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

Wu Hao
2019-08-04T12:08:30Z

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.
>
> > > 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.

One example is, FPGA image is stored in flash on the PCIe card, and the
board management controller loads the image from flash to FPGA during
power up or any time requested by software/user.

Actually I used several PCIe FPGA cards, they can do partial reconfiguration
with inband method, but they are all using side channel to do full FPGA
reconfiguration. Partial reconfiguration doesn't touch the PCIe logics
inside FPGA, but full reconfiguration case really does. So we do need some
protection for full reconfiguration case from PCIe device level.

Actually we are using the similar method as above to remove/rescan pcie dev
to make sure nobody see it during full reconfiguration. For sure we need
to make sure nobody does the rescan at the same time, and another thing is
we have to mask AER on the root port, otherwise it's easy to see system
hang/reboot. I just did some qucik tests, with this patch, after link_disable
I don't need to mask AER any more, so this patch does help on this case,
and I should be able to try more next week.

Thanks a lot for adding us to this thread. :)

Hao

> >
> > > > 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.
>
> Bjorn