Re: [PATCH net] net: stmmac: fix TX descriptor availability check for TSO traffic

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski [off-list ref]:

On Mon, 07 Sep 2026 23:46:45 +0200 you wrote:

stmmac_tso_xmit() estimates the number of free TX descriptors required by
a TSO skb as:

	(skb->len - proto_hdr_len) / TSO_MAX_BUFF_SIZE + 1

which assumes the payload is split into TSO_MAX_BUFF_SIZE chunks. This
underestimates the descriptors actually consumed by stmmac_tso_allocator(),
since each fragment is mapped individually and so it needs at least one
descriptor regardless of its size. Moreover, one descriptor is used for
the L2/L3/L4 headers and, when the MSS changes, one more is consumed for
the MSS context descriptor.
For a highly fragmented TSO skb the check can therefore pass even when the
ring has too few free slots. stmmac_tso_allocator() then writes past the
available descriptors, overwriting descriptors still owned by the DMA
engine, corrupting the TX ring.
Add stmmac_tso_get_num_desc() to compute the exact number of descriptors
needed for the header, the linear payload and each fragment, plus the MSS
context descriptor when required, and use it in the availability check.

[...]

Here is the summary with links:
  - [net] net: stmmac: fix TX descriptor availability check for TSO traffic
    https://git.kernel.org/netdev/net/c/5e38d732ec67

You are awesome, thank you!
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