linux-kernel - Re: [PATCH net-next 2/3] net: stmmac: xgmac: Correct supported speed modes

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Message-ID: <6fsqayppkyubkucghk5i6m7jjgytajtzm4wxhtdkh7i2v3znk5@vwqbzz5uffyy>
Date: Thu, 17 Jul 2025 14:47:14 +0300
From: Serge Semin <fancer.lancer@...il.com>
To: "G Thomas, Rohan" <rohan.g.thomas@...era.com>
Cc: Andrew Lunn <andrew@...n.ch>, Andrew Lunn <andrew+netdev@...n.ch>, 
	"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>, Alexandre Torgue <alexandre.torgue@...s.st.com>, 
	Romain Gantois <romain.gantois@...tlin.com>, netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Matthew Gerlach <matthew.gerlach@...era.com>
Subject: Re: [PATCH net-next 2/3] net: stmmac: xgmac: Correct supported speed
 modes

On Tue, Jul 15, 2025 at 07:03:58PM +0530, G Thomas, Rohan wrote:
> Hi Andrew,
> 
> Thanks for reviewing the patch.
> 
> On 7/14/2025 7:12 PM, Andrew Lunn wrote:
> > On Mon, Jul 14, 2025 at 03:59:18PM +0800, Rohan G Thomas via B4 Relay wrote:
> > > From: Rohan G Thomas <rohan.g.thomas@...era.com>
> > > 
> > > Correct supported speed modes as per the XGMAC databook.
> > > Commit 9cb54af214a7 ("net: stmmac: Fix IP-cores specific
> > > MAC capabilities") removes support for 10M, 100M and
> > > 1000HD. 1000HD is not supported by XGMAC IP, but it does
> > > support 10M and 100M FD mode, and it also supports 10M and
> > > 100M HD mode if the HDSEL bit is set in the MAC_HW_FEATURE0
> > > reg. This commit adds support for 10M and 100M speed modes
> > > for XGMAC IP.
> > 
> > > +++ b/drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c
> > > @@ -405,6 +405,7 @@ static int dwxgmac2_get_hw_feature(void __iomem *ioaddr,
> > >   	dma_cap->sma_mdio = (hw_cap & XGMAC_HWFEAT_SMASEL) >> 5;
> > >   	dma_cap->vlhash = (hw_cap & XGMAC_HWFEAT_VLHASH) >> 4;
> > >   	dma_cap->half_duplex = (hw_cap & XGMAC_HWFEAT_HDSEL) >> 3;
> > > +	dma_cap->mbps_10_100 = (hw_cap & XGMAC_HWFEAT_GMIISEL) >> 1;
> > 
> > The commit message does not mention this change.
> 
> Agreed. Will do in the next version.
> 
> > 
> > What does XGMAC_HWFEAT_GMIISEL mean? That a SERDES style interface is
> > not being used? Could that be why Serge removed these speeds? He was
> > looking at systems with a SERDES, and they don't support slower
> > speeds?
> > 
> > 	Andrew
> As per the XGMAC databook ver 3.10a, GMIISEL bit of MAC_HW_Feature_0
> register indicates whether the XGMAC IP on the SOC is synthesized with
> DWCXG_GMII_SUPPORT. Specifically, it states:
> "1000/100/10 Mbps Support. This bit is set to 1 when the GMII Interface
> option is selected."
> 
> So yes, it’s likely that Serge was working with a SERDES interface which
> doesn't support 10/100Mbps speeds. Do you think it would be appropriate
> to add a check for this bit before enabling 10/100Mbps speeds?

DW XGMAC IP-core of v2.x and older don't support 10/100Mbps modes
neither in the XGMII nor in the GMII interfaces. That's why I dropped
the 10/100Mbps link capabilities retaining 1G, 2.5G and 10G speeds
only (the only speeds supported for DW XGMAC 1.20a/2.11a Tx in the
MAC_Tx_Configuration.SS register field). Although I should have
dropped the MAC_5000FD too since it has been supported since v3.0
IP-core version. My bad.(

Starting from DW XGMAC v3.00a IP-core the list of the supported speeds
has been extended to: 10/100Mbps (MII), 1G/2.5G (GMII), 2.5G/5G/10G
(XGMII). Thus the more appropriate fix here should take into account
the IP-core version. Like this:
	if (dma_cap->mbps_1000 && MAC_Version.SNPSVER >= 0x30)
		dma_cap->mbps_10_100 = 1;

Then you can use the mbps_1000 and mbps_10_100 flags to set the proper
MAC-capabilities to hw->link.caps in the dwxgmac2_setup() method. I
would have added the XGMII 2.5G/5G MAC-capabilities setting up to the
dwxgmac2_setup() method too for the v3.x IP-cores and newer.

-Serge(y)

> 
> Best Regards,
> Rohan
>