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

[Vignesh R <vigneshr@ti.com>]

On Tuesday 06 March 2018 08:42 PM, Lorenzo Pieralisi wrote:
> 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 ?
> 

This is the sequence that I got from discussion with internal HW team:
 1. read CONF_IRQSTATUS_MSI in wrapper and check if MSI bit
 2. clear CONF_IRQSTATUS_MSI
 3. read, clear and handle PCIE_MSI_INTR0_STATUS vectors
 4. repeat step 3 until PCIE_MSI_INTR0_STATUS reads 0

If read of PCIE_MSI_INTR0_STATUS returns 0 at least once, then its
guaranteed that the next time any vector is set in PCIE_MSI_INTR0_STATUS
register(due to MSI IRQ), wrapper will latch it and raise an IRQ to CPU.


>> 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.
> 

It is an upper bound on how many times driver looks at
PCIE_MSI_INTR0_STATUS register, so that there is no infinite looping
when there is an IRQ flood due to misbehaving EP. count == 1000
condition should not happen and it means something is wrong in the
system.  I haven't hit this situation in testing
I can either remove this or put a WARN_ON to say this situation should
not have happened, if that makes you more comfortable with the patch.


>>> 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.
> 

Software needs to ensure that PCIE_MSI_INTR0_STATUS needs be 0 by
reading it. Then, when the next MSI IRQ is raised  CONF_IRQSTATUS_MSI
register in the wrapper will latch the next IRQ.

This is my current knowledge, let me know if you need to know anything
specifically, I will try to ask HW team.

Regards
Vignesh

> 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