Message-ID: <ZCqi8BasmmwrQ2VD@shell.armlinux.org.uk>
Date:   Mon, 3 Apr 2023 10:57:04 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Siddharth Vadapalli <s-vadapalli@...com>
Cc:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, rogerq@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        srk@...com
Subject: Re: [PATCH net-next 2/2] net: ethernet: ti: am65-cpsw: Enable
 USXGMII mode for J784S4 CPSW9G

On Mon, Apr 03, 2023 at 03:19:24PM +0530, Siddharth Vadapalli wrote:
> 
> 
> On 03/04/23 14:29, Russell King (Oracle) wrote:
> > On Mon, Apr 03, 2023 at 02:11:08PM +0530, Siddharth Vadapalli wrote:
> >>
> >>
> >> On 03/04/23 14:02, Russell King (Oracle) wrote:
> >>> On Mon, Apr 03, 2023 at 11:57:21AM +0530, Siddharth Vadapalli wrote:
> >>>> Hello Russell,
> >>>>
> >>>> On 31/03/23 19:16, Siddharth Vadapalli wrote:
> >>>>>
> >>>>>
> >>>>> On 31-03-2023 16:42, Russell King (Oracle) wrote:
> >>>>>> On Fri, Mar 31, 2023 at 04:23:16PM +0530, Siddharth Vadapalli wrote:
> >>>>>>>
> >>>>>>>
> >>>>>>> On 31/03/23 15:16, Russell King (Oracle) wrote:
> >>>>>>>> On Fri, Mar 31, 2023 at 02:55:56PM +0530, Siddharth Vadapalli wrote:
> >>>>>>>>> Russell,
> >>>>>>>>>
> >>>>>>>>> On 31/03/23 13:54, Russell King (Oracle) wrote:
> >>>>>>>>>> On Fri, Mar 31, 2023 at 01:35:10PM +0530, Siddharth Vadapalli wrote:
> >>>>>>>>>>> Hello Russell,
> >>>>>>>>>>>
> >>>>>>>>>>> Thank you for reviewing the patch.
> >>>>>>>>>>>
> >>>>>>>>>>> On 31/03/23 13:27, Russell King (Oracle) wrote:
> >>>>>>>>>>>> On Fri, Mar 31, 2023 at 12:21:10PM +0530, Siddharth Vadapalli wrote:
> >>>>>>>>>>>>> TI's J784S4 SoC supports USXGMII mode. Add USXGMII mode to the
> >>>>>>>>>>>>> extra_modes member of the J784S4 SoC data. Additionally, configure the
> >>>>>>>>>>>>> MAC Control register for supporting USXGMII mode. Also, for USXGMII
> >>>>>>>>>>>>> mode, include MAC_5000FD in the "mac_capabilities" member of struct
> >>>>>>>>>>>>> "phylink_config".
> >>>>>>>>>>>>
> >>>>>>>>>>>> I don't think TI "get" phylink at all...
> >>>>>>>>>>>>
> >>>>>>>>>>>>> diff --git a/drivers/net/ethernet/ti/am65-cpsw-nuss.c b/drivers/net/ethernet/ti/am65-cpsw-nuss.c
> >>>>>>>>>>>>> index 4b4d06199b45..ab33e6fe5b1a 100644
> >>>>>>>>>>>>> --- a/drivers/net/ethernet/ti/am65-cpsw-nuss.c
> >>>>>>>>>>>>> +++ b/drivers/net/ethernet/ti/am65-cpsw-nuss.c
> >>>>>>>>>>>>> @@ -1555,6 +1555,8 @@ static void am65_cpsw_nuss_mac_link_up(struct phylink_config *config, struct phy
> >>>>>>>>>>>>>  		mac_control |= CPSW_SL_CTL_GIG;
> >>>>>>>>>>>>>  	if (interface == PHY_INTERFACE_MODE_SGMII)
> >>>>>>>>>>>>>  		mac_control |= CPSW_SL_CTL_EXT_EN;
> >>>>>>>>>>>>> +	if (interface == PHY_INTERFACE_MODE_USXGMII)
> >>>>>>>>>>>>> +		mac_control |= CPSW_SL_CTL_XGIG | CPSW_SL_CTL_XGMII_EN;
> >>>>>>>>>>>>
> >>>>>>>>>>>> The configuration of the interface mode should *not* happen in
> >>>>>>>>>>>> mac_link_up(), but should happen in e.g. mac_config().
> >>>>>>>>>>>
> >>>>>>>>>>> I will move all the interface mode associated configurations to mac_config() in
> >>>>>>>>>>> the v2 series.
> >>>>>>>>>>
> >>>>>>>>>> Looking at the whole of mac_link_up(), could you please describe what
> >>>>>>>>>> effect these bits are having:
> >>>>>>>>>>
> >>>>>>>>>> 	CPSW_SL_CTL_GIG
> >>>>>>>>>> 	CPSW_SL_CTL_EXT_EN
> >>>>>>>>>> 	CPSW_SL_CTL_IFCTL_A
> >>>>>>>>>
> >>>>>>>>> CPSW_SL_CTL_GIG corresponds to enabling Gigabit mode (full duplex only).
> >>>>>>>>> CPSW_SL_CTL_EXT_EN when set enables in-band mode of operation and when cleared
> >>>>>>>>> enables forced mode of operation.
> >>>>>>>>> CPSW_SL_CTL_IFCTL_A is used to set the RMII link speed (0=10 mbps, 1=100 mbps).
> >>>>>>>>
> >>>>>>>> Okay, so I would do in mac_link_up():
> >>>>>>>>
> >>>>>>>> 	/* RMII needs to be manually configured for 10/100Mbps */
> >>>>>>>> 	if (interface == PHY_INTERFACE_MODE_RMII && speed == SPEED_100)
> >>>>>>>> 		mac_control |= CPSW_SL_CTL_IFCTL_A;
> >>>>>>>>
> >>>>>>>> 	if (speed == SPEED_1000)
> >>>>>>>> 		mac_control |= CPSW_SL_CTL_GIG;
> >>>>>>>> 	if (duplex)
> >>>>>>>> 		mac_control |= CPSW_SL_CTL_FULLDUPLEX;
> >>>>>>>>
> >>>>>>>> I would also make mac_link_up() do a read-modify-write operation to
> >>>>>>>> only affect the bits that it is changing.
> >>>>>>>
> >>>>>>> This is the current implementation except for the SGMII mode associated
> >>>>>>> operation that I had recently added. I will fix that. Also, the
> >>>>>>> cpsw_sl_ctl_set() function which writes the mac_control value performs a read
> >>>>>>> modify write operation.
> >>>>>>>
> >>>>>>>>
