Re: [PATCH] PCI: Add sysfs attribute for disabling PCIe link to downstream component

Bjorn Helgaas
2019-08-19T23:52:51Z

On Tue, Aug 06, 2019 at 01:12:30PM +0300, Mika Westerberg wrote:
> On Thu, Aug 01, 2019 at 04:53:39PM -0500, Bjorn Helgaas wrote:
> > [+cc FPGA folks, just FYI; I'm pretty sure PCI could do a much better
> > job supporting FPGAs, so any input is welcome!]
> >
> > On Wed, Jul 03, 2019 at 06:03:41PM +0300, Mika Westerberg wrote:
> > > On Wed, Jul 03, 2019 at 08:39:53AM -0500, Bjorn Helgaas wrote:
> > > > On Wed, May 29, 2019 at 01:49:42PM +0300, Mika Westerberg wrote:
> > > > > PCIe root and downstream ports have link control register that can be
> > > > > used disable the link from software. This can be useful for instance
> > > > > when performing "software" hotplug on systems that do not support real
> > > > > PCIe/ACPI hotplug.
> > > > >
> > > > > For example when used with FPGA card we can burn a new FPGA image
> > > > > without need to reboot the system.
> > > > >
> > > > > First we remove the FGPA device from Linux PCI stack:
> > > > >
> > > > > # echo 1 > /sys/bus/pci/devices/0000:00:01.1/0000:02:00.0/remove
> > > > >
> > > > > Then we disable the link:
> > > > >
> > > > > # echo 1 > /sys/bus/pci/devices/0000:00:01.1/link_disable
> > > > >
> > > > > By doing this we prevent the kernel from accessing the hardware while we
> > > > > burn the new FPGA image.
> > > >
> > > > What is the case where the kernel accesses the hardware? You've
> > > > already done the remove, so the pci_dev is gone. Is this to protect
> > > > against another user doing a rescan? Or is there some spurious event
> > > > during the FPGA update that causes an interrupt that causes pciehp to
> > > > rescan? Something else?
> > >
> > > Protect against another user doing rescan.
> >
> > I'm not 100% sure this is enough of an issue to warrant a new sysfs
> > file. The file is visible all the time to everybody, but it only
> > protects root from shooting him/herself in the foot.
>
> Well, only root can do rescan so in that sense it should be enough ;-)
>
> > > > I guess this particular FPGA update must be done via some side channel
> > > > (not the PCIe link)? I assume there are other FPGA arrangements where
> > > > the update *would* be done via the PCIe link, and we would just do a
> > > > reset to make the update take effect.
> > >
> > > In this setup the FPGA is programmed using side channel. I haven't seen
> > > the actual system but I think it is some sort of FPGA programmer
> > > connected to another system.
> > >
> > > > > Once the new FPGA is burned we can re-enable
> > > > > the link and rescan the new and possibly different device:
> > > > >
> > > > > # echo 0 > /sys/bus/pci/devices/0000:00:01.1/link_disable
> > > > > # echo 1 > /sys/bus/pci/devices/0000:00:01.1/rescan
> > > > >
> > > > > Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
> > > > > ---
> > > > > Documentation/ABI/testing/sysfs-bus-pci | 8 +++
> > > > > drivers/pci/pci-sysfs.c | 65 ++++++++++++++++++++++++-
> > > > > 2 files changed, 72 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
> > > > > index 8bfee557e50e..c93d6b9ab580 100644
> > > > > --- a/Documentation/ABI/testing/sysfs-bus-pci
> > > > > +++ b/Documentation/ABI/testing/sysfs-bus-pci
> > > > > @@ -324,6 +324,14 @@ Description:
> > > > > This is similar to /sys/bus/pci/drivers_autoprobe, but
> > > > > affects only the VFs associated with a specific PF.
> > > > >
> > > > > +What: /sys/bus/pci/devices/.../link_disable
> > > > > +Date: September 2019
> > > > > +Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
> > > > > +Description:
> > > > > + PCIe root and downstream ports have this attribute. Writing
> > > > > + 1 causes the link to downstream component be disabled.
> > > > > + Re-enabling the link happens by writing 0 instead.
> > > > > +
> > > > > What: /sys/bus/pci/devices/.../p2pmem/size
> > > > > Date: November 2017
> > > > > Contact: Logan Gunthorpe <logang@deltatee.com>
> > > > > diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> > > > > index 6d27475e39b2..dfcd21745192 100644
> > > > > --- a/drivers/pci/pci-sysfs.c
> > > > > +++ b/drivers/pci/pci-sysfs.c
> > > > > @@ -218,6 +218,56 @@ static ssize_t current_link_width_show(struct device *dev,
> > > > > }
> > > > > static DEVICE_ATTR_RO(current_link_width);
