linux-kernel - Re: [PATCH] pci: Handle the case when PCI_COMMAND register hasn't changed in INTx masking test

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Date:   Mon, 22 May 2017 18:38:20 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Piotr Gregor <piotrgregor@...ncme.org>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pci: Handle the case when PCI_COMMAND register hasn't
 changed in INTx masking test

Hi Piotr,

On Wed, May 10, 2017 at 01:30:01PM +0100, Piotr Gregor wrote:
> The check for interrupt masking support is done by reading
> the PCI_COMMAND config word
> 
> 	pci_read_config_word(dev, PCI_COMMAND, &orig);
> 
> then flipping the PCI_COMMAND_INTX_DISABLE bit and writing result back
> 
> 	pci_write_config_word(dev, PCI_COMMAND,
> 		orig ^ PCI_COMMAND_INTX_DISABLE);
> 
> The expected result is that following read of the PCI_COMMAND
> 
> 	pci_read_config_word(dev, PCI_COMMAND, &new);
> 
> returns PCI_COMMAND with only PCI_COMMAND_INTX_DISABLE bit changed.
> 
> There are two possible outcomes:
> 
> 	1.	Command is the same (hasn't changed)
> 	2.	Commmand has changed
> 
> And the second of them may be decoupled to:
> 
> 	2.1	Command changed only for PCI_COMMAND_INTX_DISABLE bit
> 		(hasn't changed for bits different than PCI_COMMAND_INTX_DISABLE)
> 	2.2	Command changed for bit(s) different than PCI_COMMAND_INTX_DISABLE bit
> 		(and maybe for PCI_COMMAND_INTX_DISABLE too)
> 
> The 2.1 is expected result and anything else is error.
> The 2.2 outcome is handled by pci_intx_mask_supported by printing
> appropriate message to the log, but outcome 1 is not handled.
> 
> Log the message about command not being changed at all.
> 
> Signed-off-by: Piotr Gregor <piotrgregor@...ncme.org>
> ---
>  drivers/pci/pci.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b01bd5b..67a611e 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3712,7 +3712,7 @@ EXPORT_SYMBOL_GPL(pci_intx);
>   * pci_intx_mask_supported - probe for INTx masking support
>   * @dev: the PCI device to operate on
>   *
> - * Check if the device dev support INTx masking via the config space
> + * Check if the device dev supports INTx masking via the config space
>   * command word.
>   */
>  bool pci_intx_mask_supported(struct pci_dev *dev)
> @@ -3736,11 +3736,21 @@ bool pci_intx_mask_supported(struct pci_dev *dev)
>  	 * go ahead and check it.
>  	 */
>  	if ((new ^ orig) & ~PCI_COMMAND_INTX_DISABLE) {
> +		/*
> +		 * If anything else than PCI_COMMAND_INTX_DISABLE bit has
> +		 * changed (and maybe PCI_COMMAND_INTX_DISABLE too)
> +		 */
>  		dev_err(&dev->dev, "Command register changed from 0x%x to 0x%x: driver or hardware bug?\n",
>  			orig, new);
>  	} else if ((new ^ orig) & PCI_COMMAND_INTX_DISABLE) {
> +		/*
> +		 * OK. Only PCI_COMMAND_INTX_DISABLE bit has changed
> +		 */
>  		mask_supported = true;
>  		pci_write_config_word(dev, PCI_COMMAND, orig);
> +	} else {
> +		dev_err(&dev->dev, "Command register hasn't changed when written from 0x%x to 0x%x: driver or hardware bug?\n",
> +			orig, new);

Bit 10 was a reserved bit in PCI r2.2 and was defined as
PCI_COMMAND_INTX_DISABLE in r2.3 of the spec, so I don't think we
should treat this as an error.

>  	}
>  
>  	pci_cfg_access_unlock(dev);
> @@ -3798,7 +3808,7 @@ static bool pci_check_and_set_intx_mask(struct pci_dev *dev, bool mask)
>   * @dev: the PCI device to operate on
>   *
>   * Check if the device dev has its INTx line asserted, mask it and
> - * return true in that case. False is returned if not interrupt was
> + * return true in that case. False is returned if interrupt wasn't
>   * pending.
>   */
>  bool pci_check_and_mask_intx(struct pci_dev *dev)
> -- 
> 2.1.4
>