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

From: Ikjoon Jang <ikjn@chromium.org>
To: Alan Stern <stern@rowland.harvard.edu>
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: Mon, 9 Dec 2019 11:47:47 +0800	[thread overview]
Message-ID: <CAATdQgAxCja9L33QFQOEfPjOrGc5-pTGXifsYnxAZ+LOLkcuxA@mail.gmail.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1912060958550.1618-100000@iolanthe.rowland.org>

On Fri, Dec 6, 2019 at 11:00 PM Alan Stern <stern@rowland.harvard.edu> wrote:
>
> 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?

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
>