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Date:   Fri, 31 Jan 2020 14:46:30 -0600
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Muni Sekhar <munisekharrms@gmail.com>
Cc:     linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: pcie: xilinx: kernel hang - ISR readl()
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On Fri, Jan 31, 2020 at 10:04:05PM +0530, Muni Sekhar wrote:
> On Fri, Jan 31, 2020 at 12:30 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Thu, Jan 30, 2020 at 09:37:48PM +0530, Muni Sekhar wrote:
> > > On Thu, Jan 9, 2020 at 10:05 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > > >
> > > > 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".
> > > Does kernel writes to PCIe configuration space register ‘Device
> > > Control 2 Register’ (Offset 0x28)? When I tried to read this register,
> > > I noticed bit 4 is set (which disables completion timeouts) and rest
> > > all other bits are zero. So, Completion Timeout detection mechanism is
> > > disabled, right? If so what could be the reason for this?
> >
> > To my knowledge Linux doesn't set PCI_EXP_DEVCTL2_COMP_TMOUT_DIS
> > except for one powerpc case.  You can check yourself by using cscope
> > or grep to look for PCI_EXP_DEVCTL2_COMP_TMOUT_DIS or PCI_EXP_DEVCTL2.
> >
> > If you're seeing bit 4 (PCI_EXP_DEVCTL2_COMP_TMOUT_DIS) set, it's
> > likely because firmware set it.  You can try booting with
> > "pci=earlydump" to see what's there before Linux starts changing
> > things.
> 
> [    0.000000] pci 0000:01:00.0 config space:
> 
>                  00: 56 15 55 55 07 00 10 00 00 00 00 05 10 00 00 00
>                  10: 00 00 40 d0 00 00 00 00 00 00 00 00 00 00 00 00
>                  20: 00 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00
>                  30: 00 00 00 00 40 00 00 00 00 00 00 00 0b 01 00 00
>                  40: 01 48 03 00 08 00 00 00 05 60 00 00 00 00 00 00
>                  50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>                  60: 10 00 02 00 c2 8f 00 00 00 28 01 00 21 f4 03 00
>                  70: 01 00 21 00 00 00 00 00 00 00 00 00 00 00 00 00
>                  80: 00 00 00 00 02 00 00 00 10 00 00 00 00 00 00 00
>                  90: 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00
>                  a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>                  b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>                  c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>                  d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>                  e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>                  f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> 
> 
> Device Control 2" is located @offset 0x28 in PCI Express Capability
> Structure. But where does 'PCI Express Capability Structure' located
> in the above mentioned 'PCI Express Configuration Space'?

"lspci -v" tells you the location of the capability.  For example, on
my system:

  # lspci -vxxxs1c.0
  00:1c.0 PCI bridge: Intel Corporation Sunrise Point-LP PCI Express Root Port (rev f1) (prog-if 00 [Normal decode])
	  Flags: bus master, fast devsel, latency 0, IRQ 122
	  Bus: primary=00, secondary=02, subordinate=02, sec-latency=0
	  I/O behind bridge: None
	  Memory behind bridge: f1100000-f11fffff [size=1M]
	  Prefetchable memory behind bridge: None
	  Capabilities: [40] Express Root Port (Slot+), MSI 00
	  Capabilities: [80] MSI: Enable+ Count=1/1 Maskable- 64bit-
	  Capabilities: [90] Subsystem: Lenovo Sunrise Point-LP PCI Express Root Port
	  Capabilities: [a0] Power Management version 3
	  Capabilities: [100] Advanced Error Reporting
	  Capabilities: [140] Access Control Services
	  Capabilities: [200] L1 PM Substates
	  Capabilities: [220] Secondary PCI Express
		  LnkCtl3: LnkEquIntrruptEn-, PerformEqu-
		  LaneErrStat: 0
	  Kernel driver in use: pcieport
  00: 86 80 10 9d 07 04 10 00 f1 00 04 06 00 00 81 00
  10: 00 00 00 00 00 00 00 00 00 02 02 00 f0 00 00 20
  20: 10 f1 10 f1 f1 ff 01 00 00 00 00 00 00 00 00 00
  30: 00 00 00 00 40 00 00 00 00 00 00 00 ff 01 02 00
  40: 10 80 42 01 01 80 00 00 20 00 10 00 13 48 72 01
  50: 42 00 12 70 00 b2 04 00 00 00 48 01 00 00 00 00
  60: 00 00 00 00 37 08 00 00 00 04 00 00 0e 00 00 00
  70: 03 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  80: 05 90 01 00 18 02 e0 fe 00 00 00 00 00 00 00 00
  90: 0d a0 00 00 aa 17 38 22 00 00 00 00 00 00 00 00
  a0: 01 00 03 c8 00 00 00 00 00 00 00 00 00 00 00 00
  b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  d0: 11 10 00 07 42 18 00 00 08 00 9e 8b 00 00 00 00
  e0: 00 f7 73 00 03 90 00 00 16 80 12 00 00 00 00 00
  f0: 50 01 00 00 00 03 00 40 b3 0f 30 08 04 00 00 01

The PCI Express capability is at "[40]" (0x40) and PCI_EXP_DEVCTL2 is
a 16-bit register at offset 40 (0x28) from that.  So on my system,
PCI_EXP_DEVCTL2 is at 0x68 with value 0x0400 (PCI_EXP_DEVCTL2_LTR_EN).
This matches what lspci decodes:

  # lspci -vvs1c.0 | grep -A1 DevCtl2
       DevCtl2: Completion Timeout: 50us to 50ms, TimeoutDis-, LTR+, OBFF Disabled ARIFwd-
                AtomicOpsCtl: ReqEn- EgressBlck-