net: lan966x: add PCIe FDMA MTU change support

Add MTU change support for the PCIe FDMA path: on an MTU change, the
contiguous ATU-mapped RX and TX buffers are reallocated at the new
size, falling back to the existing buffers on failure.

Cap the PCIe DCB ring at 256 (FDMA_PCI_DCB_MAX): 512 DCBs would
overflow MAX_PAGE_ORDER at jumbo MTU.

The ring must fit one MAX_PAGE_ORDER block after ATU padding, and
db_size is handed to the FDMA in the 16-bit DCB DATAL field.
Advertise the resulting limit in dev->max_mtu (FDMA_PCI_MAX_MTU)
when the FDMA is in use - a switch with no "fdma" interrupt named
uses the unconstrained register-based path instead, so the cap is
gated on lan966x->fdma, not lan966x_is_pci() alone. On a 4KB-page,
MAX_PAGE_ORDER=10 build this is MTU 15498; the overhead is shared
with lan966x_fdma_get_max_frame() via FDMA_OVERHEAD.

Skip the resize until lan966x_fdma_pci_init() has built the rings;
it runs after the netdevs register and sizes them from
DEV_MAC_MAXLEN_CFG, already programmed by the caller.

Tested-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260928-lan966x-pci-fdma-v9-12-a17caf60f89a@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions