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@microchip.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@microchip.com>

On Tue, Feb 10, 2026 at 02:44:01PM +0100, Daniel Machon wrote:
> 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@microchip.com>

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