Re: Problems (timeouts) when testing usb-ohci with qemu

[Alan Stern <stern@rowland.harvard.edu>]

On Tue, Apr 23, 2024 at 10:04:17AM -0700, Guenter Roeck wrote:
> Hi,
> 
> when testing usb-ohci

What is usb-ohci?  Do you mean ohci-hcd?

>  with qemu's pci-ohci emulation, I keep getting
> random usb interface timeouts. Sometimes the usb_hub_wq times out.
...

> Sometimes there is an i/o scheduling timeout such as
...

> This is not a new problem; I have seen it forever. Recently I spent some
> time trying to understand the problem and found that the linux driver does
> not always handle all ohci interupts.

Please be more specific: _Which_ interrupts aren't being handled?  That 
is, which flags remain set in the intrstatus register when the handler 
returns?

>  Since the interrupt is shared and
> thus level triggered, that means that interrupts are still pending when
> ohci_irq() exits. The interrupt core in Linux does not re-enter the
> interrupt handler, presumably because it is marked as shared interrupt
> and returns that the interrupt has been handled.

Isn't that behavior mistaken?  A level-triggered IRQ that remains set when 
it is re-enabled (when the various shared handlers return) should cause 
another interrupt to occur right away.

Edged-triggered interrupts would be a different story, of course.

> I found two possible fixes for the problem. One essentially mirrors the
> code from ehci_irq(), the other adds a (bad) kludge into qemu. Both "fix"
> or work around the problem.
> 
> Question is: What is actually wrong ? Something in the generic interrupt
> handling code in Linux, something in the Linux usb-ohci driver, or
> something in qemu ? Any idea how a proper fix might look like ?

To answer these questions we need more information.

Alan Stern