linux-kernel - Re: [PATCH v4 2/2] usb: overridable hub bInterval by device node

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Date:   Mon, 9 Dec 2019 11:47:47 +0800
From:   Ikjoon Jang <ikjn@...omium.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Johan Hovold <johan@...nel.org>, linux-usb@...r.kernel.org,
        GregKroah-Hartman <gregkh@...uxfoundation.org>,
        RobHerring <robh+dt@...nel.org>,
        MarkRutland <mark.rutland@....com>,
        SuwanKim <suwan.kim027@...il.com>,
        "GustavoA . R . Silva" <gustavo@...eddedor.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nicolas Boichat <drinkcat@...omium.org>
Subject: Re: [PATCH v4 2/2] usb: overridable hub bInterval by device node

On Fri, Dec 6, 2019 at 11:00 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Fri, 6 Dec 2019, Ikjoon Jang wrote:
>
> > On Thu, Dec 5, 2019 at 10:26 PM Johan Hovold <johan@...nel.org> wrote:
> > >
> > > On Thu, Dec 05, 2019 at 03:32:38PM +0800, Ikjoon Jang wrote:
> > > > On Wed, Dec 4, 2019 at 3:55 PM Johan Hovold <johan@...nel.org> wrote:
> > >
> > > > > But related to my question above, why do you need to do this during
> > > > > enumeration? Why not just set the lower interval value in the hub
> > > > > driver?
> > > >
> > > > Because I want device tree's bInterval to be checked against the same rules
> > > > defined in usb_parse_endpoint(). e.g. although hardware says its maximum
> > > > is 255, but the practical limit is still 0 to 16, so the code can
> > > > print warnings when bInterval from device node is too weird.
> > >
> > > But that could be handled refactoring the code in question or similar.
> > >
> >
> > Yes, that should be worked. I can't exactly figure out how to refactor
> > the code for now, but maybe parsed endpoint descriptors are being
> > checked with default hard wired bInterval value and after that
> > an overridden value should be checked again.
> >
> > Actually I don't care about the details of software policies. I just want
> > all devices to be handled in the same manner without any further
> > special treatments.
> >
> > > The fundamental problem here is that you're using devicetree, which is
> > > supposed to only describe the hardware, to encode policy which should be
> > > deferred to user space.
> >
> > The hub hardware has a default bInterval inside which is actually
> > adjustable. So I can think setting bInterval is to describe the hardware
> > rather than policy.
>
> If the hardware is adjustable, why don't you adjust the hardware
> instead of changing the software?

sorry, I meant "hardware has a default value but it's actually
adjustable (by software)". Adjusting hardware is the best option but
our hub doesn't allow to do that, so the current approach is patching
a hardware descriptor on enumeration stage.

>
> > > So I think you need to figure out an interface that allows user space to
> > > set the polling interval for any hub at runtime instead.
> >
> > Changing the interval at runtime is an another way to solve the
> > power consumption problem, but it's not so easy. At least xhci needs
> > to restart an endpoint and no devices are changing the interval after
> > enumeration stage.
>
> Restarting endpoints is easy; just call usb_set_interface().

I thought just changing urb->interval at runtime will be more acceptable.
Maybe I'll need an another approach if this patch is unacceptable.

Thank you!

>
> Alan Stern
>