linux-kernel - Re: [PATCH] pci: add pci_dev_is

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Date:   Wed, 26 May 2021 11:23:06 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Lambert Wang <lambert.q.wang@...il.com>
Cc:     Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pci: add pci_dev_is_alive API

On Wed, May 26, 2021 at 02:12:38PM +0800, Lambert Wang wrote:
> ...
> The user is our new PCI driver under development for WWAN devices .
> Surprise removal could happen under multiple circumstances.
> e.g. Exception, Link Failure, etc.
> 
> We wanted this API to detect surprise removal or check device recovery
> when AER and Hotplug are disabled.
> 
> I thought the API could be commonly used for many similar devices.

Be careful with this.  pci_device_is_present() is not a good way to
detect surprise removal.  Surprise removal can happen at any time, for
example, it can occur after you call pci_device_is_present() but
before you use the result:

  present = pci_device_is_present(pdev);
  /* present == true */
  /* device may be removed here */
  if (present)
    xxx; /* this operation may fail */

You have to assume that *any* operation on the device can fail because
the device has been removed.  In general, there's no response for a
PCIe write to the device, so you can't really check whether a write
has failed.

There *are* responses for reads, of course, if the device has been
removed, a read will cause a failure response.  Most PCIe controllers
turn that response into ~0 data to satisfy the read.  So the only
reliable way to detect surprise removal is to check for ~0 data when
doing an MMIO read from the device.  Of course, ~0 may be either valid
data or a symptom of a failure response, so you may have to do
additional work to distinguish those two cases.

Bjorn