Re: [PATCH] phy: core: Make 'phy_optional_get' return NULL when not implemented

[Trent Piepho <trent.piepho@igorinstitute.com>]

On Thu, Nov 4, 2021 at 1:02 PM Sascha Hauer <sha@pengutronix.de> wrote:
> On Wed, Nov 03, 2021 at 03:35:08PM -0700, Trent Piepho wrote:
> > On Tue, Nov 2, 2021 at 12:40 AM Sascha Hauer <sha@pengutronix.de> wrote:
> > >
> > > On Mon, Nov 01, 2021 at 11:33:07AM -0700, Trent Piepho wrote:
> > > >    On Mon, Nov 1, 2021 at 2:01 AM Sascha Hauer <[1]sha@pengutronix.de> wrote:
> >  >    >
> > > >    > The phy is only optional as long as none is specified in the device
> > > >    > tree. When there is one specified then it's no longer optional. We can't
> > > >    > do the right thing here without checking the device tree. Given that
> > > >    > it's simple to enable CONFIG_GENERIC_PHY I think this is the way to go.
> > > >
> > > >    But this force enables GENERIC_PHY when it's not needed.
> > > >
> > > >    There are commonly many device nodes in Linux dts files that are not used
> > > >    by an enabled Barebox driver.  It's normal to turn off a driver that might
> > > >    be or could be used.  Is it necessarily an error if a phy is present in
> > > >    the dts but we don't wish to include support for it?
> > >
> > > We need to distinguish the cases "There is a phy specified, but the
> > > reset defaults are good enough to go without a driver" and "There is a
> > > phy specified and we need driver support for it". barebox can't know
> > > this.
> >
> > Could we say that compiling barebox without CONFIG_GENERIC_PHY means
> > the driver is not needed and compiling it with the driver means that
> > is it?
>
> Please no. Enabling Kconfig options ideally gives you additional
> features, but it should not break anything. Consider the case when you
> need to enable CONFIG_GENERIC_PHY for something else like an LVDS phy or
> whatever, you don't want to end up with broken USB support then and have
> to choose whether USB or LVDS is working.

I thought your goal was to prevent less experienced users from
building a non-functional Barebox and then not understanding what they
had done.  Turning off a necessary driver and breaking USB while
producing no warnings nor errors.  But I now gather I was mistaken and
this isn't really the problem.

I think the specific situation you are concerned about is where:
A) the dts does define a phy for usb
B) This phy does not work in Barebox, e.g. no driver
C) Despite B, USB will still operate with the desired level
functionality without a working phy driver.

With the patch and CONFIG_GENERIC_PHY disabled, we get a non-fatal
return at step A and everything is good.  But once enabled, we now get
a fatal error at step B and this is not good.

Could this be fixed by making the error at B non-fatal?  This is more
how Linux works here: errors that are non-fatal in Linux's
phy_optional_get() path are fatal for Barebox.

Let phy_optional_get return NULL for any error.  Create a warning if
it appears the error was not: "no phy defined in dts".

But what if there really is an unexpected error with the PHY?  We
won't trigger the phy failure path in the driver!   I think
realistically, this is never going to make a difference for anyone.
Either way, one gets an error message and non-functional usb.  Does it
really matter where the error comes from?

> > Wouldn't it be easier to just delete the phy-names property to disable the phy?
>
> My point is that you sometimes start Linux with the barebox internal
> device tree. We should then pass a device tree Linux can handle
> properly. A barebox,status would just be ignored by Linux whereas a
> status = "disabled" property or deleted phy-names property would disable
> the phy for Linux as well.

If phy_optional_get does not make an unsupported phy an error, then
there isn't really a need to disable the phy anymore.

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