[PATCH net-next 1/2] net: stmmac: fix XDP TX DMA address for compound RX pages

[Lorenzo Bianconi]

When RX buffers are compound pages (order > 0), only the head page
carries the DMA address programmed by the page pool
(page_pool_get_dma_addr()). stmmac_xdp_xmit_xdpf() computed the head
buffer's bus address from the page that happens to contain the data:

	page = frag ? skb_frag_page(frag)
		    : virt_to_page(xdpf->data);
	dma_addr = page_pool_get_dma_addr(page);
	...

RX pages become compound as soon as dma_buf_sz needs more than one page
(pp_params.order = order_base_2(num_pages) > 0), which happens from an
MTU around 3000 bytes once dma_buf_sz reaches 4096. If an XDP program
moves the data forward with bpf_xdp_adjust_head() by more than about a
page, xdpf->data points into a tail page whose struct page has no DMA
address: page_pool_get_dma_addr() then returns a bogus bus address, and
dma_sync_single_for_device() together with the TX descriptor transmit
from the wrong location.

Use virt_to_head_page() to read the DMA base of the buffer and keep the
offset relative to the head page (sizeof(*xdpf) + xdpf->headroom), which
is correct no matter which page the data ends up in.

Fixes: be8b38a722e6 ("net: stmmac: Add support for XDP_TX action")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi at oss.qualcomm.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index b8b4de4c5b1a..ca61ab449e32 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -5301,7 +5301,7 @@ static int stmmac_xdp_xmit_xdpf(struct stmmac_priv *priv, int queue,
 			struct page *page;
 
 			page = frag ? skb_frag_page(frag)
-				    : virt_to_page(xdpf->data);
+				    : virt_to_head_page(xdpf->data);
 			dma_addr = page_pool_get_dma_addr(page);
 			if (frag)
 				dma_addr += skb_frag_off(frag);

-- 
2.56.0