> > > > >
> > > > > +static ssize_t link_disable_show(struct device *dev,
> > > > > + struct device_attribute *attr, char *buf)
> > > > > +{
> > > > > + struct pci_dev *pci_dev = to_pci_dev(dev);
> > > > > + u16 linkctl;
> > > > > + int ret;
> > > > > +
> > > > > + ret = pcie_capability_read_word(pci_dev, PCI_EXP_LNKCTL, &linkctl);
> > > > > + if (ret)
> > > > > + return -EINVAL;
> > > > > +
> > > > > + return sprintf(buf, "%d\n", !!(linkctl & PCI_EXP_LNKCTL_LD));
> > > > > +}
> > > > > +
> > > > > +static ssize_t link_disable_store(struct device *dev,
> > > > > + struct device_attribute *attr,
> > > > > + const char *buf, size_t count)
> > > > > +{
> > > > > + struct pci_dev *pci_dev = to_pci_dev(dev);
> > > > > + u16 linkctl;
> > > > > + bool disable;
> > > > > + int ret;
> > > > > +
> > > > > + ret = kstrtobool(buf, &disable);
> > > > > + if (ret)
> > > > > + return ret;
> > > > > +
> > > > > + ret = pcie_capability_read_word(pci_dev, PCI_EXP_LNKCTL, &linkctl);
> > > > > + if (ret)
> > > > > + return -EINVAL;
> > > > > +
> > > > > + if (disable) {
> > > > > + if (linkctl & PCI_EXP_LNKCTL_LD)
> > > > > + goto out;
> > > > > + linkctl |= PCI_EXP_LNKCTL_LD;
> > > > > + } else {
> > > > > + if (!(linkctl & PCI_EXP_LNKCTL_LD))
> > > > > + goto out;
> > > > > + linkctl &= ~PCI_EXP_LNKCTL_LD;
> > > > > + }
> > > > > +
> > > > > + ret = pcie_capability_write_word(pci_dev, PCI_EXP_LNKCTL, linkctl);
> > > > > + if (ret)
> > > > > + return ret;
> > > > > +
> > > > > +out:
> > > > > + return count;
> > > > > +}
> > > > > +static DEVICE_ATTR_RW(link_disable);
> > > > > +
> > > > > static ssize_t secondary_bus_number_show(struct device *dev,
> > > > > struct device_attribute *attr,
> > > > > char *buf)
> > > > > @@ -785,6 +835,7 @@ static struct attribute *pcie_dev_attrs[] = {
> > > > > &dev_attr_current_link_width.attr,
> > > > > &dev_attr_max_link_width.attr,
> > > > > &dev_attr_max_link_speed.attr,
> > > > > + &dev_attr_link_disable.attr,
> > > > > NULL,
> > > > > };
> > > > >
> > > > > @@ -1656,8 +1707,20 @@ static umode_t pcie_dev_attrs_are_visible(struct kobject *kobj,
> > > > > struct device *dev = kobj_to_dev(kobj);
> > > > > struct pci_dev *pdev = to_pci_dev(dev);
> > > > >
> > > > > - if (pci_is_pcie(pdev))
> > > > > + if (pci_is_pcie(pdev)) {
> > > > > + if (a == &dev_attr_link_disable.attr) {
> > > > > + switch (pci_pcie_type(pdev)) {
> > > > > + case PCI_EXP_TYPE_ROOT_PORT:
> > > > > + case PCI_EXP_TYPE_DOWNSTREAM:
> > > >
> > > > This is actually not completely reliable because there are weird
> > > > systems that don't identify upstream/downstream ports correctly, e.g.,
> > > > see d0751b98dfa3 ("PCI: Add dev->has_secondary_link to track
> > > > downstream PCIe links") and c8fc9339409d ("PCI/ASPM: Use
> > > > dev->has_secondary_link to find downstream links").
> > >
> > > D'oh!
> > >
> > > It never came to my mind that using pci_pcie_type() would not be
> > > reliable. Thanks for pointing it out.
> > >
> > > > I think I suggested the dev->has_secondary_link approach, but I now
> > > > think that was a mistake because it means we have to remember to look
> > > > at has_secondary_link instead of doing the obvious thing like your
> > > > code.
> > > >
> > > > set_pcie_port_type() detects those unusual topologies, and I think it
> > > > would probably be better for it to just change the cached caps reg
> > > > used by pci_pcie_type() so checking for PCI_EXP_TYPE_DOWNSTREAM does
> > > > the right thing.
> > >
> > > You mean modify set_pcie_port_type() to correct the type if it finds:
> > >
> > > type == PCI_EXP_TYPE_UPSTREAM && !pdev->has_secondary_link => type = PCI_EXP_TYPE_DOWNSTREAM
> > >
> > > or
> > >
> > > type == PCI_EXP_TYPE_DOWNSTREAM && pdev->has_secondary_link => type = PCI_EXP_TYPE_UPSTREAM
> > >
> > > ? Assuming my understanding of pdev->has_secondary_link is correct.
> >
> > I was hoping we could get rid of "has_secondary_link" completely if we
> > corrected the type, but I'm not sure that's possible.
>
> Right, it looks like we need some sort of flag there anyway.

Does this mean you're looking at getting rid of "has_secondary_link",
you think it's impossible, or you think it's not worth trying?

I'm pretty sure we could get rid of it by looking upstream, but I
haven't actually tried it.

Bjorn