linux-kernel - Re: [PATCH v2 05/13] PCI/ASPM: Add pci_enable_link

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Message-ID: <20231013164850.GA1118214@bhelgaas>
Date:   Fri, 13 Oct 2023 11:48:50 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     linux-pci@...r.kernel.org,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Lukas Wunner <lukas@...ner.de>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        ath10k@...ts.infradead.org, ath11k@...ts.infradead.org,
        ath12k@...ts.infradead.org, intel-wired-lan@...ts.osuosl.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-bluetooth@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-wireless@...r.kernel.org, Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 05/13] PCI/ASPM: Add pci_enable_link_state()

On Thu, Oct 12, 2023 at 03:53:39PM +0300, Ilpo Järvinen wrote:
> 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@...ux.intel.com>
> > > ---
> > >  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.)

This was just a comment about the doc not matching the function
behavior.

I think we have to support pci_enable_link_state() even if the driver
hasn't previously called pci_disable_link_state(), so drivers have to
be able to specify the pci_enable_link_state() @state from scratch.

Does that answer the enforcement question?  I don't think we can
really enforce anything other than that @state specifies valid ASPM
states.

Bjorn