Re: [PING][PATCH v6 0/7] pci: Work around ASMedia ASM2824 PCIe link training failures

[Lukas Wunner <lukas@wunner.de>]

[+cc Philipp]

On Sun, Feb 19, 2023 at 06:52:21PM +0000, Maciej W. Rozycki wrote:
> On Sun, 5 Feb 2023, Maciej W. Rozycki wrote:
> >  This is v6 of the change to work around a PCIe link training phenomenon 
> > where a pair of devices both capable of operating at a link speed above 
> > 2.5GT/s seems unable to negotiate the link speed and continues training 
> > indefinitely with the Link Training bit switching on and off repeatedly 
> > and the data link layer never reaching the active state.
> 
>  Ping for: 
> <https://lore.kernel.org/linux-pci/alpine.DEB.2.21.2302022022230.45310@angie.orcam.me.uk/>.

Philipp is witnessing similar issues with a Pericom PI7C9X2G404EL
switch below a Broadcom STB host controller:  On some rare occasions,
when booting the system the link trains correctly at 5 GT/s and the
switch is accessible without any issues.  But most of the time,
the switch is inaccessible on boot.  The Broadcom STB host controller
claims not to support Link Active Reporting, but in reality has a
link status indicator in a custom register.  It indicates that the
link is up, the link trains to 2.5 GT/s but the switch is inaccessible.
Due to a quirk of the Broadcom STB host controller, ECAM access to
the inaccessible switch raises an unhandled CPU exception and thus
causes a kernel panic, making the issue difficult to debug.

The switch works fine 100% when plugged into a TI Sitara AM64 board
(contains a DesignWare-derived PCIe host controller).

The switch is the same as yours, only with 4 instead of 3 ports.
Perhaps the issue you're seeing isn't specific to the ASMedia switch,
but is due to an oddity of the Pericom switch, that happens to be
triggered by the Broadcom STB host controller as well?

I've cooked up a modified version of patch 7 in your series which
performs the link retraining in the pci-brcmstb.c driver before
performing the first access to the switch.  Unfortunately it
didn't result in any kind of improvement.  Next step is to hook up
a Teledyne T28 analyzer to see what's going on.

Thanks,

Lukas