linux-kernel - Re: [PATCH v3 1/2] PCI: pciehp: Add support for async hotplug with native AER and DPC/EDR

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Message-ID: <20230622212259.GA11993@wunner.de>
Date:   Thu, 22 Jun 2023 23:22:59 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>, oohall@...il.com,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Yazen Ghannam <yazen.ghannam@....com>,
        Fontenot Nathan <Nathan.Fontenot@....com>
Subject: Re: [PATCH v3 1/2] PCI: pciehp: Add support for async hotplug with
 native AER and DPC/EDR

On Thu, Jun 22, 2023 at 02:02:03PM -0700, Smita Koralahalli wrote:
> On 6/22/2023 2:04 AM, Lukas Wunner wrote:
> > On Wed, Jun 21, 2023 at 06:51:51PM +0000, Smita Koralahalli wrote:
> > > --- a/drivers/pci/pcie/dpc.c
> > > +++ b/drivers/pci/pcie/dpc.c
> > > @@ -292,10 +292,68 @@ void dpc_process_error(struct pci_dev *pdev)
> > >   	}
> > >   }
> > > +static void pci_clear_surpdn_errors(struct pci_dev *pdev)
> > > +{
> > > +	u16 reg16;
> > > +	u32 reg32;
> > > +
> > > +	pci_read_config_dword(pdev, pdev->dpc_cap + PCI_EXP_DPC_RP_PIO_STATUS, &reg32);
> > > +	pci_write_config_dword(pdev, pdev->dpc_cap + PCI_EXP_DPC_RP_PIO_STATUS, reg32);
> > 
> > Make this read+write conditional on "if (pdev->dpc_rp_extensions)"
> > as the register otherwise doesn't exist.
> 
> I'm checking for pdev->dpc_rpc_extensions inside
> dpc_handle_surprise_removal() before calling pci_clear_surpdn_errors().
> Should I recheck it once again here?

Yes.


> > > +	pci_read_config_word(pdev, PCI_STATUS, &reg16);
> > > +	pci_write_config_word(pdev, PCI_STATUS, reg16);
> > > +
> > > +	pcie_capability_write_word(pdev, PCI_EXP_DEVSTA, PCI_EXP_DEVSTA_FED);
> > > +}
> > 
> > A code comment might be useful here saying that in practice,
> > Surprise Down errors have been observed to also set error bits
> > in the Status Register as well as the Fatal Error Detected bit
> > in the Device Status Register.
> 
> And probably move this code comment below to where this function is called
> inside dpc_handle_surprise_removal()..?

No right here would be good because that's the piece of code to which
the code comment would pertain.


> > 	if (!pcie_wait_for_link(pdev, false)) {
> > 		pci_info(pdev, "Data Link Layer Link Active not cleared in 1000 msec\n");
> > 		goto out;
> > 	}
> > 
> > here, similar to dpc_reset_link() and in accordance with PCIe r6.1
> > sec 6.2.11:
> > 
> > 	"To ensure that the LTSSM has time to reach the Disabled state
> > 	or at least to bring the Link down under a variety of error
> > 	conditions, software must leave the Downstream Port in DPC
> > 	until the Data Link Layer Link Active bit in the Link Status
> > 	Register reads 0b; otherwise, the result is undefined."
> 
> And include the above comment in code..

I'd say that's optional.  dpc_reset_link() doesn't have a code comment
for that either, so...

Thanks,

Lukas