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Date:   Fri, 6 Dec 2019 16:26:04 +0100
From:   Johan Hovold <johan@kernel.org>
To:     Ikjoon Jang <ikjn@chromium.org>
Cc:     Johan Hovold <johan@kernel.org>, linux-usb@vger.kernel.org,
        GregKroah-Hartman <gregkh@linuxfoundation.org>,
        RobHerring <robh+dt@kernel.org>,
        MarkRutland <mark.rutland@arm.com>,
        AlanStern <stern@rowland.harvard.edu>,
        SuwanKim <suwan.kim027@gmail.com>,
        "GustavoA . R . Silva" <gustavo@embeddedor.com>,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        Nicolas Boichat <drinkcat@chromium.org>,
        Mathias Nyman <mathias.nyman@linux.intel.com>
Subject: Re: [PATCH v4 2/2] usb: overridable hub bInterval by device node
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On Fri, Dec 06, 2019 at 11:57:30AM +0800, Ikjoon Jang wrote:
> On Thu, Dec 5, 2019 at 10:26 PM Johan Hovold <johan@kernel.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@kernel.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.

I'd say you're indeed trying to give a specific device special
treatment. ;)

> > 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.

No, the USB spec says bInterval is a maximum requested value and that
the host is free to poll more often. And that's policy.

> > 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.

The usb-hid driver actually supports configuring the polling rate
for devices like mice and keyboards after enumeration (through a module
parameter, but still).

Unfortunately, the xhci driver does not yet support this and always uses
the device maximum bInterval. A bug report for this was filed many years
ago, perhaps it's time to address that (adding Mathias on CC):

	https://bugzilla.kernel.org/show_bug.cgi?id=82571

Johan