Date:   Mon, 3 Apr 2023 09:32:28 +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 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?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!

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