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@xxxxxxxxxx> 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)
