Re: [PATCH] gpiolib: Avoid side effects in gpio_is_visible()

[andy . shevchenko]

Wed, May 17, 2023 at 09:30:51PM +0000, Chris Packham kirjoitti:
> On 17/05/23 20:54, Andy Shevchenko wrote:
> > On Wed, May 17, 2023 at 2:50 AM Chris Packham
> > <Chris.Packham@xxxxxxxxxxxxxxxxxxx> wrote:
> >> On 17/05/23 10:47, Kent Gibson wrote:

...

> >> The first is a userspace driver for a Power Over Ethernet Controller+PSE
> >> chipset (I'll refer to this as an MCU since the thing we talk to is
> >> really a micro controller with a vendor supplied firmware on it that
> >> does most of the PoE stuff). Communication to the MCU is based around
> >> commands sent via i2c. But there are a few extra GPIOs that are used to
> >> reset the MCU as well as provide a mechanism for quickly dropping power
> >> on certain events (e.g. if the temperature monitoring detects a problem).
> > Why does the MCU have no in-kernel driver?
> 
> There isn't any PoE PSE infrastructure in the kernel. I'm not really 
> sure what it'd look like either as the hardware designs are all highly 
> customized and often have very specialized requirements. Even the vendor 
> reference boards tend to use the i2c userspace interface and punt 
> everything to a specialist application.
> 
> Of course if anyone is thinking about adding PoE PSE support in-kernel 
> I'd be very keen to be involved.

But what do net subsystem guys know about this? Have you had a chance
to ask them?

> >> We do have a small kernel module that grabs the GPIOs based on the
> >> device tree and exports them with a known names (e.g. "poe-reset",
> >> "poe-dis") that the userspace driver can use.
> > So, besides that you repeat gpio-aggregator functionality, you already
> > have a "proxy" driver in the kernel. What prevents you from doing more
> > in-kernel?
> 
> Yes true. The main issue is that without total support for the class of 
> device in the kernel there's little more that you can do other than 
> exposing gpios (either as gpio_export_link() or some other bespoke 
> interface).
> 
> >>   Back when that code was
> >> written we did consider not exporting the GPIOs and instead having some
> >> other sysfs/ioctl interface into this kernel module but that seemed more
> >> work than just calling gpiod_export() for little gain. This is where
> >> adding the gpio-names property in our .dts would allow libgpiod to do
> >> something similar.
> >>
> >> Having the GPIOs in sysfs is also convenient as we can have a systemd
> >> ExecStopPost script that can drop power and/or reset the MCU if our
> >> application crashes.
> > I'm a bit lost. What your app is doing and how that is related to the
> > (userspace) drivers?
> 
> Probably one of the primary things it's doing is bringing the chip out 
> of reset by driving the GPIO (we don't want the PoE PSE supplying power 
> if nothing is monitoring the temperature of the system). There's also 
> some corner cases involving not resetting the PoE chipset on a hot restart.

So, do I understand correct the following?
There is a PoE PSE which has a proprietary user space driver and to make it
work reliably we have to add a quirk which utilizes the GPIO sysfs?

> >> I'm not sure if the GPIO chardev interface deals
> >> with releasing the GPIO lines if the process that requested them exits
> >> abnormally (I assume it does) and obviously our ExecStopPost script
> >> would need updating to use some of the libgpiod applications to do what
> >> it currently does with a simple 'echo 1 >.../poe-reset'
> >>
> >> The second application is a userspace driver for a L3 network switch
> >> (actually two of them for different silicon vendors). Again this needs
> >> to deal with resets for PHYs connected to the switch that the kernel has
> >> no visibility of as well as the GPIOs for the SFP cages. Again we have a
> >> slightly less simple kernel module that grabs all these GPIOs and
> >> exports them with known names. This time there are considerably more of
> >> these GPIOs (our largest system currently has 96 SFP+ ports with 4 GPIOs
> >> per port) so we're much more reliant on being able to do things like
> >> `for x in port*tx-dis; do echo 1 >$x; done`
> > Hmm... Have you talked to the net subsystem guys? I know that there is
> > a lot going on around SFP cage enumeration for some of the modules
> > (Marvell?) and perhaps they can advise something different.
> 
> Yes I'm aware of the switchdev work and I'm very enthusiastic about it 
> (as an aside I do have a fairly functional switchdev driver for some of 
> the older Marvell Poncat2 silicon, never quite got to submitting it 
> upstream before we ran out of time on the project).
> 
> Again the problem boils down to the fact that we have a userspace switch 
> driver (which uses a vendor supplied non-free SDK). So despite the 
> kernel having quite good support for SFPs I can't use it without a 
> netdev to attach it to.

That user space driver is using what from the kernel? GPIO sysfs?

> >> I'm sure both of these applications could be re-written around libgpiod
> >> but that would incur a significant amount of regression testing on
> >> existing platforms. And I still consider dealing with GPIO chips an
> >> extra headache that the applications don't need (particularly with the
> >> sheer number of them the SFP case).
> > It seems to me that having no in-kernel driver for your stuff is the
> > main point of all headache here. But I might be mistaken.
> 
> It certainly doesn't help, but I do think that is all orthogonal to the 
> fact that gpio_is_visible() changes things rather than just determining 
> if an attribute should be exported or not.

Sorry for being unhelpful here. But without understanding the issue we can't
propose better solutions.


-- 
With Best Regards,
Andy Shevchenko