On Fri, Feb 10, 2017 at 01:38:52PM -0500, Keith Busch wrote:
> On Wed, Feb 08, 2017 at 07:04:10AM +0100, Lukas Wunner wrote:
> > On Tue, Feb 07, 2017 at 02:32:35PM -0500, Keith Busch wrote:
> > > If we've detected the PCI device is disconnected, there is no need to
> > > attempt to access its config space since we know the operation will
> > > fail. This patch has all the config reads and writes return -ENODEV
> > > error immediately when in such a state.
> > >
> > > If a config read is sent to a disconnected device, the value will be
> > > set to all 1's. This is the same as what hardware is expected to return
> > > when accessing a removed device, but software can do this faster without
> > > relying on hardware.
> > >
> > > Signed-off-by: Keith Busch <keith.busch@xxxxxxxxx>
> > > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > > ---
> > > drivers/pci/access.c | 18 ++++++++++++++++++
> > > 1 file changed, 18 insertions(+)
> > >
> > > diff --git a/drivers/pci/access.c b/drivers/pci/access.c
> > > index d37b2ed..d63e9dd 100644
> > > --- a/drivers/pci/access.c
> > > +++ b/drivers/pci/access.c
> > > @@ -892,12 +892,20 @@ EXPORT_SYMBOL(pcie_capability_clear_and_set_dword);
> > >
> > > int pci_read_config_byte(const struct pci_dev *dev, int where, u8 *val)
> > > {
> > > + if (pci_dev_is_disconnected(dev)) {
> >
> > You used to have unlikely() here up until v4 but dropped it in v5.
> > Why? Seemed sensible to me.
>
> Didn't mean to remove the unlikely. The micro-optimization isn't in a
> performance path, but I'll build it back in.
Only do so if you can measure the difference, otherwise it is useless,
and can actually hurt.
thanks,
greg k-h
