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

From: Alan Stern <stern@rowland.harvard.edu>
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>,
	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>
Subject: Re: [PATCH v4 2/2] usb: overridable hub bInterval by device node
Date: Fri, 6 Dec 2019 10:00:16 -0500 (EST)	[thread overview]
Message-ID: <Pine.LNX.4.44L0.1912060958550.1618-100000@iolanthe.rowland.org> (raw)
In-Reply-To: <CAATdQgBK4gWvR06YJ3Z_y5NeqLKYY7Ajc0KG78rG2deR3Ga11A@mail.gmail.com>

On Fri, 6 Dec 2019, 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.
> 
> > 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?

> > 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().

Alan Stern