From: Johan Hovold <email@example.com>
To: Ikjoon Jang <firstname.lastname@example.org>
Cc: Johan Hovold <email@example.com>,
"GustavoA . R . Silva" <firstname.lastname@example.org>,
Nicolas Boichat <email@example.com>,
Mathias Nyman <firstname.lastname@example.org>
Subject: Re: [PATCH v4 2/2] usb: overridable hub bInterval by device node
Date: Fri, 6 Dec 2019 16:26:04 +0100 [thread overview]
Message-ID: <20191206152604.GO10631@localhost> (raw)
On Fri, Dec 06, 2019 at 11:57:30AM +0800, Ikjoon Jang wrote:
> On Thu, Dec 5, 2019 at 10:26 PM Johan Hovold <email@example.com> 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 <firstname.lastname@example.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
> > 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):
next prev parent reply other threads:[~2019-12-06 15:26 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-03 10:15 [PATCH v4 2/2] usb: overridable hub bInterval by device node Ikjoon Jang
2019-12-03 15:23 ` Alan Stern
2019-12-04 7:07 ` Ikjoon Jang
2019-12-03 16:53 ` Johan Hovold
2019-12-04 7:04 ` Ikjoon Jang
2019-12-04 7:55 ` Johan Hovold
2019-12-05 7:32 ` Ikjoon Jang
2019-12-05 14:26 ` Johan Hovold
2019-12-06 3:57 ` Ikjoon Jang
2019-12-06 15:00 ` Alan Stern
2019-12-09 3:47 ` Ikjoon Jang
2019-12-06 15:26 ` Johan Hovold [this message]
2019-12-09 4:05 ` Ikjoon Jang
2019-12-10 15:02 ` Johan Hovold
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