Re: mt7530: dsa_switch_parse_of() fails, causes probe code to run twice

[Daniel Golle <daniel@makrotopia.org>]

On Sat, Apr 15, 2023 at 03:28:55AM +0300, Arınç ÜNAL wrote:
> On 15.04.2023 02:53, Daniel Golle wrote:
> > On Sat, Apr 15, 2023 at 02:23:16AM +0300, Arınç ÜNAL wrote:
> > > On 15.04.2023 01:48, Daniel Golle wrote:
> > > > On Sat, Apr 15, 2023 at 01:41:07AM +0300, Arınç ÜNAL wrote:
> > > > > Hey there,
> > > > > 
> > > > > I've been working on the MT7530 DSA subdriver. While doing some tests, I
> > > > > realised mt7530_probe() runs twice. I moved enabling the regulators from
> > > > > mt7530_setup() to mt7530_probe(). Enabling the regulators there ends up
> > > > > with exception warnings on the first time. It works fine when
> > > > > mt7530_probe() is run again.
> > > > > 
> > > > > This should not be an expected behaviour, right? Any ideas how we can make
> > > > > it work the first time?
> > > > 
> > > > Can you share the patch or work-in-progress tree which will allow me
> > > > to reproduce this problem?
> > > 
> > > I tested this on vanilla 6.3-rc6. There's just the diff below that is
> > > applied. I encountered it on the standalone MT7530 on my Bananapi BPI-R2. I
> > > haven't tried it on MCM MT7530 on MT7621 SoC yet.
> > > 
> > > > 
> > > > It can of course be that regulator driver has not yet been loaded on
> > > > the first run and -EPROBE_DEFER is returned in that case. Knowing the
> > > > value of 'err' variable below would hence be valuable information.
> > > 
> > > Regardless of enabling the regulator on either mt7530_probe() or
> > > mt7530_setup(), dsa_switch_parse_of() always fails.
> > 
> > So dsa_switch_parse_of() can return -EPROBE_DEFER if the ethernet
> > driver responsible for the CPU port has not yet been loaded.
> > 
> > See net/dsa/dsa.c (inside function dsa_port_parse_of):
> > [...]
> > 1232)                master = of_find_net_device_by_node(ethernet);
> > 1233)                of_node_put(ethernet);
> > 1234)                if (!master)
> > 1235)                        return -EPROBE_DEFER;
> > [...]
> > 
> > Hence it would be important to include the value of 'err' in your
> > debugging printf output, as -EPROBE_DEFER can be an expected and
> > implicitely intended reality and nothing is wrong then.
> 
> Thanks Daniel. I can't do more tests soon but this is probably what's going
> on as the logs already indicate that the MediaTek ethernet driver was yet to
> load.
> 
> As acknowledged, since running the MT7530 DSA subdriver from scratch is
> expected if the ethernet driver is not loaded yet, there's not really a
> problem. Though the switch is reset twice in a short amount of time. I don't
> think that's very great.

That's true, and we should try to avoid that.

> 
> The driver initialisation seems serialised (at least for the drivers built
> into the kernel) as I tried sleeping for 5 seconds on mt7530_probe() but no
> other driver was loaded in the meantime so I got the same behaviour.
> 
> The regulator code will cause a long and nasty exception the first time.
> Though there's nothing wrong as it does what it's supposed to do on the
> second run. I'm not sure if that's negligible.
> 
> Could we at least somehow make the MT7530 DSA subdriver wait until the
> regulator driver is loaded?

I assume the regulator-related stackdump is unrelated, but caused by
cpufreq changes, which had now been fixed by commit 0883426fd07e
("cpufreq: mediatek: Raise proc and sram max voltage for MT7622/7623").

If you are using v6.3-rc6 this commit is still missing there, but
manually picking it from linux-next should fix it.

Let me know if I can help with testing on my farm of MediaTek boards.
I'm a bit nervous about fixing MT7531BE soon, so deciding if we move
PLL activation to mt7530_probe() would be essential as it makes the
fix much easier...