net: macb: never give hardware a NULL RX buffer

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The refill logic is simple: iterate over all pending rx slots, allocate
SKB (& DMA map) if needed and hand it off to the hardware by clearing
the RX_USED flag.

If the refill operation fails mid-way, it early returns leaving the
remaining slots untouched. In an initialised ring that is safe: a slot
is either owned by the hardware holding a valid buffer, or
software-owned (RX_USED set) waiting for refill to hand it a new one.
When slots have never been initialised however, we are in trouble.

After dma_alloc_coherent() of the rx ring buffer, all slots have NULL
pointers and RX_USED cleared meaning HW will try using them. Ensure
this does not happen by setting the RX_USED flag on all slots before
calling refill at buffer alloc, in gem_init_rx_ring(). That way even if
refill fails on an alloc/dma_map, the HW won't try using NULL pointers
as buffers.

Note we tweak gem_rx() and macb_rx_pending(): their previous stop
condition was only if desc was RX_USED. Now it must be either RX_USED
or we got out of the range of successfully allocated descriptors
(detected using the rx_tail and rx_prepared_head cursors).
Otherwise gem_rx() could consume unallocated buffers.

Theoretical bugfix, never encountered in practice. To reproduce,
introduce memory pressure (less than 512 SKBs of free memory) and open
the interface.

Note that this codepath also hits at resume, on HRESP errors and on
set_ringparam (while interface is running).

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Link: https://patch.msgid.link/20260925-macb-close-v2-1-360efa565914@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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This commit is contained in:

Théo Lebrun 2026-09-25 15:59:34 +02:00 • committed by Jakub Kicinski

parent 91abd44838

commit efd90b64ce

1 changed files with 16 additions and 0 deletions

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drivers/net/ethernet/cadence/macb_main.c

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