Re: [PATCH v2 1/2] usb: dwc3: host: Set XHCI_SG_TRB_CACHE_SIZE_QUIRK

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

On 26.12.2023 7.24, Prashanth K wrote:
> 
> 
> On 22-12-23 11:40 am, Greg Kroah-Hartman wrote:
>> On Fri, Dec 22, 2023 at 11:29:01AM +0530, Prashanth K wrote:
>>> On 15-12-23 06:12 pm, Greg Kroah-Hartman wrote:
>>>> On Tue, Dec 12, 2023 at 04:55:20PM +0530, Prashanth K wrote:
>>>>> Upstream commit bac1ec551434 ("usb: xhci: Set quirk for
>>>>> XHCI_SG_TRB_CACHE_SIZE_QUIRK") introduced a new quirk in XHCI
>>>>> which fixes XHC timeout, which was seen on synopsys XHCs while
>>>>> using SG buffers. But the support for this quirk isn't present
>>>>> in the DWC3 layer.
>>>>>
>>>>> We will encounter this XHCI timeout/hung issue if we run iperf
>>>>> loopback tests using RTL8156 ethernet adaptor on DWC3 targets
>>>>> with scatter-gather enabled. This gets resolved after enabling
>>>>> the XHCI_SG_TRB_CACHE_SIZE_QUIRK. This patch enables it using
>>>>> the xhci device property since its needed for DWC3 controller.
>>>>>
>>>>> In Synopsys DWC3 databook,
>>>>> Table 9-3: xHCI Debug Capability Limitations
>>>>> Chained TRBs greater than TRB cache size: The debug capability
>>>>> driver must not create a multi-TRB TD that describes smaller
>>>>> than a 1K packet that spreads across 8 or more TRBs on either
>>>>> the IN TR or the OUT TR.
>>>>>
>>>>> Cc: <stable@vger.kernel.org>
>>>>> Signed-off-by: Prashanth K <quic_prashk@quicinc.com>
>>>>
>>>> What commit id does this fix?
>>>>
>>> This doesn't fix any commit as such, but adds the support for
>>> XHCI_SG_TRB_CACHE_SIZE_QUIRK (which is present in XHCI layer) to DWC3 layer.
>>
>> So this is a new feature?
>>
>> How does this fit into the stable kernel rules?
> 
> This isn't a new feature. To give some background, upstream commit bac1ec551434 ("usb: xhci: Set quirk for XHCI_SG_TRB_CACHE_SIZE_QUIRK")
> added a XHCI quirk which converts SG lists to CMA buffers/URBS if certain conditions aren't met. But they never enabled this xhci quirk
> since no issues were hit at that time. So, the support for the above mentioned quirk is added from 5.11 kernel onwards, but was never enabled anywhere.

I remember this now.
  
Original series had three patches, two adding the feature to xhci, and one for dwc3 enabling it.
The xhci patches were fine and got in.

https://marc.info/?l=linux-usb&m=160570849625065&w=2

The last dwc3 patch had issues and never apparently got in

https://marc.info/?l=linux-usb&m=161008968009766&w=2

As this feature hasn't been enabled and code not widely run I think it would be  better
to skip stable for now. Stable can be added later once this has been successfully running
in upstream for a while.

Thanks
Mathias