linux-kernel - Re: [RFC PATCH] usb: xhci-mtk: handle bandwidth table rollover

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Message-ID: <efcd999aaf83cf73ed2f4f4b9efa1bb93efd2523.camel@mediatek.com>
Date:   Thu, 12 Aug 2021 11:49:38 +0000
From:   Chunfeng Yun (云春峰) 
        <Chunfeng.Yun@...iatek.com>
To:     "ikjn@...omium.org" <ikjn@...omium.org>
CC:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "mathias.nyman@...el.com" <mathias.nyman@...el.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] usb: xhci-mtk: handle bandwidth table rollover

On Thu, 2021-08-12 at 17:31 +0800, Ikjoon Jang wrote:
> HI,
> 
> On Wed, Aug 11, 2021 at 5:02 PM Chunfeng Yun (云春峰)
> <Chunfeng.Yun@...iatek.com> wrote:
> > 
> > On Mon, 2021-08-09 at 17:42 +0800, Ikjoon Jang wrote:
> > > On Mon, Aug 9, 2021 at 5:11 PM Greg Kroah-Hartman
> > > <gregkh@...uxfoundation.org> wrote:
> > > > 
> > > > On Mon, Aug 09, 2021 at 04:59:29PM +0800, Ikjoon Jang wrote:
> > > > > xhci-mtk has 64 slots for periodic bandwidth calculations and
> > > > > each
> > > > > slot represents byte budgets on a microframe. When an
> > > > > endpoint's
> > > > > allocation sits on the boundary of the table, byte budgets'
> > > > > slot
> > > > > should be rolled over but the current implementation doesn't.
> > > > > 
> > > > > This patch applies a 6 bits mask to the microframe index to
> > > > > handle
> > > > > its rollover 64 slots and prevent out-of-bounds array access.
> > > > > 
> > > > > Signed-off-by: Ikjoon Jang <ikjn@...omium.org>
> > > > > ---
> > > > > 
> > > > >  drivers/usb/host/xhci-mtk-sch.c | 79 +++++++++------------
> > > > > ----
> > > > > --------
> > > > >  drivers/usb/host/xhci-mtk.h     |  1 +
> > > > >  2 files changed, 23 insertions(+), 57 deletions(-)
> > > > 
> > > > Why is this "RFC"?  What needs to be addressed in this change
> > > > before it
> > > > can be accepted?
> > > 
> > > sorry, I had to mention why this is RFC:
> > > 
> > > I simply don't know about the details of the xhci-mtk internals.
> > > It was okay from my tests with mt8173 and I think this will be
> > > harmless
> > > as this is "better than before".
> > > 
> > > But when I removed get_esit_boundary(), I really have no idea why
> > > it was there. I'm wondering if there was another reason of that
> > > function
> > > other than just preventing out-of-bounds. Maybe chunfeng can
> > > answer
> > > this?
> > 
> > We use @esit to prevent out-of-bounds array access. it's not a
> > ring,
> > can't insert out-of-bounds value into head slot.
> 
> Thanks, so that function was only for out-of-bounds array access.
> then I think we just can remove that function and use it as a ring.
> Can you tell me _why_ it can't be used as a ring?
Treat it as a period, roll over slot equals to put it into the next
period.

> 
> I think a transaction (e.g. esit_boundary = 7) can start its first
> SSPLIT
> from Y_7 (offset = 7). But will that allocation be matched with this?
> 
> -               if ((offset + sch_ep->num_budget_microframes) >
> esit_boundary)
> -                       break;
> 
> I mean I'm not sure why this is needed.
Prevent out-of-bounds.

> 
> Until now, I couldn't find a way to accept the USB audio headset
> with a configuration of { INT-IN 64 + ISOC-OUT 384 + ISOC-IN 192 }
> without this patch.
what is the interval value of each endpoint?

> 
> > 
> > > 
> > > Thanks!
> > > 
> > > > 
> > > > thanks,
> > > > 
> > > > greg k-h