linux-kernel - Re: [RFC 1/1] usb: host: xhci-plat: add enable XHCI-AVOID-BEI quirk by dts

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Date: Tue, 11 Jun 2024 17:58:05 +0300
From: Mathias Nyman <mathias.nyman@...ux.intel.com>
To: joswang <joswang1221@...il.com>
Cc: mathias.nyman@...el.com, gregkh@...uxfoundation.org,
 linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
 joswang <joswang@...ovo.com>
Subject: Re: [RFC 1/1] usb: host: xhci-plat: add enable XHCI-AVOID-BEI quirk
 by dts

On 11.6.2024 17.45, joswang wrote:
> On Tue, Jun 11, 2024 at 6:01 PM Mathias Nyman
> <mathias.nyman@...ux.intel.com> wrote:
>>
>> On 6.6.2024 17.08, joswang wrote:
>>> On Wed, Jun 5, 2024 at 6:31 PM Mathias Nyman
>>> <mathias.nyman@...ux.intel.com> wrote:
>> ...
>>>>
>>>> I was thinking of turning XHCI_AVOID_BEI behavior into the new default, so no
>>>> quirk flag would be needed:
>>>>
>>>> Currently without the quirk flag:
>>>>
>>>> - ISOC TRBs trigger interrupt if TRB is the last in the TD
>>>>
>>>> Currently with XHCI_AVOID_BEI quirk flag:
>>>>
>>>> - ISOC TRBs trigger interrupt if TRB is the last in the TD
>>>> - Interrupt is additionally triggered every 32 isoc TRB (initially).
>>>> - if more than 128 events are processed in one interrupt then the
>>>>      32 is halved, and we trigger an interrupts every 16th isoc TRB, and so
>>>>      on, 16 -> 8...
>>>>
>>>> I would remove the quirk flag, and make all controllers interrupt
>>>> behave as if it was set. i.e. interrupt at least every 32 isoc TRB
>>>
>>> Thank you for your detailed analysis.
>>> Excuse me, I have a question, do you mean to set "Currently with
>>> XHCI_AVOID_BEI quirk flag" as the default behavior?
>>
>> Yes, unless it causes some issues or there are strong objections
>>
>>>>
>>>> Is there an actual real world case where interrupting every 32nd ISOC TRB is
>>>> too often?
>>>
>>> I mean that if the XHCI_AVOID_BEI quirk flag is set, an interrupt will
>>> be triggered every 8 TRBs, which makes the interrupts seem to be quite
>>> frequent.
>>> Thanks
>>> Jos
>>>
>>
>> It should start with interrupting every 32nd isoc TD, not 8th
>>
>> #define AVOID_BEI_INTERVAL_MAX   32
>>
>> ir->isoc_bei_interval = AVOID_BEI_INTERVAL_MAX;
>>
>> Thanks
>> Mathias
> 
> As you mentioned，if more than 128 events are processed in one
> interrupt then the 32 is halved, and we trigger an interrupts every
> 16th isoc TRB, and so on, 16 -> 8...
> xhci_handle_events()
>      ......
>      if (event_loop++ > TRBS_PER_SEGMENT / 2) {
>           if (ir->isoc_bei_interval > AVOID_BEI_INTERVAL_MIN)
>                ir->isoc_bei_interval = ir->isoc_bei_interval / 2;
>      }
>      .......

Yes, and to me this seems reasonable .
If there are over 128 events per interrupt then we should interrupt more often.

Would this work as the default for the Synopsys hosts you are working with?

Thanks
Mathias