Clarification of set_sel handling when dwc3 is a device (gadget)

[claus.stovgaard@gmail.com]

Hi

I have earlier worked with the possibility for disabling U1/U2 to solve
an issue where the dwc3 acting as a gadget device sometimes failing to
leave U2.

Analyzing the situation when the dwc3 failing to leave U2, it happens
when the link is in U2 after timeout from U1, and when the device wants
to leave U2. Not when the host wants to leave U2.

Going back the BOS descriptor from the device reports

bU1DevExitLat 1us
bU2DevExitLat 500us

And the SetSystemExitLatency control transfer ends with

U1 System Exit Latency 86 us
U1 Device to Host Exit Latency 1 us
U2 System Exit Latency 585 us
U2 Device to Host Exit Latency 500 us

Looking at the length of LFPS etc. it seems that the U2 exit is just
above 80 us,and it start link training just after.

So I was wandering how the exit latency of the system is communicated
to dwc3 core, and found the following code from ep0.c
dwc3_ep0_set_sel_cmpl

----
reg = dwc3_readl(dwc->regs, DWC3_DCTL);
if (reg & DWC3_DCTL_INITU2ENA)
	param = dwc->u2pel;
if (reg & DWC3_DCTL_INITU1ENA)
	param = dwc->u1pel;

/*
 * According to Synopsys Databook, if parameter is
 * greater than 125, a value of zero should be
 * programmed in the register.
 */
if (param > 125)
	param = 0;

/* now that we have the time, issue DGCMD Set Sel */
ret = dwc3_send_gadget_generic_command(dwc,
		DWC3_DGCMD_SET_PERIODIC_PAR, param);
WARN_ON(ret < 0);
----

I don't have access to Synopsys Databook, so I am puzzled about
the DWC3_DGCMD_SET_PERIODIC_PAR command. The code favor to use the
device to host exit latency, and use the u1 if present over the u2.

So the dwc3 core never get the system exit latency times, and they just
disapeer.

I hope that someone have access to the Databook, and is able to share
some details about the set periodic command and the parameter.

Regards
Claus