The blamed commit increased the needed headroom to account for
alignment. This means that the size required to always align a Tx buffer
was added inside the dpaa2_eth_needed_headroom() function. By doing
that, a manual adjustment of the pointer passed to PTR_ALIGN() was no
longer correct since the 'buffer_start' variable was already pointing
to the start of the skb's memory.

The behavior of the dpaa2-eth driver without this patch was to drop
frames on Tx even when the headroom was matching the 128 bytes
necessary. Fix this by removing the manual adjust of 'buffer_start' from
the PTR_MODE call.

Closes: https://lore.kernel.org/netdev/70f0dcd9-1906-4d13-82df-7bbbbe7194c6@app.fastmail.com/T/#u
Fixes: f422abe3f23d ("dpaa2-eth: increase the needed headroom to account for alignment")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Tested-by: Mathew McBride <matt@traverse.com.au>
---
 drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
index c96d1d6ba8fe..18d86badd6ea 100644
--- a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
+++ b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
@@ -1077,8 +1077,7 @@ static int dpaa2_eth_build_single_fd(struct dpaa2_eth_priv *priv,
 	dma_addr_t addr;
 
 	buffer_start = skb->data - dpaa2_eth_needed_headroom(skb);
-	aligned_start = PTR_ALIGN(buffer_start - DPAA2_ETH_TX_BUF_ALIGN,
-				  DPAA2_ETH_TX_BUF_ALIGN);
+	aligned_start = PTR_ALIGN(buffer_start, DPAA2_ETH_TX_BUF_ALIGN);
 	if (aligned_start >= skb->head)
 		buffer_start = aligned_start;
 	else
-- 
2.25.1

On Thu, Oct 16, 2025 at 04:58:07PM +0300, Ioana Ciornei wrote:
> The blamed commit increased the needed headroom to account for
> alignment. This means that the size required to always align a Tx buffer
> was added inside the dpaa2_eth_needed_headroom() function. By doing
> that, a manual adjustment of the pointer passed to PTR_ALIGN() was no
> longer correct since the 'buffer_start' variable was already pointing
> to the start of the skb's memory.
> 
> The behavior of the dpaa2-eth driver without this patch was to drop
> frames on Tx even when the headroom was matching the 128 bytes
> necessary. Fix this by removing the manual adjust of 'buffer_start' from
> the PTR_MODE call.
> 
> Closes: https://lore.kernel.org/netdev/70f0dcd9-1906-4d13-82df-7bbbbe7194c6@app.fastmail.com/T/#u
> Fixes: f422abe3f23d ("dpaa2-eth: increase the needed headroom to account for alignment")
> Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
> Tested-by: Mathew McBride <matt@traverse.com.au>

Reviewed-by: Simon Horman <horms@kernel.org>