > >
> > > > > I just have one open issue. The code has been like this for over 2
> > > > > years. Why has it only now started crashing?
> > > > >
> > > >
> > > > It is rare for Nvidia Jetson users to use the mainline kernel. Nvidia
> > > > provides a custom kernel package with many out of tree drivers including a
> > > > driver for the mgbe controllers.
> > > >
> > > > Also, while the Orin AGX SOC (tegra234) has 4 instances of the mgbe controller,
> > > > the Nvidia Orin AGX devkit only uses mgbe0. Connect Tech has carrier boards
> > > > that use 2 or more of the mgbe controllers which is why we found the bug.
> > >
> > > Correct. Also, this was a really stupid thing that I overlooked. I don't
> > > recall the exact circumstances, but I vaguely recall there had been
> > > discussions about adding the tegra_dev_iommu_get_stream_id() helper
> > > (that this patch uses) around the time that this driver was created. In
> > > the midst of all of this I likely forgot to update the driver after the
> > > discussions had settled.
> > >
> > > Anyway, I agree with the conclusion that we don't need a compatibility
> > > fallback for this, both because it would be actively wrong to do it and
> > > we've had the required IOMMU properties in device tree since the start,
> > > so there can't be any regressions caused by this.
> > >
> > > I don't think it's necessary to make the iommus property required,
> > > though, because there's technically no requirement for these devices to
> > > be attached to an IOMMU. They usually are, and it's better if they are,
> > > but they should be able to work correctly without an IOMMU.
> > >
> > Thanks for confirming from the Nvidia side! I wasn't sure if they would
> > work without the iommu. That said, if you did NOT want to use the iommu
> > and removed the iommu DT property then the probe will fail after my patch.
> > Would we not need a guard around the writes to MGBE_WRAP_AXI_ASID0_CTRL as well?
>
> Well... frankly, I don't know. There's an override in the memory
> controller which we set when a device is attached to an IOMMU. That's
> usually how the stream ID is programmed. If we don't do that it will
> typically default to a special passthrough stream ID (technically the
> firmware can lock down those register and force them to a specific
> stream ID all the time). I'm not sure what exactly the impact is of
> these ASID registers (there's a few other instances where those are
> needed). They are required if you want to use the IOMMU, but I don't
> know what their meaning is if the IOMMU is not enabled.
>
> There's also different cases: the IOMMU can be disabled altogether, in
> which case no page tables and such exist for any device and no
> translation should happen whatsoever. But there's also the case where an
> IOMMU is enabled, but certain devices shouldn't attach to them. I should
> make it very clear that both of these are not recommended setups and I
> don't know if they'll work. And they are mostly untested. I've been
> meaning, but haven't found the time, to test some of these cases.
>

Yes I agree, all of those situations are very niche and not recommended for
a Tegra platform that should always have the iommu enabled anyways. Adding an
iommu bypass would probably be outside of the scope of this patch.

I will not add a patch marking the iommu as required in the device tree
bindings when I submit v2 unless anyone else feels different.

Thanks again,
Parker