Re: [PATCH v2 05/13] PCI/ASPM: Add pci_enable_link_state()

[Ilpo Järvinen]

On Wed, 11 Oct 2023, Bjorn Helgaas wrote:

> On Mon, Sep 18, 2023 at 04:10:55PM +0300, Ilpo Järvinen wrote:
> > pci_disable_link_state() lacks a symmetric pair. Some drivers want to
> > disable ASPM during certain phases of their operation but then
> > re-enable it later on. If pci_disable_link_state() is made for the
> > device, there is currently no way to re-enable the states that were
> > disabled.
> 
> pci_disable_link_state() gives drivers a way to disable specified ASPM
> states using a bitmask (PCIE_LINK_STATE_L0S, PCIE_LINK_STATE_L1,
> PCIE_LINK_STATE_L1_1, etc), but IIUC the driver can't tell exactly
> what changed and can't directly restore the original state, e.g.,
> 
>   - PCIE_LINK_STATE_L1 enabled initially
>   - driver calls pci_disable_link_state(PCIE_LINK_STATE_L0S)
>   - driver calls pci_enable_link_state(PCIE_LINK_STATE_L0S)
>   - PCIE_LINK_STATE_L0S and PCIE_LINK_STATE_L1 are enabled now
> 
> Now PCIE_LINK_STATE_L0S is enabled even though it was not initially
> enabled.  Maybe that's what we want; I dunno.
> 
> pci_disable_link_state() currently returns success/failure, but only
> r8169 and mt76 even check, and only rtl_init_one() (r8169) has a
> non-trivial reason, so it's conceivable that it could return a bitmask
> instead.

It's great that you suggested this since it's actually what also I've been 
started to think should be done instead of this straightforward approach
I used in V2. 

That is, don't have the drivers to get anything directly from LNKCTL
but they should get everything through the API provided by the 
disable/enable calls which makes it easy for the driver to pass the same
value back into the enable call.

> > Add pci_enable_link_state() to remove ASPM states from the state
> > disable mask.
> > 
> > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/pci/pcie/aspm.c | 42 +++++++++++++++++++++++++++++++++++++++++
> >  include/linux/pci.h     |  2 ++
> >  2 files changed, 44 insertions(+)
> > 
> > diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> > index 91dc95aca90f..f45d18d47c20 100644
> > --- a/drivers/pci/pcie/aspm.c
> > +++ b/drivers/pci/pcie/aspm.c
> > @@ -1117,6 +1117,48 @@ int pci_disable_link_state(struct pci_dev *pdev, int state)
> >  }
> >  EXPORT_SYMBOL(pci_disable_link_state);
> >  
> > +/**
> > + * pci_enable_link_state - Re-enable device's link state
> > + * @pdev: PCI device
> > + * @state: ASPM link states to re-enable
> > + *
> > + * Enable device's link state that were previously disable so the link is
> 
> "state[s] that were previously disable[d]" alludes to the use case you
> have in mind, but I don't think it describes how this function
> actually works.  This function just makes it possible to enable the
> specified states.  The @state parameter may have nothing to do with
> any previously disabled states.

Yes, it's what I've been thinking between the lines. But I see your point 
that this API didn't make it easy/obvious as is.

Would you want me to enforce it too besides altering the API such that the 
states are actually returned from disable call? (I don't personally find
that necessary as long as the API pair itself makes it obvious what the 
driver is expect to pass there.)


-- 
 i.