Subject: Re: [PATCH net 1/1] net: stmmac: Fix zero-division error when disabling tc cbs

From: Simon Horman <[email-masked]>
Date: Thu, 12 Sep 2024 16:37:30 +0100

On Thu, Sep 12, 2024 at 09:55:41AM +0800, KhaiWenTan wrote:
> The commit b8c43360f6e4 ("net: stmmac: No need to calculate speed divider
> when offload is disabled") allows the "port_transmit_rate_kbps" to be
> set to a value of 0, which is then passed to the "div_s64" function when
> tc-cbs is disabled. This leads to a zero-division error.
> 
> When tc-cbs is disabled, the idleslope, sendslope, and credit values the
> credit values are not required to be configured. Therefore, adding a return
> statement after setting the txQ mode to DCB when tc-cbs is disabled would
> prevent a zero-division error.
> 
> Fixes: b8c43360f6e4 ("net: stmmac: No need to calculate speed divider when offload is disabled")
> Cc: <[email-masked]>
> Co-developed-by: Choong Yong Liang <[email-masked]>
> Signed-off-by: Choong Yong Liang <[email-masked]>
> Signed-off-by: KhaiWenTan <[email-masked]>
> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c
> index 996f2bcd07a2..2c3fd9c66d14 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c
> @@ -392,10 +392,10 @@ static int tc_setup_cbs(struct stmmac_priv *priv,
>  	} else if (!qopt->enable) {
>  		ret = stmmac_dma_qmode(priv, priv->ioaddr, queue,
>  				       MTL_QUEUE_DCB);
> -		if (ret)
> -			return ret;
> +		if (!ret)
> +			priv->plat->tx_queues_cfg[queue].mode_to_use = MTL_QUEUE_DCB;
>  
> -		priv->plat->tx_queues_cfg[queue].mode_to_use = MTL_QUEUE_DCB;
> +		return ret;
>  	}

Thanks,

I agree with your analysis. But I think it would
be more idomatic to write it such that the main thread
of execution is the non-error path (in any case,
it makes it easier for me to understand the intent of the code.

What I am suggesting is this (extra context provided for clarity):

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c
index 996f2bcd07a2..308ef4241768 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c
@@ -392,14 +392,15 @@ static int tc_setup_cbs(struct stmmac_priv *priv,
 	} else if (!qopt->enable) {
 		ret = stmmac_dma_qmode(priv, priv->ioaddr, queue,
 				       MTL_QUEUE_DCB);
 		if (ret)
 			return ret;
 
 		priv->plat->tx_queues_cfg[queue].mode_to_use = MTL_QUEUE_DCB;
+		return 0;
 	}
 
 	/* Final adjustments for HW */
 	value = div_s64(qopt->idleslope * 1024ll * ptr, port_transmit_rate_kbps);
 	priv->plat->tx_queues_cfg[queue].idle_slope = value & GENMASK(31, 0);
 
 	value = div_s64(-qopt->sendslope * 1024ll * ptr, port_transmit_rate_kbps);