Re: [PATCH V13 3/6] PCI: loongson: Don't access unexisting devices

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Jun 02, 2022 at 12:28:40PM +0800, Huacai Chen wrote:
> Hi, Bjorn,
> 
> On Wed, Jun 1, 2022 at 7:14 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Sat, Apr 30, 2022 at 04:48:43PM +0800, Huacai Chen wrote:
> > > On LS2K/LS7A, some unexisting devices don't return 0xffffffff when
> > > scanning. This is a hardware flaw but we can only avoid it by software
> > > now.
> >
> > What happens in other situations that normally cause Unsupported
> > Request or similar errors?  For example, memory reads/writes to a
> > device in D3hot should cause an Unsupported Request error.  I'm
> > wondering whether other error handling assumptions might be broken
> > on LS2K/LS7A.
>
> Hardware engineers told me that the problem is due to pin
> multiplexing, under some configurations, a PCI device is unusable but
> the read request doesn't return 0xffffffff.

What happens if a driver does a mem read to a device that's in D3hot?

> > > Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> > > ---
> > >  drivers/pci/controller/pci-loongson.c | 11 +++++++++--
> > >  1 file changed, 9 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/pci/controller/pci-loongson.c b/drivers/pci/controller/pci-loongson.c
> > > index adbfa4a2330f..48316daa1f23 100644
> > > --- a/drivers/pci/controller/pci-loongson.c
> > > +++ b/drivers/pci/controller/pci-loongson.c
> > > @@ -138,6 +138,8 @@ static void __iomem *pci_loongson_map_bus(struct pci_bus *bus, unsigned int devf
> > >                              int where)
> > >  {
> > >       unsigned char busnum = bus->number;
> > > +     unsigned int device = PCI_SLOT(devfn);
> > > +     unsigned int function = PCI_FUNC(devfn);
> > >       struct loongson_pci *priv = pci_bus_to_loongson_pci(bus);
> > >
> > >       if (pci_is_root_bus(bus))
> > > @@ -147,8 +149,13 @@ static void __iomem *pci_loongson_map_bus(struct pci_bus *bus, unsigned int devf
> > >        * Do not read more than one device on the bus other than
> > >        * the host bus. For our hardware the root bus is always bus 0.
> > >        */
> > > -     if (priv->data->flags & FLAG_DEV_FIX &&
> > > -                     !pci_is_root_bus(bus) && PCI_SLOT(devfn) > 0)
> > > +     if ((priv->data->flags & FLAG_DEV_FIX) && bus->self) {
> > > +             if (!pci_is_root_bus(bus) && (device > 0))
> > > +                     return NULL;
> > > +     }
> > > +
> > > +     /* Don't access unexisting devices */
> > > +     if (pci_is_root_bus(bus) && (device >= 9 && device <= 20 && function > 0))
> >
> > Yuck.  This is pretty nasty magic.  If this is something that might be
> > fixed in future versions of the hardware, maybe you should factor this
> > out into a function pointer in loongson_pci_data or something.
> OK, seems providing a pdev_is_existant() is better.
> 
> Huacai
> >
> > >               return NULL;
> > >
> > >       /* CFG0 can only access standard space */
> > > --
> > > 2.27.0
> > >