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

Thierry Reding
2025-01-27T13:29:02Z

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On Sat, Jan 25, 2025 at 11:03:47PM +0800, Furong Xu wrote:
> Hi Thierry
>
> On Sat, 25 Jan 2025 12:20:38 +0200, Ido Schimmel wrote:
>
> > 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.
> >
> > Given the problem happens when the kernel performs syncing, is it
> > possible that there is a problem with how the syncing is performed?
> >
> > I am not familiar with this driver, but it seems to allocate multiple
> > buffers per packet when split header is enabled and these buffers are
> > allocated from the same page pool (see stmmac_init_rx_buffers()).
> > Despite that, the driver is creating the page pool with a non-zero
> > offset (see __alloc_dma_rx_desc_resources()) to avoid syncing the
> > headroom, which is only present in the head buffer.
> >
> > I asked Thierry to test the following patch [1] and initial testing
> > seems OK. He also confirmed that "SPH feature enabled" shows up in the
> > kernel log.
>
> It is recommended to disable the "SPH feature" by default unless some
> certain cases depend on it. Like Ido said, two large buffers being
> allocated from the same page pool for each packet, this is a huge waste
> of memory, and brings performance drops for most of general cases.
>
> Our downstream driver and two mainline drivers disable SPH by default:
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/ethernet/stmicro/stmmac/dwmac-dwc-qos-eth.c#n357
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/ethernet/stmicro/stmmac/dwmac-intel.c#n471

Okay, that's something we can look into changing. What would be an
example of a use-case depending on SPH? Also, isn't this something
that should be a policy that users can configure?

Irrespective of that we should fix the problems we are seeing with
SPH enabled.

Thierry

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