Re: [PATCH] [RFC] xhci: Add Link TRB sync quirk for ASM3142

[Mathias Nyman <mathias.nyman@linux.intel.com>]

Hi Ole

On 16.4.2021 12.37, Ole Salscheider wrote:
> This patch adds a quirk to the xhci driver so that link TRBs are only
> given to the host controller once it has processed all previous TRBs on
> this segment.
> 
> This quirk is necessary for me on an ASMedia ASM3142 host controller.
> Without it, I get the following errors when accessing a SuperSpeed UVC
> camera:
> 
> Transfer event TRB DMA ptr not part of current TD ep_index XX comp_code XX
> 
> You can find more details in my previous mail about the problem:
> https://lkml.org/lkml/2021/3/31/355
> 
> This patch fixes my problem, but it is probably terribly wrong. I am not
> even sure if I can rely on handle_tx_event being called before each link
> TRB in the segment. Some feedback would be very welcome.

I think we need to look at the cause more closely.

We normally only get events for the last TRB of a TD, or for short transfers like in your case.
So not every transfer TRB generates events.

There are several things going on here that combined could cause this.

Last transfer TRB of a segment has some alignment requirements which  might not be handled in the isoc case.
The amount of untransferred data is large, (16388 bytes) so the TRB causing the short packet
could be far from the last TRB we expect the event on.
Due to new segment and link trb maybe the stored last_trb for this TD is just set wrong

Anyway, more detailed traces together with dynamic debug show us more: 

mount -t debugfs none /sys/kernel/debug
echo 'module xhci_hcd =p' >/sys/kernel/debug/dynamic_debug/control
echo 'module usbcore =p' >/sys/kernel/debug/dynamic_debug/control
echo 81920 > /sys/kernel/debug/tracing/buffer_size_kb
echo 1 > /sys/kernel/debug/tracing/events/xhci-hcd/enable
< trigger the issue >
Send output of dmesg
Send content of /sys/kernel/debug/tracing/trace

trace accumulates pretty fast so try to copy it as soon as the issue is seen.

Thanks
-Mathias