Boot hang with SiFive PLIC when routing I2C-HID level-triggered interrupts

[Conor Dooley]

On Fri, Mar 15, 2024 at 05:33:06AM +0200, Eva Kurchatova wrote:
> On Thu, Mar 14, 2024 at 11:46 PM Conor Dooley <conor at kernel.org> wrote:
> > This immediately seemed odd to me, but I have no reason to disbelieve
> > you, given you say this was discovered in RVVM which is an emulator and
> > you should know whether or not registers are accessed.
> > The very first action taken by the ocores i2c controller driver when it
> > gets an interrupt though is to read a register:
> >
> >         u8 stat = oc_getreg(i2c, OCI2C_STATUS);
> >
> > I would expect that this handler would be called, and therefore you'd
> > see the register read, had the probe function of that driver run to
> > completion. I'd also expect that the interrupt would not even be
> > unmasked if that probe function had failed.
> > In your case though, you can see that the interrupt is not masked,
> > since it is being raised and handled repeatedly by the PLIC driver.
> > Has the i2c controller driver probed in the period of boot that you say
> > this problem manifests?
> 
> There is not a single problem with the ocores I2C driver. I2C-HID device
> itself has a separate IRQ line which is level-triggered.

Ah, I see. I am still unsure as to how that interrupt is handled by the
PLIC driver before the user for it (i2c-hid) requests it.


> This is to signal the host that there is input available without polling,
> since I2C is a master-driven bus with no "data available" notifications.
> So in reality the I2C-HID driver should handle the interrupt; Then it
> uses the I2C controller to access I2C-HID slave registers (via I2C) to
> read the incoming HID input report. I2C controller interrupts are unrelated;
> it's the link between the HID device and the host and it doesn't seem
> to be touched at all inside the I2C-HID IRQ handler (So it's just a pair
> of Claim/Complete actions). I2C ocores interrupts are not generated
> (and shouldn't) at that point, because no I2C transfer was initiated at all.
> 
> There is a way to make I2C-HID device edge-triggered, in RVVM
> emulation code, but it's not actually spec compliant. It gets rid of the
> hang too; However the same Claim/Complete actions without any
> handling inside the IRQ handler are observed at least once, which
> technically means a lost interrupt (Pressing a key somewhere early in
> boot thus doesn't propagate the keypress to the guest until you press
> another key later, after which both HID reports are read), so it's not
> a way how I'd like to mitigate this in the emulator code.
> I, and another developer from Haiku OS team (X512), are almost sure
> this is a kernel bug related to level triggered IRQs with PLIC or a
> specific incompatibility of PLIC/I2C-HID (Not the ocores I2C controller).
> 
> This hang is not reproducible with a Haiku OS guest in any way and
> most of the drivers involved seem to be FreeBSD based or written by
> X512 (Specifically the PLIC and I2C-HID drivers are). This person also
> believes that this Claim/Complete behavior on PLIC side without any
> other actions made in between is erroneous kernel behavior too.

Maybe it is, I've re-added Thomas and Samuel to CC as I said before that
they know much more than I about the code in question. I was just
pointing out that the behaviour you were seeing is inconsistent with
what I have come across myself for interrupts that have not had a user
request them.

> I am open to discussions what specifically could be wrong with the VM
> since one of my end goals is to just make HID devices work without
> issues there; However if a simple 2-line patch (which I'm not entirely
> sure of it's implications) that removes return path at line 223 in PLIC
> driver resolves the issue (I kept a guest in a 24 hr reboot loop whilst
> spamming fake I2C-HID input and no hang was observed), then it does
> lead me to belief that it's at least not some blatant emulation issue.
> 
> I came here to collect some kernel devs opinions since we are
> debugging this for some 2 weeks already. Your initial understanding
> that something is wrong with ocores I2C controller is not what I meant,
> sorry for lacking in my explanation.
> 
> >Are interrupts unmasked by default on RVVM?
> 
> By default all PLIC ENABLE registers are set to zero. All PRIO,
> THRESHOLD registers are zero on reset. So all PLIC state is
> simply zeroed on reset, as can be seen here:
> https://github.com/LekKit/RVVM/blob/f81df57a2af77cbae25fd3cc65d07106d9505e23/src/devices/plic.c#L265
> 
> > Have you checked that this actually affects any actual hardware?
> 
> I might very soon if no one has immediate ideas what is wrong;
> Problem is that I don't have hardware that exposes PLIC IRQ lines to
> the user. It might be possible to use some FPGA or at least reproduce
> in other simulators.
> 
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