Fwd: Micrel KSZ9031RN PHY problem

[Sascha Hauer]

On Tue, May 03, 2016 at 04:40:02PM +0200, Guillermo Rodriguez Garcia wrote:
> Hello,
> 
> 2016-04-29 20:18 GMT+02:00 Trent Piepho <tpiepho at kymetacorp.com>:
> > On Fri, 2016-04-29 at 13:00 +0200, Guillermo Rodriguez Garcia wrote:
> >> 2016-04-28 23:09 GMT+02:00 Trent Piepho <tpiepho at kymetacorp.com>:
> >> >
> >> > The first anreg start call will also un-powerdown the PHY if BMCR_PDOWN
> >> > was set.  I wonder if that is happening?
> >>
> >> That was a very good hint and it looks like this is exactly what is happening.
> >>
> >> genphy_restart_aneg() clears the BMCR_PDOWN bit and would get the phy
> >> out of powerdown mode. I have added a trace right at the beginning of
> >> genphy_restart_aneg and verified that BMCR_PDOWN bit was set before
> >> genphy_restart_aneg clears it.
> >>
> >> Then, the datasheet for the ksz9031 [1], page 44, says:
> >>
> >> After this bit is changed from '1' to '0', an internal global reset is
> >> automatically generated. Wait a minimum of 1ms before read/write
> >> access to the PHY registers.
> >
> > Mystery solved!
> 
> Indeed. Although it's strange that the problem can only be reproduced
> with certain routers. I can reproduce it everytime when the board is
> connected with a ComTrend VG-8050, but not with other routers..
> 
> >
> >> So this seems to be what is causing the problem. At least on the
> >> ksz9031 (don't know about others), a delay of 1ms is required when
> >> coming out of powerdown mode.
> >
> > The kernel will take the phy in/out of powerdown mode as part of the PM
> > suspend/resume calls, which is supported on all micrel phys since 2013.
> > I don't see a delay in the kernel code and wonder why this hasn't been a
> > problem?
> 
> Perhaps this is due to the fact that it does not happen with every router.
> 
> >  Might be worth asking on net-dev if this is a known issue with
> > some phys and how it is solved?  Maybe it's an undiscovered cause of
> > network flakiness after a resume.
> 
> Would you be willing to help here? (i.e. report/ask about this on net-dev)
> 
> >
> >> What is the best way to fix this? We can add a 1ms delay in
> >> genphy_restart_aneg (this is probably the easiest, and the delay is
> >> small enough that it shouldn't make a difference for other phys that
> >> might not need it). Or if this is not acceptable, perhaps add a custom
> >> restart_aneg function for the ksz9031.
> >
> > Could add a custom init function that un-powerdowns the phy and does the
> > wait.
> >
> > Or have restart_aneg check if the powerdown bit was set before it clears
> > it, and only delay in that case.
> 
> This would be easy, would solve the problem at hand, and would only
> introduce a (perhaps unnecessary) 1ms delay for phys that don't need
> this.
> 
> >
> > Having the un-powerdown in the restart_aneg isn't really the right place
> > for it.  If there is no reason the restart aneg, then the phy will not
> > be powered up.
> 
> Yes but I would say that that's a different issue. I must say I don't feel
> confident enough to move this code to somewhere else myself. Perhaps
> Sascha (as the original author of this change [1]) could comment.
> 
> I would suggest so separate these two issues: 1) Adding the missing
> 1ms delay as described in the Micrel datasheet, 2) Consider whether
> the code should be refactored / reorganized.
> 
> Does this make sense?

Yes, makes sense.

Currently I don't know a better place for clearing the BMCR_PDOWN bit.
genphy_config_init would be a candidate, but it's not called for phys
which have a custom .config_init hook.
If I'm lucky I can find the ethernet adapter which motivated me to
create ac48b10467ffb, it would be interesting to see which phy type the
adapter has.

Sascha

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