linux-kernel - Re: [PATCH 1/2] PCI: save and restore device state during bus reset

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Message-ID: <20160913215315.GJ4138@localhost>
Date:   Tue, 13 Sep 2016 16:53:15 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Sinan Kaya <okaya@...eaurora.org>
Cc:     linux-pci@...r.kernel.org, timur@...eaurora.org,
        cov@...eaurora.org, alex.williamson@...hat.com,
        vikrams@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] PCI: save and restore device state during bus reset

On Thu, Sep 01, 2016 at 07:00:00PM -0400, Sinan Kaya wrote:
> A secondary bus reset causes settings to be lost by all downstream
> devices on the tree. The code is currently saving and restoring device
> states only when called from the VFIO path via pci_probe_reset_bus
> and pci_reset_bus functions.
> 
> Moving the save and restore into pci_reset_bridge_secondary_bus
> so that all users of the API have the same behavior.
> 
> Signed-off-by: Sinan Kaya <okaya@...eaurora.org>
> ---
>  drivers/pci/pci.c | 36 +++++++++++++++---------------------
>  1 file changed, 15 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index aab9d51..b209378 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3860,19 +3860,6 @@ void __weak pcibios_reset_secondary_bus(struct pci_dev *dev)
>  	pci_reset_secondary_bus(dev);
>  }
>  
> -/**
> - * pci_reset_bridge_secondary_bus - Reset the secondary bus on a PCI bridge.
> - * @dev: Bridge device
> - *
> - * Use the bridge control register to assert reset on the secondary bus.
> - * Devices on the secondary bus are left in power-on state.
> - */
> -void pci_reset_bridge_secondary_bus(struct pci_dev *dev)
> -{
> -	pcibios_reset_secondary_bus(dev);
> -}
> -EXPORT_SYMBOL_GPL(pci_reset_bridge_secondary_bus);
> -
>  static int pci_parent_bus_reset(struct pci_dev *dev, int probe)
>  {
>  	struct pci_dev *pdev;
> @@ -4362,6 +4349,21 @@ static void pci_slot_restore(struct pci_slot *slot)
>  	}
>  }
>  
> +/**
> + * pci_reset_bridge_secondary_bus - Reset the secondary bus on a PCI bridge.
> + * @dev: Bridge device
> + *
> + * Use the bridge control register to assert reset on the secondary bus.
> + * Devices on the secondary bus are left in power-on state.
> + */
> +void pci_reset_bridge_secondary_bus(struct pci_dev *dev)
> +{
> +	pci_bus_save_and_disable(dev->bus);
> +	pcibios_reset_secondary_bus(dev);
> +	pci_bus_restore(dev->bus);

This path eventually writes the Bridge Control register:

  pci_reset_bridge_secondary_bus
    pcibios_reset_secondary_bus
      pci_reset_secondary_bus
        pci_write_config_word(dev, PCI_BRIDGE_CONTROL, ctrl)

But I think it'd be easy to call this on a non-bridge device, and I
don't think there's anything in the path that checks whether this is
actually a bridge.  I wonder if we should check that somewhere, or
maybe even change the interface so it takes a struct pci_bus instead
of a pci_dev.

> +}
> +EXPORT_SYMBOL_GPL(pci_reset_bridge_secondary_bus);
> +
>  static int pci_slot_reset(struct pci_slot *slot, int probe)
>  {
>  	int rc;
> @@ -4504,12 +4506,8 @@ int pci_reset_bus(struct pci_bus *bus)
>  	if (rc)
>  		return rc;
>  
> -	pci_bus_save_and_disable(bus);
> -
>  	rc = pci_bus_reset(bus, 0);
>  
> -	pci_bus_restore(bus);
> -
>  	return rc;
>  }
>  EXPORT_SYMBOL_GPL(pci_reset_bus);
> @@ -4528,8 +4526,6 @@ int pci_try_reset_bus(struct pci_bus *bus)
>  	if (rc)
>  		return rc;
>  
> -	pci_bus_save_and_disable(bus);
> -
>  	if (pci_bus_trylock(bus)) {
>  		might_sleep();
>  		pci_reset_bridge_secondary_bus(bus->self);
> @@ -4537,8 +4533,6 @@ int pci_try_reset_bus(struct pci_bus *bus)
>  	} else
>  		rc = -EAGAIN;
>  
> -	pci_bus_restore(bus);
> -
>  	return rc;
>  }
>  EXPORT_SYMBOL_GPL(pci_try_reset_bus);
> -- 
> 1.9.1
>