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From:   Muni Sekhar <munisekharrms@gmail.com>
Date:   Tue, 7 Jan 2020 21:45:13 +0530
Message-ID: <CAHhAz+ijBTp55gZYAejWthnvdmR_qyQJpVV4r1gyQ-Kud6t9qg@mail.gmail.com>
Subject: pcie: xilinx: kernel hang - ISR readl()
To:     linux-pci@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org
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Sender: linux-kernel-owner@vger.kernel.org
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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 =E2=80=98readl()=E2=
=80=99 as
given below.
    status =3D readl(ctrl->reg + INT_STATUS);


And then clears the pending interrupts using =E2=80=98writel()=E2=80=99 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?


Snippet of the ISR code is given blow:

https://pastebin.com/WdnZJZF5


static irqreturn_t pcie_isr(int irq, void *dev_id)

{

        struct test_device *ctrl =3D data;

        u32 status;

=E2=80=A6


        status =3D 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 =3D readl(ctrl->reg + INT_STATUS);

                ....

        }

..

        return IRQ_HANDLED;
}


--=20
Thanks,
Sekhar