Re: [PATCH net 2/2] net: stmmac: qcom-ethqos: advertise supported SerDes interfaces

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

Hi,

On 9/16/26 14:17, Mohd Ayaan Anwar wrote:
> On Wed, Sep 16, 2026 at 08:39:50PM +1000, Thomas Karpiniec wrote:
>> From: Thomas Karpiniec <tom@wendy.sh>
>>
>> With a QCA8081 PHY and phy-mode = "2500base-x", stmmac reports only
>> 2500BASE-X in phylink's supported_interfaces. Phylink consequently removes
>> 10/100/1000BASE-T modes from the PHY's advertisement, preventing a link
>> with a gigabit switch.
>>
>> The QCA8081 uses SGMII for lower copper speeds and 2500BASE-X for 2.5Gbps.
>> ETHQOS already reconfigures the SerDes in ethqos_mac_finish_serdes() when
>> the PHY changes interface, but does not advertise this capability.
>>
>> Provide a get_interfaces() callback for serial interfaces. Add SGMII and
>> 2500BASE-X when the SerDes validates them, allowing phylink to retain the
>> corresponding copper link modes. Leave the existing stmmac fallback to
>> the firmware interface in place when no interfaces are reported. RGMII
>> configurations are unchanged.
>>
>> Fixes: 61e9be0efbe8 ("net: stmmac: qcom-ethqos: add support for 2.5G BASEX mode")
>> Cc: stable@vger.kernel.org # 7.1+
>> Assisted-by: LLM sparse
>> Signed-off-by: Thomas Karpiniec <tom@wendy.sh>
>> ---
>>  .../stmicro/stmmac/dwmac-qcom-ethqos.c        | 22 +++++++++++++++++++
>>  1 file changed, 22 insertions(+)
>>
>> diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-qcom-ethqos.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-qcom-ethqos.c
>> index ac7d6d3e205a..3493e090f1a8 100644
>> --- a/drivers/net/ethernet/stmicro/stmmac/dwmac-qcom-ethqos.c
>> +++ b/drivers/net/ethernet/stmicro/stmmac/dwmac-qcom-ethqos.c
>> @@ -1,6 +1,7 @@
>>  // SPDX-License-Identifier: GPL-2.0
>>  // Copyright (c) 2018-19, Linaro Limited
>>  
>> +#include <linux/bitops.h>
>>  #include <linux/module.h>
>>  #include <linux/of.h>
>>  #include <linux/of_net.h>
>> @@ -564,6 +565,26 @@ static void ethqos_pcs_set_inband(struct qcom_ethqos *ethqos, bool enable)
>>  	stmmac_pcs_ctrl_ane(priv, enable, 0);
>>  }
>>  
>> +static void ethqos_get_interfaces_serdes(struct stmmac_priv *priv, void *bsp_priv,
>> +					 unsigned long *interfaces)
>> +{
>> +	struct qcom_ethqos *ethqos = bsp_priv;
>> +
>> +	if (!ethqos->serdes_phy)
>> +		return;
>> +
>> +	/* PHYs such as QCA8081 switch between SGMII and 2500BASE-X with
>> +	 * the negotiated copper speed. mac_finish reconfigures the SerDes
>> +	 * accordingly; let phylink validate all modes that path supports.
>> +	 */
>> +	if (!phy_validate(ethqos->serdes_phy, PHY_MODE_ETHERNET,
>> +			  PHY_INTERFACE_MODE_SGMII, NULL))
>> +		__set_bit(PHY_INTERFACE_MODE_SGMII, interfaces);
>> +	if (!phy_validate(ethqos->serdes_phy, PHY_MODE_ETHERNET,
>> +			  PHY_INTERFACE_MODE_2500BASEX, NULL))
>> +		__set_bit(PHY_INTERFACE_MODE_2500BASEX, interfaces);
>> +}
>> +
> 
> So, I had proposed this exact change a while back:
> https://lore.kernel.org/netdev/aMgootkPQ%2FGcdiXX@oss.qualcomm.com/
> 
> As Russell noted, this behaviour is due to the messed-up integrated
> PCS support in STMMAC/QCOM-ETHQOS.
> 
> I would defer to him for this if he's around.

Indeed, but we haven't heard from him in a while, let's see if we can
figure this out. 

Does this IP have an integrated PCS ? (i.e. dma_cap.pcs is set)
you can check that in debugfs :

mount -t debugfs none /sys/kernel/debug
cat /sys/kernel/debug/stmmaceth/eth0/dma_cap | grep PCS

if it's Y, then you have the integrated one.

Russell worked hard on that, it looks like a lot of the plumbing went
through.

I think the right approach here is to have the PCS itself report the
list of supported interfaces, instead of relying on the glue. looking
at stmmac_pcs.c :

220 int stmmac_integrated_pcs_init(struct stmmac_priv *priv,                                
221                                const struct stmmac_pcs_info *pcs_info)                  
222 {                                                                                       
223         struct stmmac_pcs *spcs;                                                        
224                                                                                         
225         spcs = devm_kzalloc(priv->device, sizeof(*spcs), GFP_KERNEL);                   
226         if (!spcs)                                                                      
227                 return -ENOMEM;                                                         

[...]
                                                             
246         /* Only allow 2500BASE-X if the SerDes has support. */                          
247         if (priv->plat->flags & STMMAC_FLAG_SERDES_SUPPORTS_2500M)                      
248                 __set_bit(PHY_INTERFACE_MODE_2500BASEX,                                 
249                           spcs->pcs.supported_interfaces);                              
250                                                                                         
251         priv->integrated_pcs = spcs;                                                    
252                                                                                         
253         return 0;                                                                       
254 }        

Can you test settung the STMMAC_FLAG_SERDES_SUPPORTS_2500M flag in
dwmac-qcom-ethqos ?

Maxime