Re: [PATCH] usb: dwc2: Fix shutdown callback in platform

[Doug Anderson <dianders@chromium.org>]

Hi,

On Fri, May 29, 2020 at 12:00 PM Alan Stern <stern@rowland.harvard.edu> wrote:
>
> On Fri, May 29, 2020 at 11:45:53AM -0700, Doug Anderson wrote:
> > Hi,
> >
> > On Fri, May 29, 2020 at 11:21 AM Frank Mori Hess <fmh6jj@gmail.com> wrote:
> > >
> > > On Fri, May 29, 2020 at 1:53 PM Doug Anderson <dianders@chromium.org> wrote:
> > > > >
> > > > > I don't get it.  A hypothetical machine could have literally anything
> > > > > sharing the IRQ line, right?
> > > >
> > > > It's not a real physical line, though?  I don't think it's common to
> > > > have a shared interrupt between different IP blocks in a given SoC.
> > > > Even if it existed, all the drivers should disable their interrupts?
> > >
> > > I don't know, it's a hypothetical machine so it can be whatever you
> > > want.  The driver requests shared irqs, if it doesn't actually support
> > > irq sharing, it shouldn't request them.
> >
> > I guess?  As I understood it drivers have to be very carefully coded
> > up to support sharing their IRQ with someone else and I'm not
> > convinced dwc2 does that anyway.  Certainly it doesn't hurt to keep
> > dwc2 clean, but until I see someone that's actually sharing dwc2's
> > interrupt and I can actually see an example I'm not sure I'm going to
> > spend too much time thinking about it.
>
> This is silly.  If the driver says it supports shared IRQs, then it
> should actually support them.

Ah.  The IRQ here is "shared" because of the way that the dwc2 driver
is architected.  The "gadget" mode and "host" mode driver "share" the
IRQ.  ...but there is no non-dwc2 device sharing.  In other words,
it's not like there's a UART or and i2c device that would share it.


> > > > > Anyways, my screaming interrupt occurs after a a new kernel has been
> > > > > booted with kexec.  In this case, it doesn't matter if the old kernel
> > > > > called disable_irq or not.  As soon as the new kernel re-enables the
> > > > > interrupt line, the kernel immediately disables it again with a
> > > > > backtrace due to the unhandled screaming interrupt.  That's why the
> > > > > dwc2 hardware needs to have its interrupts turned off when the old
> > > > > kernel is shutdown.
> > > >
> > > > Isn't that a bug with your new kernel?  I've seen plenty of bugs where
> > > > drivers enable their interrupt before their interrupt handler is set
> > > > to handle it.  You never know what state the bootloader (or previous
> > > > kernel) might have left things in and if an interrupt was pending it
> > > > shouldn't kill you.
> > >
> > > It wouldn't hurt to add disabling of the dwc2 irq early in dwc2
> > > initialization,
> >
> > More than it not hurting, I'd consider it a bug in the driver (and a
> > much more serious one than shutdown not disabling the interrupt).
>
> Normally the first thing a driver would do is reset the hardware, and
> that reset should disable any interrupt source.

Yup.


> > > but why leave the irq screaming after shutdown?
> >
> > Sure.  So I guess the answer is to just do both disable the interrupt
> > and make sure that the interrupt handler has finished.
> >
> > dwc2_disable_global_interrupts(hsotg);
> > disable_irq(hsotg->irq);
>
> Drivers with shared IRQs don't call disable_irq(); they call
> synchronize_irq().  It will do what you want (wait until all running
> handlers have returned).

OK.

-Doug