Message-ID: <CA+h21hrsosGVQczMWy1+WfyNGZCpeMFerUwvWb-z+TTjrSOP1Q@mail.gmail.com>
Date:   Wed, 26 Jun 2019 02:13:25 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     René van Dorst <opensource@...rst.com>,
        sean.wang@...iatek.com, Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>, matthias.bgg@...il.com,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        frank-w@...lic-files.de, netdev <netdev@...r.kernel.org>,
        linux-mediatek@...ts.infradead.org, linux-mips@...r.kernel.org
Subject: Re: [PATCH RFC net-next 1/5] net: dsa: mt7530: Convert to PHYLINK API

On Wed, 26 Jun 2019 at 02:10, Vladimir Oltean <olteanv@...il.com> wrote:
>
> On Wed, 26 Jun 2019 at 01:58, Russell King - ARM Linux admin
> <linux@...linux.org.uk> wrote:
> >
> > On Wed, Jun 26, 2019 at 01:14:59AM +0300, Vladimir Oltean wrote:
> > > On Wed, 26 Jun 2019 at 00:53, Russell King - ARM Linux admin
> > > <linux@...linux.org.uk> wrote:
> > > >
> > > > On Tue, Jun 25, 2019 at 11:24:01PM +0300, Vladimir Oltean wrote:
> > > > > Hi Russell,
> > > > >
> > > > > On 6/24/19 6:39 PM, Russell King - ARM Linux admin wrote:
> > > > > > This should be removed - state->link is not for use in mac_config.
> > > > > > Even in fixed mode, the link can be brought up/down by means of a
> > > > > > gpio, and this should be dealt with via the mac_link_* functions.
> > > > > >
> > > > >
> > > > > What do you mean exactly that state->link is not for use, is that true in
> > > > > general?
> > > >
> > > > Yes.  mac_config() should not touch it; it is not always in a defined
> > > > state.  For example, if you set modes via ethtool (the
> > > > ethtool_ksettings_set API) then state->link will probably contain
> > > > zero irrespective of the true link state.
> > > >
> > >
> > > Experimentally, state->link is zero at the same time as state->speed
> > > is -1, so just ignoring !state->link made sense. This is not in-band
> > > AN. What is your suggestion? Should I proceed to try and configure the
> > > MAC for SPEED_UNKNOWN?
> >
> > What would you have done with a PHY when the link is down, what speed
> > would you have configured in the phylib adjust_link callback?  phylib
> > also sets SPEED_UNKNOWN/DUPLEX_UNKNOWN when the link is down.
> >
>
> With phylib, I'd make the driver ignore the speed and do nothing.
> With phylink, I'd make the core not call mac_config.
> But what happened is I saw phylink call mac_config anyway, said
> 'weird' and proceeded to ignore it as I would have for phylib.
> I'm just not understanding your position - it seems like you're
> implying there's a bug in phylink and the function call with
> MLO_AN_FIXED, state->link=0 and state->speed=-1 should not have taken

I meant MLO_AN_PHY, sorry.

> place, which is what I wanted to confirm.
>
> > What we do in Marvell drivers is set to the lowest speed (10M) in such
> > cases, which is fine as the MAC supports 10M.
> >
> > It wouldn't be appropriate for phylink to force something on MAC
> > drivers, it's easier if the MAC just defaults SPEED_UNKNOWN to something
> > itself.
> >
> > >
> > > > It exists in this structure because it was convenient to just use one
> > > > structure to store all the link information in various parts of the
> > > > code, and when requesting the negotiated in-band MAC configuration.
> > > >
> > > > I've come to the conclusion that that decision was a mistake, based
> > > > on patches such as the above mistakenly thinking that everything in
> > > > the state structure is fair game.  I've since updated the docs to
> > > > explicitly spell it out, but I'm also looking at the feasibility of
> > > > changing the mac_config() interface entirely - splitting it into two
> > > > (mac_config_fixed() and mac_config_inband()) and passing only the
> > > > appropriate parameters to each.
> > > >
> > > > However, having looked at that, I think such a change will make some
> > > > MAC drivers quite a bit more complicated - having all the config
> > > > steps in one method appears to make the configuration of MAC drivers
> > > > easier (eg, mvneta, mvpp2.)
> > > >
> > > > > In drivers/net/dsa/sja1105/sja1105_main.c, if I remove the "if
> > > > > (!state->link)" guard, I see PHYLINK calls with a SPEED_UNKNOWN argument for
> > > > > ports that are BR_STATE_DISABLED. Is that normal?
> > > >
> > > > This looks like another driver which has been converted to phylink
> > > > without my review; I certainly wasn't aware of it.  It gets a few
> > > > things wrong, such as:
> > > >
> > > > 1) not checking state->interface in the validate callback - so it
> > > >    is basically saying that it can support any PHY interface mode
> > > >    that the kernel happens to support.
> > > >
> > >
> > > Partially true. It does check the DT bindings for supported MII modes
> > > in sja1105_init_mii_settings (for fundamental reasons... the switch
> > > expects an 'all-in-one' configuration buffer with the operating modes
> > > of all MACs - don't ask me to delay the uploading of this static
> > > config until all ports collected their interface_mode from phylink via
> > > the mac_config callback - it's a deadlock).
> >
> > Ok, so you need to reject interface modes that are not compatible
> > with the currently configured mode in the validate() callback, but
> > please keep PHY_INTERFACE_MODE_NA reporting back the capabilities.
> > (this is now documented.)
> >
>
> Ok.
>
> > > It is a gigabit switch with MII/RMII/RGMII MACs - I have never seen
> > > any PHY wired for these modes that can change system interface type.
> >
> > It is unlikely that MII/RMII/RGMII will switch modes, but in terms of
> > correct implementation, sticking to the way the function is expected
> > to behave means that I don't get surprises when changing phylink layer
> > in the future.
> >
> > >
> > > > 2) if phylink is configured to use in-band, then state->speed is
> > > >    undefined; this driver will fail.  (If folk don't want to support
> > > >    that, we ought to have a way to tell phylink to reject anything
> > > >    that attempts to set it to in-band mode!)
> > > >
> > >
> > > Ok.
> > >
> > > > 3) it doesn't implement phylink_mac_link_state DSA ops, so it doesn't
> > > >    support SGMII or 802.3z phy interface modes (see 1).
> > > >
> > >
> > > No, it doesn't.
> > > Some odd switch in this device family supports SGMII on 1 of its
> > > ports, however I haven't put my hands on it.
> > > When I do I'll add checks for strange scenarios, like connecting it to
> > > an Aquantia PHY that can switch between SGMII and USXGMII (although
> > > why would anyone pair a 10G capable PHY to a 1G capable MAC...)
> >
> > It's unlikely that it would switch between SGMII and USXGMII
> > dynamically, as USXGMII supports speeds from 10G down to 10M.
> >
> > Where interface mode switching tends to be used is with modes such
> > as 10GBASE-R, which doesn't support anything except 10G.  In order
> > for the PHY to operate at slower speeds, it has a few options:
> >
> > 1) perform rate adaption.
> > 2) dynamically switch interface type to an interface type that
> >    supports the desired speed.
> > 3) just not support slower speeds.
> >
>
> So am I reading this correctly - it kind of makes sense for gigabit
> MAC drivers to not check for the MII interface changing protocol?
>
> > --
> > RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> > FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
> > According to speedtest.net: 11.9Mbps down 500kbps up
>
> Thanks,
> -Vladimir

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