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

From: Ikjoon Jang <ikjn@chromium.org>
To: Johan Hovold <johan@kernel.org>
Cc: 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>
Subject: Re: [PATCH v4 2/2] usb: overridable hub bInterval by device node
Date: Fri, 6 Dec 2019 11:57:30 +0800	[thread overview]
Message-ID: <CAATdQgBK4gWvR06YJ3Z_y5NeqLKYY7Ajc0KG78rG2deR3Ga11A@mail.gmail.com> (raw)
In-Reply-To: <20191205142641.GL10631@localhost>

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.

>
> 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.

Thanks!

>
> Johan