> >>>>>>>> Now, for SGMII, I would move setting CPSW_SL_CTL_EXT_EN to mac_config()
> >>>>>>>> to enable in-band mode - don't we want in-band mode enabled all the
> >>>>>>>> time while in SGMII mode so the PHY gets the response from the MAC?
> >>>>>>>
> >>>>>>> Thank you for pointing it out. I will move that to mac_config().
> >>>>>>>
> >>>>>>>>
> >>>>>>>> Lastly, for RGMII at 10Mbps, you seem to suggest that you need RGMII
> >>>>>>>> in-band mode enabled for that - but if you need RGMII in-band for
> >>>>>>>> 10Mbps, wouldn't it make sense for the other speeds as well? If so,
> >>>>>>>> wouldn't that mean that CPSW_SL_CTL_EXT_EN can always be set for
> >>>>>>>> RGMII no matter what speed is being used?
> >>>>>>>
> >>>>>>> The CPSW MAC does not support forced mode at 10 Mbps RGMII. For this reason, if
> >>>>>>> RGMII 10 Mbps is requested, it is set to in-band mode.
> >>>>>>
> >>>>>> What I'm saying is that if we have in-band signalling that is reliable
> >>>>>> for a particular interface mode, why not always use it, rather than
> >>>>>> singling out one specific speed as an exception? Does it not work in
> >>>>>> 100Mbps and 1Gbps?
> >>>>
> >>>> While the CPSW MAC supports RGMII in-band status operation, the link partner
> >>>> might not support it. I have also observed that forced mode is preferred to
> >>>> in-band mode as implemented for another driver:
> >>>> commit ade64eb5be9768e40c90ecb01295416abb2ddbac
> >>>> net: dsa: microchip: Disable RGMII in-band status on KSZ9893
> >>>>
> >>>> and in the mail thread at:
> >>>> https://lore.kernel.org/netdev/20200905160647.GJ3164319@lunn.ch/
> >>>> based on Andrew's suggestion, using forced mode appears to be better.
> >>>>
> >>>> Additionally, I have verified that switching to in-band status causes a
> >>>> regression. Thus, I will prefer keeping it in forced mode for 100 and 1000 Mbps
> >>>> RGMII mode which is the existing implementation in the driver. Please let me know.
> >>>
> >>> Okay, so what this seems to mean is if you have a PHY that does not
> >>> support in-band status in RGMII mode, then 10Mbps isn't possible -
> >>> because the MAC requires in-band status mode to select 10Mbps.
> >>> To put it another way, in such a combination, 10Mbps link modes
> >>> should not be advertised, nor should they be reported to userspace
> >>> as being supported.
> >>>
> >>> Is that correct?
> >>
> >> Yes, if the PHY does not support in-band status, 10 Mbps RGMII will not work,
> >> despite the MAC supporting 10 Mbps in-band RGMII. However, I notice the following:
> >> If the RGMII interface speed is set to 10 Mbps via ethtool, but the:
> >> managed = "in-band-status";
> >> property is not mentioned in the device-tree, the interface is able to work with
> >> 10 Mbps mode with the PHY. This is with the CPSW MAC configured for in-band mode
> >> of operation at 10 Mbps RGMII mode. Please let me know what this indicates,
> >> since it appears to me that 10 Mbps is functional in this special case (It might
> >> be an erroneous configuration).
> > 
> > I think you need to check carefully what is going on.
> > 
> > Firstly, if you as the MAC is choosing to enable in-band status mode,
> > but phylink isn't using in-band status mode, that is entirely a matter
> > for your MAC driver.
> > 
> > Secondly, you need to research what the PHY does during the inter-frame
> > time (when in-band status would be transferred). This is when RX_CTL
> > is 0,0, RX_DV is 0, RX_ER is 0.
> > 
> > For in-band 10Mbps mode to work, RXD nibbles would need to be x001
> > (middle two bits indicate RX clock = 2.5MHz clock for 10Mbps, lsb
> > indicates link up). MSB determines duplex. Remember that 10Mbps can
> > appear to work with mismatched duplex settings but can cause chaos on
> > networks when it disagrees with what the rest of the network is doing.
> > 
> > So, I think before one says "setting in-band mode for 10Mbps with a
> > PHY that doesn't support in-band" really needs caution and research
> > to check what _actually_ ends up happening, and whether it is really
> > correct to do this.
> 
> Thank you for the detailed explanation. I will analyze it and fix this. In the
> meanwhile, is it acceptable for me to post the v2 of this series, with the other
> suggestions implemented, while maintaining the status quo for the 10 Mbps RGMII
> configuration in the driver? Please let me know.

Yes, but I would like a comment against the bit of code that enables
in-band mode indicating that it's questionable whether it is correct.

Thanks.

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