Re: [PATCH v2] pci-bridge/xio3130_downstream: fix invalid link speed and link width

[Nam Cao <namcao@linutronix.de>]

On Fri, May 31, 2024 at 11:14:00AM +0100, Jonathan Cameron wrote:
> On Wed, 29 May 2024 22:17:44 +0200
> Nam Cao <namcao@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 <namcao@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