Re: [PATCH net-next v3 1/4] net: stmmac: Switch to zero-copy in non-XDP RX path

Thierry Reding
2025-01-24T13:16:02Z

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On Fri, Jan 24, 2025 at 10:42:56AM +0800, Furong Xu wrote:
> On Thu, 23 Jan 2025 22:48:42 +0100, Andrew Lunn <andrew@lunn.ch> wrote:
>
> > > Just to clarify, the patch that you had us try was not intended as an actual
> > > fix, correct? It was only for diagnostic purposes, i.e. to see if there is
> > > some kind of cache coherence issue, which seems to be the case? So perhaps
> > > the only fix needed is to add dma-coherent to our device tree?
> >
> > That sounds quite error prone. How many other DT blobs are missing the
> > property? If the memory should be coherent, i would expect the driver
> > to allocate coherent memory. Or the driver needs to handle
> > non-coherent memory and add the necessary flush/invalidates etc.
>
> stmmac driver does the necessary cache flush/invalidates to maintain cache lines
> explicitly.
>
> See dma_sync_single_for_cpu():
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/include/linux/dma-mapping.h#n297
>
> dma_dev_need_sync() is supposed to return false for Tegra234, since the ethernet
> controller on Tegra234 is dma coherent by SoC design as Brad said their
> downstream device tree has dma-coherent turned on by default, and after add
> dma-coherent to mainline ethernet node, stmmac driver works fine.
> But dma-coherent property is missing in mainline Tegra234 ethernet device tree
> node, dma_dev_need_sync() returns true and this is not the expected behavior.

My understanding is that the Ethernet controller itself is not DMA
coherent. Instead, it is by accessing memory through the IOMMU that the
accesses become coherent. It's technically possible for the IOMMU to be
disabled via command-line, or simply compile out the driver for it, and
the devices are supposed to keep working with it (though it's not a
recommended configuration), so exclusively relying on the presence of
an IOMMU node or even a dma-coherent property is bound to fail in these
corner cases. We don't currently have a good way of describing this in
device tree, but a discussion with the DT maintainers is probably
warranted.

> The dma-coherent property in device tree node is SoC specific, so only the
> vendors know if their stmmac ethernet controller is dma coherent and
> whether their device tree are missing the critical dma-coherent property.

What I fail to understand is how dma-coherent can make a difference in
this case. If it's not present, then the driver is supposed to maintain
caches explicitly. But it seems like explicit cache maintenance actually
causes things to break. So do we need to assume that DMA coherent
devices in generally won't work if the driver manages caches explicitly?

I always figured dma-coherent was more of an optimization, but this
seems to indicate it isn't.

Thierry

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