Subject: Re: [PATCH net-next] net: stmmac: qcom-ethqos: remove MAC_CTRL_REG modification

From: Mohd Ayaan Anwar <[email-masked]>
Date: Thu, 30 Oct 2025 15:26:25 +0530

On Wed, Oct 29, 2025 at 10:18:36AM +0000, Russell King (Oracle) wrote:
> When operating in "SGMII" mode (Cisco SGMII or 2500BASE-X), qcom-ethqos
> modifies the MAC control register in its ethqos_configure_sgmii()
> function, which is only called from one path:
> 
> stmmac_mac_link_up()
> +- reads MAC_CTRL_REG
> +- masks out priv->hw->link.speed_mask
> +- sets bits according to speed (2500, 1000, 100, 10) from priv->hw.link.speed*
> +- ethqos_fix_mac_speed()
> |  +- qcom_ethqos_set_sgmii_loopback(false)
> |  +- ethqos_update_link_clk(speed)
> |  `- ethqos_configure(speed)
> |     `- ethqos_configure_sgmii(speed)
> |        +- reads MAC_CTRL_REG,
> |        +- configures PS/FES bits according to speed
> |        `- writes MAC_CTRL_REG as the last operation
> +- sets duplex bit(s)
> +- stmmac_mac_flow_ctrl()
> +- writes MAC_CTRL_REG if changed from original read
> ...
> 
> As can be seen, the modification of the control register that
> stmmac_mac_link_up() overwrites the changes that ethqos_fix_mac_speed()
> does to the register. This makes ethqos_configure_sgmii()'s
> modification questionable at best.
> 
> Analysing the values written, GMAC4 sets the speed bits as:
> speed_mask = GMAC_CONFIG_FES | GMAC_CONFIG_PS
> speed2500 = GMAC_CONFIG_FES                     B14=1 B15=0
> speed1000 = 0                                   B14=0 B15=0
> speed100 = GMAC_CONFIG_FES | GMAC_CONFIG_PS     B14=1 B15=1
> speed10 = GMAC_CONFIG_PS                        B14=0 B15=1
> 
> Whereas ethqos_configure_sgmii():
> 2500: clears ETHQOS_MAC_CTRL_PORT_SEL           B14=X B15=0
> 1000: clears ETHQOS_MAC_CTRL_PORT_SEL           B14=X B15=0
> 100: sets ETHQOS_MAC_CTRL_PORT_SEL |            B14=1 B15=1
>           ETHQOS_MAC_CTRL_SPEED_MODE
> 10: sets ETHQOS_MAC_CTRL_PORT_SEL               B14=0 B15=1
>     clears ETHQOS_MAC_CTRL_SPEED_MODE
> 
> Thus, they appear to be doing very similar, with the exception of the
> FES bit (bit 14) for 1G and 2.5G speeds.

Makes sense.

> 
> Given that stmmac_mac_link_up() will write the MAC_CTRL_REG after
> ethqos_configure_sgmii(), remove the unnecessary update in the
> glue driver's ethqos_configure_sgmii() method, simplifying the code.
> 
> Signed-off-by: Russell King (Oracle) <[email-masked]>
> ---
> Ayaan, please can you also test this patch? I believe that this
> code is unnecessary as per the analysis in the commit message.
> Thanks.
> 
>  .../ethernet/stmicro/stmmac/dwmac-qcom-ethqos.c  | 16 +---------------
>  1 file changed, 1 insertion(+), 15 deletions(-)
>

Tested on top of net-next on the Qualcomm QCS9100 Ride R3 board and
found no issues, so:

Tested-by: Mohd Ayaan Anwar <[email-masked]>