Re: DWC eDMA weirdness

On Wed, Jul 29, 2026 at 10:49:56PM +0200, Niklas Cassel wrote:
> On Wed, Jul 29, 2026 at 10:28:22PM +0200, Niklas Cassel wrote:
> > Getting two IRQs when we are only expecting a single IRQ is a very
> > serious error, and suggests that something is very wrong in the EPC
> > driver.
> > 
> > Scream loudly so that EPC driver developers can see the problem,
> > which might otherwise be very hard to detect.
> > 
> > Signed-off-by: Niklas Cassel <[email protected]>
> > ---
> >  drivers/misc/pci_endpoint_test.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> > 
> > diff --git a/drivers/misc/pci_endpoint_test.c b/drivers/misc/pci_endpoint_test.c
> > index 3635741c3e7a..a212e53654c6 100644
> > --- a/drivers/misc/pci_endpoint_test.c
> > +++ b/drivers/misc/pci_endpoint_test.c
> > @@ -178,6 +178,19 @@ static irqreturn_t pci_endpoint_test_irqhandler(int irq, void *dev_id)
> >  	if (reg & STATUS_IRQ_RAISED) {
> >  		test->last_irq = irq;
> >  		complete(&test->irq_raised);
> > +		/*
> > +		 * The endpoint test driver performs all testing sequentially.
> > +		 * This means that test->irq_raised.done should never exceed 1.
> > +		 * If it does, then we received two IRQs in a row, without a
> > +		 * successful wait_for_completion_timeout() call in between.
> > +		 *
> > +		 * While complete() increases test->irq_raised.done by one,
> > +		 * wait_for_completion_timeout() reduces test->irq_raised.done
> > +		 * by one on success.
> > +		 *
> > +		 * Please debug your EPC driver if you see this warning.
> > +		 */
> > +		WARN_ON(test->irq_raised.done > 1);
> >  	}
> >  
> >  	return IRQ_HANDLED;
> > -- 
> > 2.55.0
> > 
> 
> Hello Koichiro,
> 
> I created this patch in response to a problem that I saw on v7.2-rc5.
> 
> When running the pci_endpoint selftest:
> # /pcitest
> (snip)
> #  RUN           pcie_ep_doorbell.DOORBELL_TEST ...
> [  215.256772] pci-endpoint-test 0000:01:00.0: Failed to trigger doorbell in endpoint
> #            OK  pcie_ep_doorbell.DOORBELL_TEST
> ok 23 pcie_ep_doorbell.DOORBELL_TEST
> # FAILED: 19 / 23 tests passed.
> # 2 skipped test(s) detected. Consider enabling relevant config options to improve coverage.
> # Totals: pass:17 fail:4 xfail:0 xpass:0 skip:2 error:0
> 
> 
> I saw the weird error:
> "Failed to trigger doorbell in endpoint"
> 
> Ignore that the test case leaves things to be desired, since it passes even
> though we see this warning.
> 

How come this is possible? Doorbell timeout occured, but still Doorbell got
triggered in EP?

> 
> The problem seems to be that the embedded doorbell IRQ handler is executed twice,
> in response to a single writel() which rings the doorbell.
> 
> diff --git a/drivers/pci/endpoint/functions/pci-epf-test.c b/drivers/pci/endpoint/functions/pci-epf-test.c
> index 4802d4f80f78..7f4360579ffb 100644
> --- a/drivers/pci/endpoint/functions/pci-epf-test.c
> +++ b/drivers/pci/endpoint/functions/pci-epf-test.c
> @@ -714,6 +714,7 @@ static irqreturn_t pci_epf_test_doorbell_handler(int irq, void *data)
>  
>         status |= STATUS_DOORBELL_SUCCESS;
>         reg->status = cpu_to_le32(status);
> +       trace_printk("calling pci_epf_test_raise_irq()\n");
>         pci_epf_test_raise_irq(epf_test, reg);
>  
>         return IRQ_HANDLED;
> 
> 
> As you can see:
>  irq/58-pci-ep-t-221     [006] .....    58.601020: pci_epf_test_doorbell_handler: calling pci_epf_test_raise_irq()
>  irq/58-pci-ep-t-221     [006] .....    58.601421: pci_epf_test_doorbell_handler: calling pci_epf_test_raise_irq()
> 
> It is called twice for a single doorbell ring.
> 

Which doorbell is used here? Platform MSI or Embedded one?

- Mani

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