Re: [PATCH net-next v7 10/14] net: lan966x: add PCIe FDMA support

[Simon Horman <horms@kernel.org>]

On Wed, Sep 23, 2026 at 10:11:55PM +0200, Daniel Machon wrote:
> > On Wed, Sep 23, 2026 at 10:45:38AM +0200, Daniel Machon wrote:
> > 
> > ...
> > 
> > > > [Severity: Medium]
> > > > Is this path expected to work on a big-endian host?
> > > >
> > > > Until now the descriptor producer and the FDMA engine were both on the
> > > > little-endian SoC, so the native-endian descriptor layout in
> > > > drivers/net/ethernet/microchip/fdma/fdma_api.h was fine:
> > > >
> > > >   struct fdma_db {
> > > >         u64 dataptr;
> > > >         u64 status;
> > > >   };
> > > >
> > > >   struct fdma_dcb {
> > > >         u64 nextptr;
> > > >         u64 info;
> > > >         struct fdma_db db[FDMA_DB_MAX];
> > > >   };
> > > >
> > > > With PCIe FDMA the descriptors live in host memory and are fetched by the
> > > > endpoint, so the host and the engine can disagree on byte order. The
> > > > translated addresses written here, and the status/info words written by
> > > > __fdma_db_add()/__fdma_dcb_add(), all go out in host byte order, and the
> > > > HW-written DONE/BLOCKL fields are read back the same way in
> > > > lan966x_fdma_pci_get_next_dcb() and lan966x_fdma_pci_rx_check_frame().
> > > >
> > > > CONFIG_MCHP_LAN966X_PCI in drivers/misc/Kconfig only has
> > > >
> > > >   depends on PCI
> > > >   depends on OF_OVERLAY
> > > >
> > > > so a big-endian host (e.g. powerpc) can select it. On such a host, would
> > > > the engine see byte-swapped nextptr/dataptr and length fields, i.e. would
> > > > RX/TX not work at all? Should the descriptor fields become explicit
> > > > little-endian types with accessors at the hardware boundary, or should the
> > > > Kconfig entry gain a little-endian dependency?
> > >
> > > Hmm. Never tested on PowerPC, not this driver nor the DT overlay mechanism it
> > > relies on for PCI deployment, which has only been exercised on ARM and x86 so
> > > far (AFAICT) [1].
> > >
> > > [1] https://lore.kernel.org/all/20240430183301.46568e35@bootlin.com/
> > 
> > I don't think that the baseline expectation is that drivers are tested
> > on big-endian systems, but rather that reasonable effort is made to ensure
> > that byte-order usage is correct.
> > 
> > In this case I suggest that will involve using __le64 instead of u64,
> > coupled with calling le64_to_cpu() and cpu_to_le64() as appropriate.
> > 
> > Sparse can aid with getting this right.
> 
> Ack.
> 
> I am a little reluctant, when I cant really test it properly. Sashiko gives
> another solution - adding a LE dependency to the Kconfig of the misc PCI driver,
> maybe that'll do?

I think that should be a last resort. I won't hold out on this,
but I would prefer the approach I suggested.

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