linux-kernel - Re: [PATCH v2] PCI: Restore the original INTX_DISABLE bit by pcim

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Message-ID: <61ef07331f843c25b19e5a6f68419e0a607a1b0b.camel@redhat.com>
Date: Mon, 04 Nov 2024 10:14:28 +0100
From: Philipp Stanner <pstanner@...hat.com>
To: Takashi Iwai <tiwai@...e.de>, Bjorn Helgaas <bhelgaas@...gle.com>
Cc: linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] PCI: Restore the original INTX_DISABLE bit by
 pcim_intx()

On Thu, 2024-10-31 at 14:42 +0100, Takashi Iwai wrote:
> pcim_intx() tries to restore the INTx bit at removal via devres, but
> there is a chance that it restores a wrong value.
> Because the value to be restored is blindly assumed to be the
> negative
> of the enable argument, when a driver calls pcim_intx() unnecessarily
> for the already enabled state, it'll restore to the disabled state in
> turn.  That is, the function assumes the case like:
> 
>   // INTx == 1
>   pcim_intx(pdev, 0); // old INTx value assumed to be 1 -> correct
> 
> but it might be like the following, too:
> 
>   // INTx == 0
>   pcim_intx(pdev, 0); // old INTx value assumed to be 1 -> wrong
> 
> Also, when a driver calls pcim_intx() multiple times with different
> enable argument values, the last one will win no matter what value it
> is.  This can lead to inconsistency, e.g.
> 
>   // INTx == 1
>   pcim_intx(pdev, 0); // OK
>   ...
>   pcim_intx(pdev, 1); // now old INTx wrongly assumed to be 0
> 
> This patch addresses those inconsistencies by saving the original
> INTx state at the first pcim_intx() call.  For that,
> get_or_create_intx_devres() is folded into pcim_intx() caller side;
> it allows us to simply check the already allocated devres and record
> the original INTx along with the devres_alloc() call.
> 
> Fixes: 25216afc9db5 ("PCI: Add managed pcim_intx()")
> Cc: stable@...r.kernel.org # 6.11+
> Link: https://lore.kernel.org/87v7xk2ps5.wl-tiwai@suse.de
> Signed-off-by: Takashi Iwai <tiwai@...e.de>

Reviewed-by: Philipp Stanner <pstanner@...hat.com>

Nice!

> ---
> v1->v2: refactoring, fold get_or_create_intx_devres() into the caller
> instead of retrieving the original INTx there.
> Also add comments and improve the patch description.
> 
>  drivers/pci/devres.c | 34 +++++++++++++++++++---------------
>  1 file changed, 19 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/pci/devres.c b/drivers/pci/devres.c
> index b133967faef8..c93d4d4499a0 100644
> --- a/drivers/pci/devres.c
> +++ b/drivers/pci/devres.c
> @@ -438,19 +438,12 @@ static void pcim_intx_restore(struct device
> *dev, void *data)
>  	__pcim_intx(pdev, res->orig_intx);
>  }
>  
> -static struct pcim_intx_devres *get_or_create_intx_devres(struct
> device *dev)
> +static void save_orig_intx(struct pci_dev *pdev, struct
> pcim_intx_devres *res)
>  {
> -	struct pcim_intx_devres *res;
> +	u16 pci_command;
>  
> -	res = devres_find(dev, pcim_intx_restore, NULL, NULL);
> -	if (res)
> -		return res;
> -
> -	res = devres_alloc(pcim_intx_restore, sizeof(*res),
> GFP_KERNEL);
> -	if (res)
> -		devres_add(dev, res);
> -
> -	return res;
> +	pci_read_config_word(pdev, PCI_COMMAND, &pci_command);
> +	res->orig_intx = !(pci_command & PCI_COMMAND_INTX_DISABLE);
>  }
>  
>  /**
> @@ -466,12 +459,23 @@ static struct pcim_intx_devres
> *get_or_create_intx_devres(struct device *dev)
>  int pcim_intx(struct pci_dev *pdev, int enable)
>  {
>  	struct pcim_intx_devres *res;
> +	struct device *dev = &pdev->dev;
>  
> -	res = get_or_create_intx_devres(&pdev->dev);
> -	if (!res)
> -		return -ENOMEM;
> +	/*
> +	 * pcim_intx() must only restore the INTx value that existed
> before the
> +	 * driver was loaded, i.e., before it called pcim_intx() for
> the
> +	 * first time.
> +	 */
> +	res = devres_find(dev, pcim_intx_restore, NULL, NULL);
> +	if (!res) {
> +		res = devres_alloc(pcim_intx_restore, sizeof(*res),
> GFP_KERNEL);
> +		if (!res)
> +			return -ENOMEM;
> +
> +		save_orig_intx(pdev, res);
> +		devres_add(dev, res);
> +	}
>  
> -	res->orig_intx = !enable;
>  	__pcim_intx(pdev, enable);
>  
>  	return 0;