Re: pcie: xilinx: kernel hang - ISR readl()

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Jan 09, 2020 at 08:47:51AM +0530, Muni Sekhar wrote:
> On Thu, Jan 9, 2020 at 1:45 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Tue, Jan 07, 2020 at 09:45:13PM +0530, Muni Sekhar wrote:
> > > Hi,
> > >
> > > I have module with Xilinx FPGA. It implements UART(s), SPI(s),
> > > parallel I/O and interfaces them to the Host CPU via PCI Express bus.
> > > I see that my system freezes without capturing the crash dump for
> > > certain tests. I debugged this issue and it was tracked down to the
> > > below mentioned interrupt handler code.
> > >
> > >
> > > In ISR, first reads the Interrupt Status register using ‘readl()’ as
> > > given below.
> > >     status = readl(ctrl->reg + INT_STATUS);
> > >
> > >
> > > And then clears the pending interrupts using ‘writel()’ as given blow.
> > >         writel(status, ctrl->reg + INT_STATUS);
> > >
> > >
> > > I've noticed a kernel hang if INT_STATUS register read again after
> > > clearing the pending interrupts.
> > >
> > > Can someone clarify me why the kernel hangs without crash dump incase
> > > if I read the INT_STATUS register using readl() after clearing the
> > > pending bits?
> > >
> > > Can readl() block?
> >
> > readl() should not block in software.  Obviously at the hardware CPU
> > instruction level, the read instruction has to wait for the result of
> > the read.  Since that data is provided by the device, i.e., your FPGA,
> > it's possible there's a problem there.
> 
> Thank you very much for your reply.
> Where can I find the details about what is protocol for reading the
> ‘memory mapped IO’? Can you point me to any useful links..
> I tried locate the exact point of the kernel code where CPU waits for
> read instruction as given below.
> readl() -> __raw_readl() -> return *(const volatile u32 __force *)add
> Do I need to check for the assembly instructions, here?

The C pointer dereference, e.g., "*address", will be some sort of a
"load" instruction in assembly.  The CPU wait isn't explicit; it's
just that when you load a value, the CPU waits for the value.

> > Can you tell whether the FPGA has received the Memory Read for
> > INT_STATUS and sent the completion?
> 
> Is there a way to know this with the help of software debugging(either
> enabling dynamic debugging or adding new debug prints)? Can you please
> point some tools\hw needed to find this?

You could learn this either via a PCIe analyzer (expensive piece of
hardware) or possibly some logic in the FPGA that would log PCIe
transactions in a buffer and make them accessible via some other
interface (you mentioned it had parallel and other interfaces).

> > On the architectures I'm familiar with, if a device doesn't respond,
> > something would eventually time out so the CPU doesn't wait forever.
> 
> What is timeout here? I mean how long CPU waits for completion? Since
> this code runs from interrupt context, does it causes the system to
> freeze if timeout is more?

The Root Port should have a Completion Timeout.  This is required by
the PCIe spec.  The *reporting* of the timeout is somewhat
implementation-specific since the reporting is outside the PCIe
domain.  I don't know the duration of the timeout, but it certainly
shouldn't be long enough to look like a "system freeze".

> lspci output:
> $ lspci
> 00:00.0 Host bridge: Intel Corporation Atom Processor Z36xxx/Z37xxx
> Series SoC Transaction Register (rev 11)
> 00:02.0 VGA compatible controller: Intel Corporation Atom Processor
> Z36xxx/Z37xxx Series Graphics & Display (rev 11)
> 00:13.0 SATA controller: Intel Corporation Atom Processor E3800 Series
> SATA AHCI Controller (rev 11)
> 00:14.0 USB controller: Intel Corporation Atom Processor
> Z36xxx/Z37xxx, Celeron N2000 Series USB xHCI (rev 11)
> 00:1a.0 Encryption controller: Intel Corporation Atom Processor
> Z36xxx/Z37xxx Series Trusted Execution Engine (rev 11)
> 00:1b.0 Audio device: Intel Corporation Atom Processor Z36xxx/Z37xxx
> Series High Definition Audio Controller (rev 11)
> 00:1c.0 PCI bridge: Intel Corporation Atom Processor E3800 Series PCI
> Express Root Port 1 (rev 11)
> 00:1c.2 PCI bridge: Intel Corporation Atom Processor E3800 Series PCI
> Express Root Port 3 (rev 11)
> 00:1c.3 PCI bridge: Intel Corporation Atom Processor E3800 Series PCI
> Express Root Port 4 (rev 11)
> 00:1d.0 USB controller: Intel Corporation Atom Processor Z36xxx/Z37xxx
> Series USB EHCI (rev 11)
> 00:1f.0 ISA bridge: Intel Corporation Atom Processor Z36xxx/Z37xxx
> Series Power Control Unit (rev 11)
> 00:1f.3 SMBus: Intel Corporation Atom Processor E3800 Series SMBus
> Controller (rev 11)
> 01:00.0 RAM memory: PLDA Device 5555

Is this 01:00.0 device the FPGA?

> 03:00.0 Ethernet controller: Intel Corporation I210 Gigabit Network
> Connection (rev 03)