[RFC net-next] net: stmmac: should not modify RX descriptor when STMMAC resume

[Joakim Zhang <qiangqing.zhang@nxp.com>]

When system resume back, STMMAC will clear RX descriptors:
stmmac_resume()
	->stmmac_clear_descriptors()
		->stmmac_clear_rx_descriptors()
			->stmmac_init_rx_desc()
				->dwmac4_set_rx_owner()
				//p->des3 |= cpu_to_le32(RDES3_OWN | RDES3_BUFFER1_VALID_ADDR);
It only assets OWN and BUF1V bits in desc3 field, doesn't clear desc0/1/2 fields.

Let's take a case into account, when system suspend, it is possible that
there are packets have not received yet, so the RX descriptors are wrote
back by DMA, e.g.
008 [0x00000000c4310080]: 0x0 0x40 0x0 0x34010040

When system resume back, after above process, it became a broken
descriptor:
008 [0x00000000c4310080]: 0x0 0x40 0x0 0xb5010040

The issue is that it only changes the owner of this descriptor, but do nothing
about desc0/1/2 fields. The descriptor of STMMAC a bit special, applicaton
prepares RX descriptors for DMA, after DMA recevie the packets, it will write
back the descriptors, so the same field of a descriptor have different
meanings to application and DMA. It should be a software bug there, and may
not easy to reproduce, but there is a certain probability that it will
occur.

Commit 9c63faaa931e ("net: stmmac: re-init rx buffers when mac resume back") tried
to re-init desc0/desc1 (buffer address fields) to fix this issue, but it
is not a proper solution, and made regression on Jetson TX2 boards.

It is unreasonable to modify RX descriptors outside of stmmac_rx_refill() function,
where it will clear all desc0/desc1/desc2/desc3 fields together.

This patch removes RX descriptors modification when STMMAC resume.

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 9f396648d76f..b784304a22e8 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -7186,6 +7186,8 @@ static void stmmac_reset_queues_param(struct stmmac_priv *priv)
 		tx_q->mss = 0;
 
 		netdev_tx_reset_queue(netdev_get_tx_queue(priv->dev, queue));
+
+		stmmac_clear_tx_descriptors(priv, queue);
 	}
 }
 
@@ -7250,7 +7252,6 @@ int stmmac_resume(struct device *dev)
 	stmmac_reset_queues_param(priv);
 
 	stmmac_free_tx_skbufs(priv);
-	stmmac_clear_descriptors(priv);
 
 	stmmac_hw_setup(ndev, false);
 	stmmac_init_coalesce(priv);
-- 
2.17.1