linux-kernel - Re: [PATCH V2] PCI/MSI: Set device flag indicating only 32-bit MSI support

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Date:   Thu, 3 Dec 2020 13:54:04 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Vidya Sagar <vidyas@...dia.com>
Cc:     bhelgaas@...gle.com, lorenzo.pieralisi@....com,
        thierry.reding@...il.com, jonathanh@...dia.com,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        kthota@...dia.com, mmaddireddy@...dia.com, sagar.tv@...il.com
Subject: Re: [PATCH V2] PCI/MSI: Set device flag indicating only 32-bit MSI
 support

On Fri, Dec 04, 2020 at 12:33:45AM +0530, Vidya Sagar wrote:
> On 12/3/2020 11:54 PM, Bjorn Helgaas wrote:
> > On Tue, Nov 24, 2020 at 04:20:35PM +0530, Vidya Sagar wrote:
> > > There are devices (Ex:- Marvell SATA controller) that don't support
> > > 64-bit MSIs and the same is advertised through their MSI capability
> > > register. Set no_64bit_msi flag explicitly for such devices in the
> > > MSI setup code so that the msi_verify_entries() API would catch
> > > if the MSI arch code tries to use 64-bit MSI.
> > 
> > This seems good to me.  I'll post a possible revision to set
> > dev->no_64bit_msi in the device enumeration path instead of in the IRQ
> > allocation path, since it's really a property of the device, not of
> > the msi_desc.
> > 
> > I like the extra checking this gives us.  Was this prompted by
> > tripping over something, or is it something you noticed by code
> > reading?  If the former, a hint about what was wrong and how it's
> > being fixed would be useful.
> I observed functionality issue with Marvell SATA controller (1b4b:9171) when
> the allocated MSI target address was a 64-bit address. I mentioned the
> Marvell SATA controller as an example in the commit message.

I know you mentioned the Marvell controller, but apparently that
device is working perfectly correctly: it does not support 64-bit MSI,
and it does not advertise support for 64-bit MSI.

So if there's a functionality issue, that means something is wrong in
Linux that caused us to assign a 64-bit MSI address to it.  *That*
issue is what I want to know about.  Your patch only warns about the
issue; it doesn't fix it.

I don't think there's any point in specifically mentioning the Marvell
device if it is working correctly, because the same issue should
affect *any* device that doesn't support 64-bit MSI.  But if there's
some arch code that incorrectly assigns a 64-bit address, it would
definitely be useful to specify the arch.  And hopefully there's a fix
for that arch code, too.

> > > Signed-off-by: Vidya Sagar <vidyas@...dia.com>
> > > ---
> > > V2:
> > > * Addressed Bjorn's comment and changed the error message
> > > 
> > >   drivers/pci/msi.c | 11 +++++++----
> > >   1 file changed, 7 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> > > index d52d118979a6..8de5ba6b4a59 100644
> > > --- a/drivers/pci/msi.c
> > > +++ b/drivers/pci/msi.c
> > > @@ -581,10 +581,12 @@ msi_setup_entry(struct pci_dev *dev, int nvec, struct irq_affinity *affd)
> > >        entry->msi_attrib.multi_cap     = (control & PCI_MSI_FLAGS_QMASK) >> 1;
> > >        entry->msi_attrib.multiple      = ilog2(__roundup_pow_of_two(nvec));
> > > 
> > > -     if (control & PCI_MSI_FLAGS_64BIT)
> > > +     if (control & PCI_MSI_FLAGS_64BIT) {
> > >                entry->mask_pos = dev->msi_cap + PCI_MSI_MASK_64;
> > > -     else
> > > +     } else {
> > >                entry->mask_pos = dev->msi_cap + PCI_MSI_MASK_32;
> > > +             dev->no_64bit_msi = 1;
> > > +     }
> > > 
> > >        /* Save the initial mask status */
> > >        if (entry->msi_attrib.maskbit)
> > > @@ -602,8 +604,9 @@ static int msi_verify_entries(struct pci_dev *dev)
> > >        for_each_pci_msi_entry(entry, dev) {
> > >                if (!dev->no_64bit_msi || !entry->msg.address_hi)
> > >                        continue;
> > > -             pci_err(dev, "Device has broken 64-bit MSI but arch"
> > > -                     " tried to assign one above 4G\n");
> > > +             pci_err(dev, "Device has either broken 64-bit MSI or "
> > > +                     "only 32-bit MSI support but "
> > > +                     "arch tried to assign one above 4G\n");
> > >                return -EIO;
> > >        }
> > >        return 0;
> > > --
> > > 2.17.1
> > >