Re: [PATCH net v2] net: fec: reject oversized fragments before bounce-buffer memcpy

On Wed, Sep 23, 2026 at 06:22:01PM -0300, Aldo Ariel wrote:

Hi Andrew,

PKT_MAXBUF_SIZE = round_down(2048 - 64, 64) = 1984
max_mtu (non-jumbo) = 1984 - VLAN_ETH_HLEN - ETH_FCS_LEN = 1962

So for standard MTU, max_mtu < FEC_ENET_TX_FRSIZE and individual
fragments should not exceed the bounce buffer.

However, when the device supports jumbo frames,
max_buf_size = MAX_JUMBO_BUF_SIZE (~16256) and max_mtu reaches ~16234.
Scatter-gather fragments in that case can exceed FEC_ENET_TX_FRSIZE
while still needing the bounce path for alignment or byte-swap.

Should I respin with a check only for the jumbo path, or is the
unconditional guard acceptable?

Can bounce buffers and jumbo be used at the same time?

How is the scatter-gather size determined? Maybe the better fix is to
set the fragment size based on the size of the bounce buffer when it
is in use?

	Andrew