Subject: Re: [PATCH net-next] net: pcs: lynxi: fully reconfigure if link is down

From: "Russell King (Oracle)" <[email-masked]>
Date: Thu, 17 Aug 2023 14:03:40 +0100

On Thu, Aug 17, 2023 at 01:04:06PM +0100, Daniel Golle wrote:
> On MT7988 When switching from 10GBase-R/5GBase-R/USXGMII to one of the
> interface modes provided by mtk-pcs-lynxi we need to make sure to
> always perform a full configuration of the PHYA.
> As the idea behind not doing that was mostly to prevent an existing link
> going down without any need for it to do so. Hence we can just always
> perform a full confinguration in case the link is down.

And this is racy - because in the case with inband signalling, the link
can come up between reading the status and acting on it. It could even
be already up, but the link status indicates it is not. Lastly, reading
the BMSR has side effects: the link status bit latches low until a read.

Basically, do not read the BMSR here, it's buggy to read it any place
other than pcs_get_state.

I think what we need to do instead are:

1) mtk_mac_select_pcs() returns the SGMII PCS or NULL. Presumably this
   is the driver which supports 10GBase-R/5GBase-R/USXGMII, and thus
   this returns NULL for 10GBase-R/5GBase-R/USXGMII.

   Phylink doesn't cater for mac_select_pcs() returning non-NULL for
   some modes and NULL for others, mainly because the presence of a PCS
   _used_ to cause phylink to change its behaviour (see
   https://lore.kernel.org/netdev/[email-masked]/).
   That has now changed (we've got rid of the legacy stuff at last!) so
   there is no technical reason not to now allow that.

   Vladimir did have some arguments for not allowing it when we had the
   phylink_set_pcs() interface:
   https://lore.kernel.org/netdev/20211123181515.qqo7e4xbuu2ntwgt@skbuf/
   I'm assuming that your requirement now provides sufficient
   justification for allowing this.

   There is one bug that does need fixing first:
   phylink_change_inband_advert() checks pl->pcs->neg_mode without
   first checking whether pl->pcs is non-NULL.

   To allow this, phylink_major_config() needs:

   	pcs_changed = pcs && pl->pcs != pcs;

   to become:

   	pcs_changed = pl->pcs != pcs;

2) with (1) solved, there are a couple of callbacks that can be used to
   solve this - I think pcs_disable() is the one you want, which will
   be called when we switch to a mode where _this_ PCS will no longer
   be used (thus you can reset mpcs->interface to _NA, ready for when
   it is next brought into use.)

Would that work for you?

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