Message-ID: <5b2911489844f6a970da053ebfc126eddf7c896c.camel@redhat.com>
Date: Fri, 25 Oct 2024 16:28:42 +0200
From: Philipp Stanner <pstanner@...hat.com>
To: Takashi Iwai <tiwai@...e.de>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: Restore the original INTX_DISABLE bit by
 pcim_intx()

On Fri, 2024-10-25 at 12:44 +0200, Takashi Iwai wrote:
> On Fri, 25 Oct 2024 11:26:18 +0200,
> Philipp Stanner wrote:
> > 
> > Hi,
> > 
> > On Thu, 2024-10-24 at 17:55 +0200, Takashi Iwai wrote:
> > > pcim_intx() tries to restore the INTX_DISABLE 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.
> > 
> > It depends on how it is called, no?
> > 
> > // INTx == 1
> > pcim_intx(pdev, 0); // old INTx value assumed to be 1 -> correct
> > 
> > ---
> > 
> > // INTx == 0
> > pcim_intx(pdev, 0); // old INTx value assumed to be 1 -> wrong
> > 
> > Maybe it makes sense to replace part of the commit text with
> > something
> > like the example above?
> 
> If it helps better understanding, why not.
> 
> > >   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.
> > 
> > Means
> > 
> > // INTx == 0
> > pcim_intx(pdev, 0); // orig_INTx == 1, INTx == 0
> > pcim_intx(pdev, 1); // orig_INTx == 0, INTx == 1
> > pcim_intx(pdev, 0); // orig_INTx == 1, INTx == 0
> > 
> > So in this example the first call would cause a wrong orig_INTx,
> > but
> > the last call – the one "who will win" – seems to do the right
> > thing,
> > dosen't it?
> 
> Yes and no.  The last call wins to write the current value, but
> shouldn't win for setting the original value.  The original value
> must
> be recorded only from the first call.

Alright, so you think that pcim_intx() should always restore the INTx
state that existed before the driver was loaded.

> > > This patch addresses those inconsistencies by saving the original
> > > INTX_DISABLE state at the first devres_alloc(); this assures that
> > > the
> > > original state is restored properly, and the later pcim_intx()
> > > calls
> > > won't overwrite res->orig_intx any longer.
> > > 
> > > Fixes: 25216afc9db5 ("PCI: Add managed pcim_intx()")
> > 
> > That commit is also in 6.11, so we need:
> > 
> > Cc: stable@...r.kernel.org # 6.11+
> 
> OK.
> 
> > > Link: https://lore.kernel.org/87v7xk2ps5.wl-tiwai@suse.de
> > > Signed-off-by: Takashi Iwai <tiwai@...e.de>
> > > ---
> > >  drivers/pci/devres.c | 18 ++++++++++++++----
> > >  1 file changed, 14 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/pci/devres.c b/drivers/pci/devres.c
> > > index b133967faef8..aed3c9a355cb 100644
> > > --- a/drivers/pci/devres.c
> > > +++ b/drivers/pci/devres.c
> > > @@ -438,8 +438,17 @@ 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)
> > >  {
> > > +	u16 pci_command;
> > > +
> > > +	pci_read_config_word(pdev, PCI_COMMAND, &pci_command);
> > > +	res->orig_intx = !(pci_command &
> > > PCI_COMMAND_INTX_DISABLE);
> > > +}
> > > +
> > > +static struct pcim_intx_devres *get_or_create_intx_devres(struct
> > > pci_dev *pdev)
> > > +{
> > > +	struct device *dev = &pdev->dev;
> > >  	struct pcim_intx_devres *res;
> > >  
> > >  	res = devres_find(dev, pcim_intx_restore, NULL, NULL);
> > > @@ -447,8 +456,10 @@ static struct pcim_intx_devres
> > > *get_or_create_intx_devres(struct device *dev)
> > >  		return res;
> > >  
> > >  	res = devres_alloc(pcim_intx_restore, sizeof(*res),
> > > GFP_KERNEL);
> > > -	if (res)
> > > +	if (res) {
> > > +		save_orig_intx(pdev, res);
> > 
> > This is not the correct place – get_or_create_intx_devres() should
> > get
> > the resource if it exists, or allocate it if it doesn't, but its
> > purpose is not to modify the resource.
> 
> The behavior of the function makes the implementation a bit harder,
> because the initialization of res->orig_intx should be done only once
> at the very first call.
> 
> > >  		devres_add(dev, res);
> > > +	}
> > >  
> > >  	return res;
> > >  }
> > > @@ -467,11 +478,10 @@ int pcim_intx(struct pci_dev *pdev, int
> > > enable)
> > >  {
> > >  	struct pcim_intx_devres *res;
> > >  
> > > -	res = get_or_create_intx_devres(&pdev->dev);
> > > +	res = get_or_create_intx_devres(pdev);
> > >  	if (!res)
> > >  		return -ENOMEM;
> > >  
> > > -	res->orig_intx = !enable;
> > 
> > Here is the right place to call save_orig_intx(). That way you also
> > won't need the new variable struct device *dev above :)
> 
> The problem is that, at this place, we don't know whether it's a
> freshly created devres or it's an inherited one.  So, we'd need to
> modify get_or_create_intx_devres() to indicate that it's a new
> creation.  Or, maybe simpler would be rather to flatten
> get_or_create_intx_devres() into pcim_intx().  It's a small function,
> and it wouldn't be worsen the readability so much.

That might be the best solution. If it's done that way it should
include a comment detailing the problem.

Looking at the implementation of pci_intx() before
25216afc9db53d85dc648aba8fb7f6d31f2c8731 probably indicates that you're
right:

	if (dr && !dr->restore_intx) {
		dr->restore_intx = 1;
		dr->orig_intx = !enable;
	}


So they used a boolean to only take the first state. Although that
still wouldn't have necessarily been the pre-driver INTx state.


> 
> That is, something like below.
> 
> 
> thanks,
> 
> Takashi
> 
> --- a/drivers/pci/devres.c
> +++ b/drivers/pci/devres.c
> @@ -438,21 +438,6 @@ 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)
> -{
> -	struct pcim_intx_devres *res;
> -
> -	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;
> -}
> -
>  /**
>   * pcim_intx - managed pci_intx()
>   * @pdev: the PCI device to operate on
> @@ -466,12 +451,21 @@ 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;
> +	u16 pci_command;
>  
> -	res = get_or_create_intx_devres(&pdev->dev);
> -	if (!res)
> -		return -ENOMEM;
> +	res = devres_find(dev, pcim_intx_restore, NULL, NULL);

sth like:

/*
 * 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.
 */

> +	if (!res) {
> +		res = devres_alloc(pcim_intx_restore, sizeof(*res),
> GFP_KERNEL);
> +		if (!res)
> +			return -ENOMEM;
> +
> +		pci_read_config_word(pdev, PCI_COMMAND,
> &pci_command);
> +		res->orig_intx = !(pci_command &
> PCI_COMMAND_INTX_DISABLE);
> +
> +		devres_add(dev, res);
> +	}
>  
> -	res->orig_intx = !enable;
>  	__pcim_intx(pdev, enable);

Looks like a good idea to me

The only thing I'm wondering about right now is the following: In the
old days, there was only pci_intx(), which either did devres or didn't.

Now you have two functions, pcim_intx() and pci_intx().

The thing is that the driver could theoretically still intermingle them
and for example call pci_intx() before pcim_intx(), which would lead
the latter to still restore the wrong value.

But that's very unlikely and I'm not sure whether we can do something
about it.


Regards,
P.

>  
>  	return 0;
> 

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