[PATCH V2 14/16] WIP: usb: dwc2: Implement recovery after PM domain off

[Doug Anderson]

Hi,

On Wed, Aug 14, 2024 at 2:48 PM Stefan Wahren <wahrenst at gmx.net> wrote:
>
> >> You're saying that your
> >> registers get saved _unless_ the power domain gets turned off, right?
> On BCM2835 there is no need to store the registers because there is no
> power management supported by USB core except of the power domain. So
> DWC2 don't expect a register loss.
> >> ...and the device core keeps power domains on for suspended devices if
> >> they are wakeup sources, which makes sense.
> >>
> >> So with that, your patch sounds like a plausible way to do it. I guess
> >> one other way to do it would be some sort of "canary" approach. You
> >> could _always_ save registers and then, at resume time, you could
> >> detect if some "canary" register had reset to its power-on default. If
> >> you see this then you can assume power was lost and re-init all the
> >> registers. This could be pretty much any register that you know won't
> >> be its power on default. In some ways a "canary" approach is uglier
> >> but it also might be more reliable across more configurations?
> I don't have enough knowledge about DWC2 and i also don't have the
> databook to figure out if there is a magic register which could be used
> for the canary approach. But all these different platforms, host vs
> gadget role, different low modes let me think the resulting solution
> would be also fragile and ugly.

I won't admit to having a DWC2 databook. ;-)

...but don't think it's too hard to find a good canary. What about
"GAHBCFG_GLBL_INTR_EN" ? From a quick glance it looks like the driver
seems to set that bit during driver startup and then it stays on until
driver shutdown. The databook that I definitely won't admit to having
almost certainly says that this register resets to 0 on all hardware
and it's applicable to both host and device. I think you could say
that if the register is 0 at resume time that registers must have been
lost and you can restore them.

I guess if that doesn't work then "GUSBCFG_TOUTCAL" could be used (I
think that resets to 0 but must be initted to non-0 by the driver).

Yet another register that could probably work as a canary would be
"GINTMSK". I believe that inits to all 0 (everything is masked) and
obviously to use the device we've got to unmask _some_ interrupts.

I can look for more, if need be.

-Doug