On Fri, May 31, 2024 at 11:14:00AM +0100, Jonathan Cameron wrote:
> On Wed, 29 May 2024 22:17:44 +0200
> Nam Cao <nam...@linutronix.de> wrote:
>
> > Set link width to x1 and link speed to 2.5 Gb/s as specified by the
> > datasheet. Without this, these fields in the link status register read
> > zero, which is incorrect.
> >
> > This problem appeared since 3d67447fe7c2 ("pcie: Fill PCIESlot link fields
> > to support higher speeds and widths"), which allows PCIe slot to set link
> > width and link speed. However, if PCIe slot does not explicitly set these
> > properties, they will be zero. Before this commit, the width and speed
> > default to x1 and 2.5 Gb/s.
> >
> > Fixes: 3d67447fe7c2 ("pcie: Fill PCIESlot link fields to support higher
> > speeds and widths")
> > Signed-off-by: Nam Cao <nam...@linutronix.de>
> Hi Nam,
>
> I'm feeling a bit guilty about this one a known it was there for a while.
>
> I was lazy when fixing the equivalent CXL case a while back on
> basis no one had noticed and unlike CXL (where migration is broken for a lot
> of reasons) fixing this may need to take into account migration from broken to
> fixed versions. Have you tested that?
I tested this patch with Linux kernel.
I noticed this bug when Linux complained that the PCI link was broken.
Linux determines weather a link is up by checking if these speed/width
fields have valid value.
Repro:
qemu-system-x86_64 \
-machine pc-q35-2.10 \
-kernel bzImage \
-drive "file=img,format=raw" \
-m 2048 -smp 1 -enable-kvm \
-append "console=ttyS0 root=/dev/sda debug" \
-nographic \
-device pcie-root-port,bus=pcie.0,slot=1,id=rp1,bus-reserve=253 \
-device x3130-upstream,id=up1,bus=rp1 \
-device xio3130-downstream,id=dp1,bus=up1,chassis=1,slot=1
Then after Linux has booted:
device_add device_add e1000,bus=dp1,id=eth0
Then Linux complains that something is wrong with the link:
pcieport 0000:02:00.0: pciehp: Slot(1-1): Cannot train link: status 0x2000
This patch gets rid of Linux's complain, and the hot-plug now works fine.
> I did the CXL fix slightly differently. Can't remember why though - looking
> at the fact it uses an instance_post_init, is there an issue with accidentally
> overwriting the parameters? Or did I just over engineer the fix?
I would say over engineer. I think CXL does not take link speed and link
width as parameters.
Best regards,
Nam
