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

[Jonathan Cameron via <qemu-devel@nongnu.org>]

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 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?

https://gitlab.com/jic23/qemu/-/commit/314f5033c639ebe8218078a17513935747f15d9d

> ---
> v2: implement this in .realize() instead
> ---
>  hw/pci-bridge/xio3130_downstream.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/hw/pci-bridge/xio3130_downstream.c b/hw/pci-bridge/xio3130_downstream.c
> index 38a2361fa2..2df1ee203d 100644
> --- a/hw/pci-bridge/xio3130_downstream.c
> +++ b/hw/pci-bridge/xio3130_downstream.c
> @@ -72,6 +72,9 @@ static void xio3130_downstream_realize(PCIDevice *d, Error **errp)
>      pci_bridge_initfn(d, TYPE_PCIE_BUS);
>      pcie_port_init_reg(d);
>  
> +    s->speed = QEMU_PCI_EXP_LNK_2_5GT;
> +    s->width = QEMU_PCI_EXP_LNK_X1;
> +
>      rc = msi_init(d, XIO3130_MSI_OFFSET, XIO3130_MSI_NR_VECTOR,
>                    XIO3130_MSI_SUPPORTED_FLAGS & PCI_MSI_FLAGS_64BIT,
>                    XIO3130_MSI_SUPPORTED_FLAGS & PCI_MSI_FLAGS_MASKBIT,