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: Wed, 4 Dec 2019 15:04:53 +0800	[thread overview]
Message-ID: <CAATdQgCqYrd_aXN5GDsso+F3WadNx3DQKK3Efk3tgkrv2VXjyw@mail.gmail.com> (raw)
In-Reply-To: <20191203165301.GH10631@localhost>

On Wed, Dec 4, 2019 at 12:52 AM Johan Hovold <johan@kernel.org> wrote:
>
> On Tue, Dec 03, 2019 at 06:15:52PM +0800, Ikjoon Jang wrote:
> > This patch enables hub device to override its own endpoint descriptor's
> > bInterval when the hub has a device node with "hub,interval" property.
> >
> > When we know reducing autosuspend delay for built-in HIDs is better for
> > power saving, we can reduce it to the optimal value. But if a parent hub
> > has a long bInterval, mouse lags a lot from more frequent autosuspend.
> > So this enables overriding bInterval for a hard wired hub device only
> > when we know that reduces the power consumption.
>
> I think I saw you argue about why this shouldn't simply be configured at
> runtime. Please include that here too, I can't seem to remember why...

Okay.

>
> > Signed-off-by: Ikjoon Jang <ikjn@chromium.org>
> > Acked-by: Alan Stern <stern@rowland.harvard.edu>
> > ---
> >  drivers/usb/core/config.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/drivers/usb/core/config.c b/drivers/usb/core/config.c
> > index 5f40117e68e7..95ec5af42a1c 100644
> > --- a/drivers/usb/core/config.c
> > +++ b/drivers/usb/core/config.c
> > @@ -6,6 +6,7 @@
> >  #include <linux/usb.h>
> >  #include <linux/usb/ch9.h>
> >  #include <linux/usb/hcd.h>
> > +#include <linux/usb/of.h>
> >  #include <linux/usb/quirks.h>
> >  #include <linux/module.h>
> >  #include <linux/slab.h>
> > @@ -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.

>
> > +             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.

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

I will send v5 soon, thank you.