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Message-ID: <87ldycs097.wl-tiwai@suse.de>
Date: Fri, 25 Oct 2024 16:52:36 +0200
From: Takashi Iwai <tiwai@...e.de>
To: Philipp Stanner <pstanner@...hat.com>
Cc: Takashi Iwai <tiwai@...e.de>,
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, 25 Oct 2024 16:28:42 +0200,
Philipp Stanner wrote:
>
> 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.
IIRC, this code path is reached only after checking that the INTx
state is changed. Hence "!enable" is assured to be the pre-driver
INTx state in the old code.
> >
> > 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.
> */
OK, will add it.
> > + 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.
Right, pcim_intx() assures to restore INTx value back to the moment it
was called. And that should be enough and consistent behavior.
BTW, a possible optimization would be to skip the devres if the value
isn't really changed from the current state (which is similar like the
old code before pcim_intx()). OTOH, this can lead to inconsistencies
when INTx is changed manually after pcim_intx() somehow. So maybe
it's not worth.
thanks,
Takashi
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