Re: read the memory mapped address - pcie - kernel hangs

[Muni Sekhar <munisekharrms@gmail.com>]

On Fri, Jan 10, 2020 at 4:46 PM Primoz Beltram <primoz.beltram@kate.si> wrote:
>
> Hi,
> Have read also other replays to this topic.
> I have seen-debug such deadlock problems with FPGA based PCIe endpoint
> devices (Xilinx chips) and usually (if not signal integrity problems),
> the problem was in wrong AXI master/slave bus handling in FPGA design.
> I guess you have FPGA Xilinx PCIe endpoint IP core attached as AXI
> master to FPGA internal AXI bus (access to AXI slaves inside FPGA design).
> If FPGA code in your design does not handle correctly AXI master
> read/write requests, e.g. FPGA AXI slave does not generate bus ACK in
> correct way, the PCIe bus will stay locked (no PCIe completion sent
> back), resulting in complete system lock. Some PCIe root chips have
> diagnostic LEDs to help decode PCIe problems.
>  From your notice about doing two 32bit reads on 64bit CPU, I would
> guess the problem is in handling AXI transfer size signals in FPGA slave
> code.
> I would suggest you to check the code in FPGA design. You can use FPGA
> test bench simulation to check the behaviour of PCIe endpoint originated
> AXI read/write requests.
> Xilinx provides test bench simulation code for their PCIe IP's.
> They provide also PCIe root port model, so you can simulate AXI
> read/writes accesses as they would come from CPU I/O memory requests via
> PCIe TLPs.
Thank you so much for sharing valuable information, will work on this.

> WBR Primoz
>
> On 8. 01. 20 20:00, 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.
> >
> > My system freezes only after executing the same ISR code after
> > millions of interrupts. Basically reading the memory mapped register
> > in ISR resulting this behavior.
> > If I comment “status = readl(ctrl->reg + INT_STATUS);” after clearing
> > the pending interrupts then system is stable .
> >
> > As a temporary workaround I avoided reading the INT_STATUS register
> > after clearing the pending bits, and this code change works fine.
> >
> > Can someone clarify me why the kernel hangs without crash dump incase
> > if I read the INT_STATUS register using readl() after
> > clearing(writel()) the pending bits?
> >
> > To read the memory mapped IO kernel provides {read}{b,w,l,q}() API’s.
> > If PCIe card is not responsive , can call to readl() from interrupt
> > context makes system freeze?
> >
> > Thanks for any suggestions and solutions to this problem!
> >
> > Snippet of the ISR code is given blow:
> > https://pastebin.com/as2tSPwE
> >
> >
> > static irqreturn_t pcie_isr(int irq, void *data)
> >
> > {
> >
> >          struct test_device *ctrl = (struct test_device *)data;
> >
> >          u32 status;
> >
> > …
> >
> >
> >
> >          status = readl(ctrl->reg + INT_STATUS);
> >
> >          /*
> >
> >           * Check to see if it was our interrupt
> >
> >           */
> >
> >          if (!(status & 0x000C))
> >
> >                  return IRQ_NONE;
> >
> >
> >
> >          /* Clear the interrupt */
> >
> >          writel(status, ctrl->reg + INT_STATUS);
> >
> >
> >
> >          if (status & 0x0004) {
> >
> >                  /*
> >
> >                   * Tx interrupt pending.
> >
> >                   */
> >
> >                   ....
> >
> >         }
> >
> >
> >
> >          if (status & 0x0008) {
> >
> >                  /* Rx interrupt Pending */
> >
> >                  /* The system freezes if I read again the INT_STATUS
> > register as given below */
> >
> >                  status = readl(ctrl->reg + INT_STATUS);
> >
> >                  ....
> >
> >          }
> >
> > ..
> >
> >          return IRQ_HANDLED;
> > }
> >
>


-- 
Thanks,
Sekhar

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