Re: [PATCH] usb: hub: Per-port setting to use old enumeration scheme

[Greg Kroah-Hartman]

On Fri, May 25, 2018 at 06:05:16AM +0800, Nicolas Boichat wrote:
> On Fri, May 25, 2018 at 12:21 AM, Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, May 24, 2018 at 07:42:00AM +0800, Nicolas Boichat wrote:
> >> On Thu, May 24, 2018 at 12:39 AM, Greg Kroah-Hartman
> >> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> >> > On Wed, May 23, 2018 at 10:03:55AM -0400, Alan Stern wrote:
> >> >> On Wed, 23 May 2018, Nicolas Boichat wrote:
> >> >>
> >> >> > The "old" enumeration scheme is considerably faster (it takes
> >> >> > ~294ms instead of ~439ms to get the descriptor).
> >> >> >
> >> >> > It is currently only possible to use the old scheme globally
> >> >> > (/sys/module/usbcore/parameters/old_scheme_first), which is not
> >> >> > desirable as the new scheme was introduced to increase compatibility
> >> >> > with more devices.
> >> >> >
> >> >> > However, in our case, we care about time-to-active for a specific
> >> >> > USB device (which we make the firmware for), on a specific port
> >> >> > (that is pogo-pin based: not a standard USB port). This new
> >> >> > sysfs option makes it possible to use the old scheme on a single
> >> >> > port only.
> >> >> >
> >> >> > Signed-off-by: Nicolas Boichat <drinkcat@xxxxxxxxxxxx>
> >> >> > ---
> >> >> >
> >> >> > There are other "quirks" that we could add to reduce further
> >> >> > enumeration time (e.g. reduce USB debounce time, reduce TRSTRCY
> >> >> > to 10ms instead of 50ms as used currently), but the logic is quite
> >> >> > similar, so it'd be good to have this reviewed first.
> >> >>
> >> >> I'm not opposed to the idea in principle, although I don't like your
> >> >> implementation because it breaks the original old_scheme_first
> >> >> parameter.
> >>
> >> I don't think it breaks the original parameter? I mean,
> >> /sys/module/usbcore/parameters/old_scheme_first is still a global
> >> default, while bit 0 of /sys/bus/usb/devices/x/y/z/quirks becomes a
> >> port-specific override.
> >>
> >> >> Let's see what some other people think.
> >> >>
> >> >> Yours is a rather special case, because you know exactly what device
> >> >> will be attached to a specific port.  Still, I can see that sort of
> >> >> thing happening in constrained and special-purpose settings.
> >> >>
> >> >> How do you arrange to set the new quirk before the device is
> >> >> discovered?
> >> >
> >> > Yeah, this last question is what I had when looking at this.  Or does it
> >> > not matter at first boot and only matters for wake-up?
> >>
> >> It does not matter on boot, we have plenty of time to enumerate the
> >> device. We use USB (auto-)suspend and remote wake, so no
> >> re-enumeration there either. It only matters on unplug/replug where
> >> the device needs to be re-enumerated.
> >
> > How does this device get unplugged/replugged if it is connected directly
> > to the device?
> 
> It is external. Essentially, this is a tablet with a detachable
> keyboard/touchpad. The interface between tablet and base is USB, over
> pogo pins. The port is non-standard (pogo, not normal USB), and we
> fully control the firmware on the base side as well, which allows us
> to take shortcuts like this: we know exactly what device will be
> connected on that port.

Ah, ok, that makes more sense, thanks for the explanation.  I'll go
queue this up in a bit.

greg k-h