linux-kernel - Re: [PATCH 2/2] PCI: Do not wait for disconnected devices when resuming

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Date: Mon, 29 Jan 2024 12:55:44 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
	"Maciej W. Rozycki" <macro@...am.me.uk>,
	linux-kernel@...r.kernel.org,
	Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: [PATCH 2/2] PCI: Do not wait for disconnected devices when
 resuming

On Mon, Jan 29, 2024 at 01:27:10PM +0200, Ilpo Järvinen wrote:
> On runtime resume, pci_dev_wait() is called:
>   pci_pm_runtime_resume()
>     pci_pm_bridge_power_up_actions()
>       pci_bridge_wait_for_secondary_bus()
>         pci_dev_wait()
> 
> While a device is runtime suspended along with its PCIe hierarchy, the
> device could get disconnected. In such case, the link will not come up
> no matter how log pci_dev_wait() waits for it.

s/PCIe/PCI/ (unless this is a PCIe-specific thing)
s/log/long/

> Besides the above mentioned case, there could be other ways to get the
> device disconnected while pci_dev_wait() is waiting for the link to
> come up.
> 
> Make pci_dev_wait() to exit if the device is already disconnected to
> avoid unnecessary delay. As disconnected device is not really even a
> failure in the same sense as link failing to come up for whatever
> reason, return 0 instead of errno.

The device being disconnected is not the same as a link failure.  Do
all the callers do the right thing if pci_dev_wait() returns success
when there's no device there?

> Also make sure compiler does not become too clever with
> dev->error_state and use READ_ONCE() to force a fetch for the
> up-to-date value.

I think we should have a comment there to say why READ_ONCE() is
needed.  Otherwise it's hard to know whether a future change might
make it unnecessary.

> Reported-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Tested-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> ---
>  drivers/pci/pci.c | 5 +++++
>  drivers/pci/pci.h | 4 +++-
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index d8f11a078924..ec9bf6c90312 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1250,6 +1250,11 @@ static int pci_dev_wait(struct pci_dev *dev, char *reset_type, int timeout)
>  	for (;;) {
>  		u32 id;
>  
> +		if (pci_dev_is_disconnected(dev)) {
> +			pci_dbg(dev, "disconnected; not waiting\n");
> +			return 0;
> +		}
> +
>  		pci_read_config_dword(dev, PCI_COMMAND, &id);
>  		if (!PCI_POSSIBLE_ERROR(id))
>  			break;
> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> index 2336a8d1edab..563a275dff67 100644
> --- a/drivers/pci/pci.h
> +++ b/drivers/pci/pci.h
> @@ -4,6 +4,8 @@
>  
>  #include <linux/pci.h>
>  
> +#include <asm/rwonce.h>
> +
>  /* Number of possible devfns: 0.0 to 1f.7 inclusive */
>  #define MAX_NR_DEVFNS 256
>  
> @@ -370,7 +372,7 @@ static inline int pci_dev_set_disconnected(struct pci_dev *dev, void *unused)
>  
>  static inline bool pci_dev_is_disconnected(const struct pci_dev *dev)
>  {
> -	return dev->error_state == pci_channel_io_perm_failure;
> +	return READ_ONCE(dev->error_state) == pci_channel_io_perm_failure;
>  }
>  
>  /* pci_dev priv_flags */
> -- 
> 2.39.2
>