linux-kernel - Re: [PATCH V7 1/3] PCI: limit FLR wait time to 100ms maximum

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Date:   Wed, 2 Aug 2017 17:22:08 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     linux-pci@...r.kernel.org, timur@...eaurora.org,
        alex.williamson@...hat.com, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V7 1/3] PCI: limit FLR wait time to 100ms maximum

On 8/2/2017 4:27 PM, Bjorn Helgaas wrote:
> On Wed, Aug 02, 2017 at 03:15:15PM -0400, Sinan Kaya wrote:
> My hypothesis was that CRS isn't useful on VFs because of sec 2.2.2
> says "FLR ... does not affect its existence in PCI Configuration
> Space".  But I think that hypothesis is wrong because sec 3.3.3.1
> does talk about a VF returning CRS.
> 

hmm, good catch. I'll drop the first patch.

> The SR-IOV spec (sec 3.4.1.1) says a VF's Vendor ID is read-only
> 0xffff.  But I expect CRS visibility (PCIe r3.1, sec 2.3.2) would work
> normally and return a Vendor ID of 0x0001 to indicate CRS for a VF.
> 
> Of course, not all Root Ports support CRS software visibility, so
> whatever we do has to work when it's absent.

How about a mixture of old code and new code as follows?

static void pci_flr_wait(struct pci_dev *dev)
{
	u32 id;
	bool ret = false;
	int i = 0;

	if (CRS supported) {
		/* don't touch the HW before waiting 100ms */
		msleep(100);

		ret = pci_bus_read_dev_vendor_id(dev->bus, dev->devfn, &id, 60000);
		if (ret)
			return;
	}

	do {
		msleep(100);
		pci_read_config_dword(dev, PCI_COMMAND, &id);
	} while (i++ < 10 && id == ~0);
 
	if (id == ~0)
 		dev_warn(&dev->dev, "Failed to return from FLR\n");
	else if (i > 1)
		dev_info(&dev->dev, "Required additional %dms to return from FLR\n",
			 (i - 1) * 100);
}


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.