[0/3] usb: gadget: Add support for disabling U1 and U2 entries

["Claus H. Stovgaard" <cst@phaseone.com>]

Hi Anurag

On Fri, 2019-05-03 at 07:34 +0000, Anurag Kumar Vulisha wrote:
> Hi Claus,
> Thanks for testing and voting for this patch.

I have first tested your patches today. My test setup is a ZynqMP
device, running kernel 4.14 (Xilinx version) with your patches.
I then create an overlay for the devicetree with the new parameters,
and unbind/bind the dwc3 driver.

Next I have a host running Windows 10 and a MacBook pro with Type-C
ports. For logging the communication I use a Total Phase Beagle USB3
5000 V2 analyzer.

The test showed that OS-X does as expected. When BOS descriptor
(bU1DevExtLat and bU2DevExtLat) returns 0, it does not enable LPM.

Windows 10 on the other hand does not, and even though it received 0 as
bU1DevExtLat and bU2DevExtLat it send Set Sel with U1SEL 85 us, U1PEL 0
us, U2SEL 85 us and U2PEL 0 us.
Next the Windows 10 host sends the U1 Enable and the U2 enable as Set
Device Feature, resulting in the system entering U2.

> > 
> > Just today I was making another solution for this feature, using
> > the
> > configfs instead of the devicetree. Though thinks your solution is
> > better, as it uses the U1DevExitLat and U2DevExitLat instead. I
> > just
> > added my solution to the bottem of the mail for reference.
> > 
> > [1] https://www.spinics.net/lists/linux-usb/msg179393.html
> > 
> Your approach below is also good, but you are just avoiding the
> gadget dwc3
> controller from entering into U1 and U2 states by disabling the
> ACCEPTU1ENA
> and ACCEPTU2ENA bits in DCTL but not preventing the host from sending
> the
> LG0_U1 and LGO_U2 link command signaling to the gadget. The host will
> keep
> on trying to get the link into U1 or U2 by sending LGO_U1 or LGO_U2
> and the
> gadget rejects these signals by sending LXU link command. To avoid
> this extra
> overhead I thought that sending zero  value in the BOS descriptor's
> U1DevExitLat and U2DevExitLat fields would be the best option. Host
> on seeing
> U 1 & U2 Exit Latencies doesn't initiate LPM U1 and U2 commands.
> 
> Thanks,
> Anurag Kumar Vulisha

Correct that it does not prevent the host from sending LG0_U1 and
LG0_U2, and there is your solution better on hosts using the BOS
descriptor for disabling LPM. So based on my test with Windows 10, I
think we should combine the solutions. To prevent LG0_U1/LG0_U2 when
possible and still being able to completely disable U1/U2.

Regarding interface for controlling it. I am very novice regarding
Linux kernel development, but would think the BOS descriptor control
would be better from a configfs interface then devicetree as I don't
see BOS descriptor as hardware specific. I am more in doubt about the
forcing of U1/U2 as I did with setting hardware registers, as it
control hardware registers. So will like to hear from other more
experienced developers.

Regards
Claus