Re: [PATCH v5] PCI/ASPM: Add back L1 PM Substate save and restore

["David E. Box" <david.e.box@linux.intel.com>]

On Thu, 2023-12-28 at 18:30 -0600, Bjorn Helgaas wrote:
> [+cc Michael]
> 
> On Thu, Dec 28, 2023 at 04:31:12PM -0600, Bjorn Helgaas wrote:
> > On Wed, Dec 20, 2023 at 05:12:50PM -0800, David E. Box wrote:
> > ...
> 
> > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > index 55bc3576a985..3c4b2647b4ca 100644
> > > --- a/drivers/pci/pci.c
> > > +++ b/drivers/pci/pci.c
> 
> > > @@ -1579,7 +1579,7 @@ static void pci_restore_pcie_state(struct pci_dev
> > > *dev)
> > >  {
> > > ...
> 
> > > +        So we restore here only the
> > > +	 * LNKCTL register with the ASPM control field clear. ASPM will
> > > +	 * be restored in pci_restore_aspm_state().
> > > +	 */
> > > +	val = cap[i++] & ~PCI_EXP_LNKCTL_ASPMC;
> > > +	pcie_capability_write_word(dev, PCI_EXP_LNKCTL, val);
> > 
> > When CONFIG_PCIEASPM is not set, we will clear ASPMC here and never
> > restore it.  I don't know if this ever happens.  Do we need to worry
> > about this?  Might firmware restore ASPMC itself before we get here?
> > What do we want to happen in this case?

I just checked this. L1 does get disabled which we don't want. We need to save
and restore the BIOS ASPM configuration even when CONFIG_PCIEASPM is not set.

> > 
> > Since ASPM is intertwined with the PCIe Capability, can we call
> > pci_restore_aspm_state() from here instead of from
> > pci_restore_state()?
> > 
> > Calling it here would make it easier to see the required ordering
> > (LNKCTL with ASPMC cleared, restore ASPM L1SS, restore ASPMC) and
> > it would be obvious that none of the other stuff in
> > pci_restore_state() is relevant (PASID, PRI, ATS, VC, etc).
> > 
> > If that could be done, I think it would make sense to do the same with
> > pci_save_aspm_state() even though it's a little more independent.
> 

Makes sense

> The lspci output in Michael's report at
> https://lore.kernel.org/r/76c61361-b8b4-435f-a9f1-32b716763d62@5challer.de
> reminded me that LTR is important for L1.2, and we currently have
> this:
> 
>   pci_restore_state
>     pci_restore_ltr_state
>     pci_restore_pcie_state
> 
> I wonder if pci_restore_ltr_state() should be called from
> pci_restore_pcie_state() as well?  It's intimately connected to ASPM,
> and that connection isn't very clear in the current code.

Make sense too since LTR is a required capability for L1.2. I'll send updated
patches after the merge window.

David