Subject: Re: [PATCH v2 3/3] net: stmmac: Bring down the clocks to lower frequencies when mac link goes down

From: "Russell King (Oracle)" <[email-masked]>
Date: Fri, 28 Jun 2024 23:16:53 +0100

On Tue, Jun 25, 2024 at 04:49:30PM -0700, Sagar Cheluvegowda wrote:
> When mac link goes down we don't need to mainitain the clocks to operate
> at higher frequencies, as an optimized solution to save power when
> the link goes down we are trying to bring down the clocks to the
> frequencies corresponding to the lowest speed possible.

I thought I had already commented on a similar patch, but I can't find
anything in my mailboxes to suggest I had.

> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index ec7c61ee44d4..f0166f0bc25f 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -996,6 +996,9 @@ static void stmmac_mac_link_down(struct phylink_config *config,
>  {
>  	struct stmmac_priv *priv = netdev_priv(to_net_dev(config->dev));
>  
> +	if (priv->plat->fix_mac_speed)
> +		priv->plat->fix_mac_speed(priv->plat->bsp_priv, SPEED_10, mode);
> +
>  	stmmac_mac_set(priv, priv->ioaddr, false);
>  	priv->eee_active = false;
>  	priv->tx_lpi_enabled = false;
> @@ -1004,6 +1007,11 @@ static void stmmac_mac_link_down(struct phylink_config *config,
>  
>  	if (priv->dma_cap.fpesel)
>  		stmmac_fpe_link_state_handle(priv, false);
> +
> +	stmmac_set_icc_bw(priv, SPEED_10);
> +
> +	if (priv->plat->fix_mac_speed)
> +		priv->plat->fix_mac_speed(priv->plat->bsp_priv, SPEED_10, mode);

Two things here:

1) Why do we need to call fix_mac_speed() at the start and end of this
   stmmac_mac_link_down()?

2) What if the MAC doesn't support 10M operation? For example, dwxgmac2
   and dwxlgmac2 do not support anything below 1G. It feels that this
   is storing up a problem for the future, where a platform that uses
   e.g. xlgmac2 also implements fix_mac_speed() and then gets a
   surprise when it's called with SPEED_10.

Personally, I don't like "fix_mac_speed", and I don't like it even more
with this change. I would prefer to see link_up/link_down style
operations so that platforms can do whatever they need to on those
events, rather than being told what to do by a single call that may
look identical irrespective of whether the link came up or went down.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!