Now for dwmac-loongson {tx,rx}_fifo_size are uninitialised, which means
zero. This means dwmac-loongson doesn't support changing MTU because in
stmmac_change_mtu() it requires the fifo size be no less than MTU. Thus,
set the correct tx_fifo_size and rx_fifo_size for it (16KB multiplied by
queue counts).

Here {tx,rx}_fifo_size is initialised with the initial value (also the
maximum value) of {tx,rx}_queues_to_use. So it will keep as 16KB if we
don't change the queue count, and will be larger than 16KB if we change
(decrease) the queue count. However stmmac_change_mtu() still work well
with current logic (MTU cannot be larger than 16KB for stmmac).

Note: the Fixes tag picked here is the oldest commit and key commit of
the dwmac-loongson series "stmmac: Add Loongson platform support".

Cc: stable@vger.kernel.org
Fixes: ad72f783de06 ("net: stmmac: Add multi-channel support")
Acked-by: Yanteng Si <si.yanteng@linux.dev>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Chong Qiao <qiaochong@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
---
V2: Update commit message and CC list.

 drivers/net/ethernet/stmicro/stmmac/dwmac-loongson.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-loongson.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-loongson.c
index bfe6e2d631bd..79acdf38c525 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwmac-loongson.c
+++ b/drivers/net/ethernet/stmicro/stmmac/dwmac-loongson.c
@@ -574,6 +574,9 @@ static int loongson_dwmac_probe(struct pci_dev *pdev, const struct pci_device_id
 	if (ret)
 		goto err_disable_device;
 
+	plat->tx_fifo_size = SZ_16K * plat->tx_queues_to_use;
+	plat->rx_fifo_size = SZ_16K * plat->rx_queues_to_use;
+
 	if (dev_of_node(&pdev->dev))
 		ret = loongson_dwmac_dt_config(pdev, plat, &res);
 	else
-- 
2.47.1

On Mon, 10 Feb 2025 21:43:28 +0800 Huacai Chen wrote:
> Now for dwmac-loongson {tx,rx}_fifo_size are uninitialised, which means
> zero. This means dwmac-loongson doesn't support changing MTU because in
> stmmac_change_mtu() it requires the fifo size be no less than MTU. Thus,
> set the correct tx_fifo_size and rx_fifo_size for it (16KB multiplied by
> queue counts).

Not all drivers support changing MTU. Supporting jumbo frames 
is a feature, so this commit enables the use of a feature. 
As such it is not a fix. I'm applying this to net-next.