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

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Message-ID: <CAMtoTm2G-w+yRjechji7scJO-JZo6N==VK6Wun3ATRc7do9rEg@mail.gmail.com>
Date: Tue, 11 Jun 2024 22:45:26 +0800
From: joswang <joswang1221@...il.com>
To: Mathias Nyman <mathias.nyman@...ux.intel.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 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;
    }
    .......

Thanks
Jos Wang