linux-kernel - Re: [PATCH net v2] net: stmmac: Set MAC's flow control register to reflect current settings

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Message-ID: <Y30ayCTyVpjjyEzh@shell.armlinux.org.uk>
Date:   Tue, 22 Nov 2022 18:54:00 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     "Goh, Wei Sheng" <wei.sheng.goh@...el.com>
Cc:     Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Ong Boon Leong <boon.leong.ong@...el.com>,
        Voon Wei Feng <weifeng.voon@...el.com>,
        Tan Tee Min <tee.min.tan@...el.com>,
        Ahmad Tarmizi Noor Azura <noor.azura.ahmad.tarmizi@...el.com>,
        Looi Hong Aun <hong.aun.looi@...el.com>
Subject: Re: [PATCH net v2] net: stmmac: Set MAC's flow control register to
 reflect current settings

On Tue, Nov 22, 2022 at 06:40:33PM +0000, Russell King (Oracle) wrote:
> On Tue, Nov 22, 2022 at 02:39:35PM +0800, Goh, Wei Sheng wrote:
> > diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac4_core.c b/drivers/net/ethernet/stmicro/stmmac/dwmac4_core.c
> > index c25bfecb4a2d..369db308b1dd 100644
> > --- a/drivers/net/ethernet/stmicro/stmmac/dwmac4_core.c
> > +++ b/drivers/net/ethernet/stmicro/stmmac/dwmac4_core.c
> > @@ -748,6 +748,9 @@ static void dwmac4_flow_ctrl(struct mac_device_info *hw, unsigned int duplex,
> >  	if (fc & FLOW_RX) {
> >  		pr_debug("\tReceive Flow-Control ON\n");
> >  		flow |= GMAC_RX_FLOW_CTRL_RFE;
> > +	} else {
> > +		pr_debug("\tReceive Flow-Control OFF\n");
> > +		flow &= ~GMAC_RX_FLOW_CTRL_RFE;
> >  	}
> >  	writel(flow, ioaddr + GMAC_RX_FLOW_CTRL);
> 
> This doesn't look correct to me. The function starts off:
> 
>         unsigned int flow = 0;
> 
> flow is not written to before the context above. So, the code you've
> added effectively does:
> 
> 	flow = 0 & ~GMAC_RX_FLOW_CTRL_RFE;
> 
> which is still zero. So, I don't think this hunk is meaningful apart
> from adding the pr_debug().

It also should be noted that it looks like:

dwxgmac2_core.c::dwxgmac2_flow_ctrl() is definitely buggy - it will
only set flow control, never clearing it.

dwmac100_core.c::dwmac100_flow_ctrl() looks potentially buggy - always
enabling flow control irrespective of anything.

The other two implmentations look sane to me.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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