linux-kernel - [PATCH net] net: sparx5/lan969x: fix DWRR cost max to match hardware register width

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Message-ID: <20260210-sparx5-fix-dwrr-cost-max-v1-1-58fbdbc25652@microchip.com>
Date: Tue, 10 Feb 2026 14:44:01 +0100
From: Daniel Machon <daniel.machon@...rochip.com>
To: Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
	<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Steen Hegelund
	<Steen.Hegelund@...rochip.com>, <UNGLinuxDriver@...rochip.com>
CC: <netdev@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>
Subject: [PATCH net] net: sparx5/lan969x: fix DWRR cost max to match
 hardware register width

DWRR (Deficit Weighted Round Robin) scheduling distributes bandwidth
across traffic classes based on per-queue cost values, where lower cost
means higher bandwidth share.

The SPX5_DWRR_COST_MAX constant is 63 (6 bits) but the hardware
register field HSCH_DWRR_ENTRY_DWRR_COST is GENMASK(24, 20), only
5 bits wide (max 31). This causes sparx5_weight_to_hw_cost() to
compute cost values that silently overflow via FIELD_PREP, resulting
in incorrect scheduling weights.

Set SPX5_DWRR_COST_MAX to 31 to match the hardware register width.

Fixes: 211225428d65 ("net: microchip: sparx5: add support for offloading ets qdisc")
Signed-off-by: Daniel Machon <daniel.machon@...rochip.com>
---
 drivers/net/ethernet/microchip/sparx5/sparx5_qos.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/microchip/sparx5/sparx5_qos.h b/drivers/net/ethernet/microchip/sparx5/sparx5_qos.h
index 1231a80335d7..04f76f1e23f6 100644
--- a/drivers/net/ethernet/microchip/sparx5/sparx5_qos.h
+++ b/drivers/net/ethernet/microchip/sparx5/sparx5_qos.h
@@ -35,7 +35,7 @@
 #define SPX5_SE_BURST_UNIT 4096
 
 /* Dwrr */
-#define SPX5_DWRR_COST_MAX 63
+#define SPX5_DWRR_COST_MAX 31
 
 struct sparx5_shaper {
 	u32 mode;

---
base-commit: 9384a1d1897a809f073a0f8cc252b7a41f9c05ac
change-id: 20260210-sparx5-fix-dwrr-cost-max-f86d986fdc50

Best regards,
-- 
Daniel Machon <daniel.machon@...rochip.com>