read the memory mapped address - pcie - kernel hangs

Muni Sekhar munisekharrms at
Thu Jan 9 06:14:16 EST 2020

On Thu, Jan 9, 2020 at 1:15 AM Greg KH <greg at> wrote:
> On Thu, Jan 09, 2020 at 12:30:20AM +0530, Muni Sekhar wrote:
> > Hi All,
> >
> > 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 ‘readl()’ in
> > 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.
> Why would you read that register again after writing to it?
> And are you sure you are reading/writing the correct size of the irq
> field?  I thought it was a "word" not "long"?  But that might depend on
> your hardware, do you have a pointer to the kernel driver source you are
> using for all of this?
Actually no need to read that register again. But reading that
register again should not freeze the system, right?

INT_STATUS register is 32-bit width, so readl() API is used(my system
is x86_64, Intel(R) Atom(TM) CPU). Instead of readl(), do I need to
use readw() twice? If so what is reason for this code change?

I’m trying to understand why system freezes without any crash dump
while reading the memory mapped IO from interrupt context?

FPGA code might be buggy, it may not send the completion for Memory
Read request. But CPU should not get stuck at LOAD instruction level..

When it hung, it does not even respond for SYSRQ button(SYSRQ is
enabled – in normal scenario it works), only way to recover is reboot
the system. I enabled almost all the kernel.panic* variables. I set
the kernel.panic to positive, so it should reboot after panic instead
of just hang. But it’s not rebooting by itself. Even 'pstore\ramoops’
also not helped.
After reboot I looked at the kern.log and most of the times it has
“^@^@^@^ ...“ line just before reboot.

Okay, I will write the minimalistic code to reproduce this one and
then share with you guys.

> thanks,
> greg k-h


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