Re: [PATCH 2/3] PCI: dwc: pci-dra7xx: Improve MSI IRQ handling

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Thu, Feb 15, 2018 at 09:59:21AM +0530, Vignesh R wrote:
> Hi,
> 
> On Monday 12 February 2018 11:28 PM, Lorenzo Pieralisi wrote:
> > On Fri, Feb 09, 2018 at 05:34:14PM +0530, Vignesh R wrote:
> >> We need to ensure that there are no pending MSI IRQ vector set (i.e
> >> PCIE_MSI_INTR0_STATUS reads 0 at least once) before exiting
> >> dra7xx_pcie_msi_irq_handler(). Else, the dra7xx PCIe wrapper will not
> >> register new MSI IRQs even though PCIE_MSI_INTR0_STATUS shows IRQs are
> >> pending. Therefore, keep calling dra7xx_pcie_msi_irq_handler() until it
> >> returns IRQ_NONE, which suggests that PCIE_MSI_INTR0_STATUS is 0.
> >> 
> >> This fixes a bug, where PCIe wifi cards with 4 DMA queues like Intel
> >> 8260 used to throw following error and stall during ping/iperf3 tests.
> >> 
> >> [   97.776310] iwlwifi 0000:01:00.0: Queue 9 stuck for 2500 ms.
> >> 
> >> Signed-off-by: Vignesh R <vigneshr@ti.com>
> >> ---
> >>  drivers/pci/dwc/pci-dra7xx.c | 21 ++++++++++++++++++---
> >>  1 file changed, 18 insertions(+), 3 deletions(-)
> >> 
> >> diff --git a/drivers/pci/dwc/pci-dra7xx.c b/drivers/pci/dwc/pci-dra7xx.c
> >> index ed8558d638e5..3420cbf7b60a 100644
> >> --- a/drivers/pci/dwc/pci-dra7xx.c
> >> +++ b/drivers/pci/dwc/pci-dra7xx.c
> >> @@ -254,14 +254,31 @@ static irqreturn_t dra7xx_pcie_msi_irq_handler(int irq, void *arg)
> >>        struct dra7xx_pcie *dra7xx = arg;
> >>        struct dw_pcie *pci = dra7xx->pci;
> >>        struct pcie_port *pp = &pci->pp;
> >> +     int count = 0;
> >>        unsigned long reg;
> >>        u32 virq, bit;
> >>  
> >>        reg = dra7xx_pcie_readl(dra7xx, PCIECTRL_DRA7XX_CONF_IRQSTATUS_MSI);
> >> +     dra7xx_pcie_writel(dra7xx, PCIECTRL_DRA7XX_CONF_IRQSTATUS_MSI, reg);
> >>  
> >>        switch (reg) {
> >>        case MSI:
> >> -             dw_handle_msi_irq(pp);
> >> +             /*
> >> +              * Need to make sure no MSI IRQs are pending before
> >> +              * exiting handler, else the wrapper will not catch new
> >> +              * IRQs. So loop around till dw_handle_msi_irq() returns
> >> +              * IRQ_NONE
> >> +              */
> >> +             while (dw_handle_msi_irq(pp) != IRQ_NONE && count < 1000)
> >> +                     count++;
> >> +
> >> +             if (count == 1000) {
> >> +                     dev_err(pci->dev, "too much work in msi irq\n");
> >> +                     dra7xx_pcie_writel(dra7xx,
> >> +                                        PCIECTRL_DRA7XX_CONF_IRQSTATUS_MSI,
> >> +                                        reg);
> >> +                     return IRQ_HANDLED;
> > 
> > I am not merging any code patching this IRQ handling routine anymore
> > unless you thoroughly explain to me how this CONF_IRQSTATUS_MSI register
> > works (and how it is related to DW registers) and why this specific host
> > controller needs handling that is not required by any other host
> > controller relying on dw_handle_msi_irq().
> 
> Unlike other DW PCIe controllers, TI implementation has a wrapper on top
> of DW core. This wrapper latches the DW core level MSI and legacy
> interrupts and then propagates it to GIC.
> PCIECTRL_DRA7XX_CONF_IRQSTATUS_MSI register is present in this TI
> wrapper which aggregates all the MSI IRQs(PCIE_MSI_INTR0_STATUS) of DW
> level. They are mapped on the MSI interrupt line of PCIe controller,
> using a single status bit in the PCIECTRL_TI_CONF_IRQSTATUS_MSI register.
> 
> So, the irq handler, dra7xx_pcie_msi_irq_handler(), first needs to look
> at PCIECTRL_DRA7XX_CONF_IRQSTATUS_MSI[4] to know that its MSI IRQ and
> then call dw_handle_msi_irq() to handle individual MSI vectors.
> Driver has to make sure there are no pending vectors in DW core MSI

How can it make *sure* ? And what makes the wrapper latch MSI IRQs
again ?

> status register before exiting handler. Otherwise next MSI IRQ will not
> be latched by the wrapper.

I am sorry but I do not understand how this works - what is the
condition that makes wrapper latch IRQs again ? This is at least
racy, if not outright broken.

That count == 1000 is a symptom there is something broken on how this
driver handles IRQs and I have the impression that we are applying
plasters on top of plasters to make it less broken than it actually is.

> > I suspect there is a code design flaw with the way this host handles
> > IRQs and we are going to find it and fix it the way it should, not with
> > any plaster like this patch.
> > 
> 
> I agree there has been some churn wrt this wrapper level IRQ handler.
> But, that was because hardware documentation/TRM did not match
> actual behavior and so it took some time to understand how the
> hardware is working.

How does HW work :) ? Please explain in detail how this works in HW
then we will get to the code.

Thanks,
Lorenzo

> I have extensively tested this series on multiple problematic PCIe USB
> cards and PCIe WiFi cards over week long stress tests. And also had
> some agreement with internal hardware designers.  Hardware
> documentations will also be updated.
> 
> 
> > Lorenzo
> > 
> >> +             }
> >>                break;
> >>        case INTA:
> >>        case INTB:
> >> @@ -275,8 +292,6 @@ static irqreturn_t dra7xx_pcie_msi_irq_handler(int irq, void *arg)
> >>                break;
> >>        }
> >>  
> >> -     dra7xx_pcie_writel(dra7xx, PCIECTRL_DRA7XX_CONF_IRQSTATUS_MSI, reg);
> >> -
> >>        return IRQ_HANDLED;
> >>  }
> >>  
> >> -- 
> >> 2.16.1
> >> 
> 
> -- 
> Regards
> Vignesh