linux-kernel - Re: [PATCH] pci: Make DWORD accesses while saving / restoring LTR state

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Date:   Tue, 11 Jan 2022 14:08:47 -0800
From:   Rajat Jain <rajatxjain@...il.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Rajat Jain <rajatja@...gle.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pci: Make DWORD accesses while saving / restoring LTR state

Hello Bjorn,

On Tue, Jan 11, 2022 at 8:39 AM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> On Tue, Dec 21, 2021 at 05:21:05PM -0800, Rajat Jain wrote:
> > Some devices have an errata such that they only support DWORD accesses
> > to some registers.
> >
> > For e.g. this Bayhub O2 device ([VID:DID] = [0x1217:0x8621]) only
> > supports DWORD accesses to LTR latency registers and L1 PM substates
> > control registers:
> > https://github.com/rajatxjain/public_shared/blob/main/OZ711LV2_appnote.pdf
> >
> > Since L1 PM substate control registers are DWORD sized, and hence their
> > access in the kernel is already DWORD sized, so we don't need to do
> > anything for them.
> >
> > However, the LTR registers being WORD sized, are in need of a solution.
> > This patch converts the WORD sized accesses to these registers, into
> > DWORD sized accesses, while saving and restoring them.
> >
> > Signed-off-by: Rajat Jain <rajatja@...gle.com>
>
> Applied to pci/enumeration for v5.17, thanks, Rajat!

Thank you.

>
> The app note suggests that this erratum only affects the registers at
> 0x234, 0x248, and 0x24c, i.e., the LTR snoop registers and the L1 SS
> control registers.
>
> Can you confirm that is true?  Byte and word accesses to other parts
> of config space work correctly?
>
> I *assume* the other parts work correctly, because if byte and word
> accesses were broken, all sorts of things would not work, like
> PCI_COMMAND, PCI_STATUS, searching the capability list, etc.


Yes, that is correct. The Bayhub SD controller works fine otherwise,
so only these registers needed the quirk.

Thanks & Best Regards,

Rajat


>
> Bjorn
>
> > ---
> >  drivers/pci/pci.c       | 24 ++++++++++++++++--------
> >  drivers/pci/pcie/aspm.c |  1 +
> >  2 files changed, 17 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index 3d2fb394986a..efa8cd16827f 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -1556,7 +1556,7 @@ static void pci_save_ltr_state(struct pci_dev *dev)
> >  {
> >       int ltr;
> >       struct pci_cap_saved_state *save_state;
> > -     u16 *cap;
> > +     u32 *cap;
> >
> >       if (!pci_is_pcie(dev))
> >               return;
> > @@ -1571,25 +1571,33 @@ static void pci_save_ltr_state(struct pci_dev *dev)
> >               return;
> >       }
> >
> > -     cap = (u16 *)&save_state->cap.data[0];
> > -     pci_read_config_word(dev, ltr + PCI_LTR_MAX_SNOOP_LAT, cap++);
> > -     pci_read_config_word(dev, ltr + PCI_LTR_MAX_NOSNOOP_LAT, cap++);
> > +     /*
> > +      * We deliberately do a dword access to save both PCI_LTR_MAX_SNOOP_LAT
> > +      * and PCI_LTR_MAX_NOSNOOP_LAT together since some devices only support
> > +      * dword accesses to these registers.
> > +      */
> > +     cap = &save_state->cap.data[0];
> > +     pci_read_config_dword(dev, ltr + PCI_LTR_MAX_SNOOP_LAT, cap);
> >  }
> >
> >  static void pci_restore_ltr_state(struct pci_dev *dev)
> >  {
> >       struct pci_cap_saved_state *save_state;
> >       int ltr;
> > -     u16 *cap;
> > +     u32 *cap;
> >
> >       save_state = pci_find_saved_ext_cap(dev, PCI_EXT_CAP_ID_LTR);
> >       ltr = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_LTR);
> >       if (!save_state || !ltr)
> >               return;
> >
> > -     cap = (u16 *)&save_state->cap.data[0];
> > -     pci_write_config_word(dev, ltr + PCI_LTR_MAX_SNOOP_LAT, *cap++);
> > -     pci_write_config_word(dev, ltr + PCI_LTR_MAX_NOSNOOP_LAT, *cap++);
> > +     /*
> > +      * We deliberately do a dword access to restore both
> > +      * PCI_LTR_MAX_SNOOP_LAT and PCI_LTR_MAX_NOSNOOP_LAT together since
> > +      * some devices only support dword accesses to these registers.
> > +      */
> > +     cap = &save_state->cap.data[0];
> > +     pci_write_config_dword(dev, ltr + PCI_LTR_MAX_SNOOP_LAT, *cap);
> >  }
> >
> >  /**
> > diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> > index 52c74682601a..083f47a7b69b 100644
> > --- a/drivers/pci/pcie/aspm.c
> > +++ b/drivers/pci/pcie/aspm.c
> > @@ -496,6 +496,7 @@ static void aspm_calc_l1ss_info(struct pcie_link_state *link,
> >       encode_l12_threshold(l1_2_threshold, &scale, &value);
> >       ctl1 |= t_common_mode << 8 | scale << 29 | value << 16;
> >
> > +     /* Always make DWORD sized accesses to these registers */
> >       pci_read_config_dword(parent, parent->l1ss + PCI_L1SS_CTL1, &pctl1);
> >       pci_read_config_dword(parent, parent->l1ss + PCI_L1SS_CTL2, &pctl2);
> >       pci_read_config_dword(child, child->l1ss + PCI_L1SS_CTL1, &cctl1);
> > --
> > 2.34.1.307.g9b7440fafd-goog
> >