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

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Message-ID: <e903cb0f-3970-4ad2-a0a2-ee58551779dc@altera.com>
Date: Tue, 15 Jul 2025 19:03:58 +0530
From: "G Thomas, Rohan" <rohan.g.thomas@...era.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: 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>,
 Serge Semin <fancer.lancer@...il.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

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?

Best Regards,
Rohan