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: Thu, 5 Dec 2019 15:32:38 +0800	[thread overview]
Message-ID: <CAATdQgBcuJenS2VSm+y4Yhn5mWE1P0CGJQ3NRdoe68dd2SRPGg@mail.gmail.com> (raw)
In-Reply-To: <20191204075533.GI10631@localhost>

On Wed, Dec 4, 2019 at 3:55 PM Johan Hovold <johan@kernel.org> wrote:

> > > > @@ -257,6 +258,14 @@ static int usb_parse_endpoint(struct device *ddev, int cfgno, int inum,
> > > >       memcpy(&endpoint->desc, d, n);
> > > >       INIT_LIST_HEAD(&endpoint->urb_list);
> > > >
> > > > +     /* device node property overrides bInterval */
> > > > +     if (usb_of_has_combined_node(to_usb_device(ddev))) {
> > >
> > > Not only hubs have combined nodes so you probably need to check
> > > bDeviceClass here instead.
> >
> > yes, you're right, I didn't think of that case:
> > if (to_usb_device(ddev)->descriptor.bDeviceClass == USB_CLASS_HUB &&
> > ddev->of_node && !of_property_read_u32(...))
> >
> > Or is it better to check bInterfaceClass, for composite devices with a
> > hub interface inside?
> > if (ifp->desc.bInterfaceClass == USB_CLASS_HUB && ddev->of_node &&
> > !of_property_read_u32(...))
> >
> > I think checking bInterfaceClass is better.
>
> Yep, that seems better (but please use two conditionals for
> readability).
>
> 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.

> > > > +             u32 interval = 0;
> > > > +             if (!of_property_read_u32(ddev->of_node, "hub,interval",
> > > > +                                 &interval))
> > > > +                     d->bInterval = min_t(u8, interval, 255);
> > >
> > > You want min_t(u32, ...) here to avoid surprises when someone specifies
> > > a value > 255.
> >
> > yes, thanks.
>
> And I guess you should really be honouring bInterval as a maximum value,
> right?

Yes, right, not masking.

>
> > > > +     }
> > > > +
> > > >       /*
> > > >        * Fix up bInterval values outside the legal range.
> > > >        * Use 10 or 8 ms if no proper value can be guessed.
>
> Johan