Re: [PATCH v4 5/8] PCI/DPC: Converge EDR and DPC Path of clearing AER registers

[Naveen Naidu <naveennaidu479@gmail.com>]

On 20/10, Bjorn Helgaas wrote:
> [+cc Keith, Sinan, Oza]
> 
> On Tue, Oct 05, 2021 at 10:48:12PM +0530, Naveen Naidu wrote:
> > In the EDR path, AER registers are cleared *after* DPC error event is
> > processed. The process stack in EDR is:
> > 
> >   edr_handle_event()
> >     dpc_process_error()
> >     pci_aer_raw_clear_status()
> >     pcie_do_recovery()
> > 
> > But in DPC path, AER status registers are cleared *while* processing
> > the error. The process stack in DPC is:
> > 
> >   dpc_handler()
> >     dpc_process_error()
> >       pci_aer_clear_status()
> >     pcie_do_recovery()
> 
> These are accurate but they both include dpc_process_error(), so we
> need a hint to show why the one here is different from the one in the
> EDR path, e.g.,
> 
>   dpc_handler
>     dpc_process_error
>       if (reason == 0)
>         pci_aer_clear_status    # uncorrectable errors only
>     pcie_do_recovery
> 
> > In EDR path, AER status registers are cleared irrespective of whether
> > the error was an RP PIO or unmasked uncorrectable error. But in DPC, the
> > AER status registers are cleared only when it's an unmasked uncorrectable
> > error.
> > 
> > This leads to two different behaviours for the same task (handling of
> > DPC errors) in FFS systems and when native OS has control.
> 
> FFS?
>

Firmware First Systems

> I'd really like to have a specific example of how a user would observe
> this difference.  I know you probably don't have two systems to
> compare like that, but maybe we can work it out manually.
> 

Apologies again! Reading through the code again and the specification, I
realize that my understanding was very incorrect at the time of making
this patch. I grossly oversimplified EDR and DPC when I was learning
about it.

I'll drop this patch when I send the v5 for the series.

Apologies again ^^'

> I guess you're saying the problem is in the native DPC handling, and
> we don't clear the AER status registers for ERR_NONFATAL,
> ERR_NONFATAL, etc., right?
> 

But yes, I did have this question though (I wasn't able to find the
answers to it when reading the spec). Why do we not clear the entire
ERR_NONFATAL and ERR_FATAL registers in the DPC path just like EDR does
using the pci_aer_raw_clear_status() before going to pcie_do_recovery()

I am sure I might have missed something in the spec. I guess I'll
look/re-read these bits again.

Thanks for the review :)

> I think the current behavior is from 8aefa9b0d910 ("PCI/DPC: Print AER
> status in DPC event handling"), where Keith explicitly mentions those
> cases.  The commit log here should connect back to that and explain
> whether something has changed.
> 
> I cc'd Keith and the reviewers of that change in case any of them have
> time to dig into this again.
> 
> > Bring the same semantics for clearing the AER status register in EDR
> > path and DPC path.
> > 
> > Signed-off-by: Naveen Naidu <naveennaidu479@gmail.com>
> > ---
> >  drivers/pci/pcie/dpc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pci/pcie/dpc.c b/drivers/pci/pcie/dpc.c
> > index faf4a1e77fab..68899a3db126 100644
> > --- a/drivers/pci/pcie/dpc.c
> > +++ b/drivers/pci/pcie/dpc.c
> > @@ -288,7 +288,6 @@ void dpc_process_error(struct pci_dev *pdev)
> >  		 dpc_get_aer_uncorrect_severity(pdev, &info) &&
> >  		 aer_get_device_error_info(pdev, &info)) {
> >  		aer_print_error(pdev, &info);
> > -		pci_aer_clear_status(pdev);
> >  	}
> >  }
> >  
> > @@ -297,6 +296,7 @@ static irqreturn_t dpc_handler(int irq, void *context)
> >  	struct pci_dev *pdev = context;
> >  
> >  	dpc_process_error(pdev);
> > +	pci_aer_clear_status(pdev);
> >  
> >  	/* We configure DPC so it only triggers on ERR_FATAL */
> >  	pcie_do_recovery(pdev, pci_channel_io_frozen, dpc_reset_link);
> > -- 
> > 2.25.1
> > 
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