Re: [PATCH 2/2] dt-bindings: usb: cdns,usb3: Add support for USBSSP

[Peter Chen]

On 26-03-04 10:02:07, Pawel Laszczak wrote:
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> 
> >> >>>
> >> >>> +description:
> >> >>> +  Cadence USB dual-role controllers. USBSS (cdns,usb3) supports
> >> >>> +up to
> >> >>> +  SuperSpeed (USB 3.0). USBSSP (cdns,usbssp) is the next
> >> >>> +generation with
> >> >>> +  SuperSpeed Plus (USB 3.1 gen2x1) and XHCI-based device
> >> >>> +controller. Both
> >> >>> +  share the same register layout and resource model.
> >> >>
> >> >> So are compatible or not?
> >> >>
> >> >
> >> > Sorry for the misleading description. They are NOT fully compatible.
> >> > The register layout (OTG/XHCI/Device) and interrupts
> >> > (OTG/XHCI/Device/Wakeup) are the same, but register contents are
> >>
> >> Layout cannot be the same if contents is different. Same layout means
> >> same register is at the same place. If you have different register
> >> with different contents at given place, how is it "same layout"?
> >
> >Sorry. I mean the USBSS and USBSSP share the same resource model (three
> >memory regions for OTG, XHCI and Device, plus three to four interrupts). But in
> >each region, eg in OTG region, the layout for each register are different for both
> >controllers.
> >
> >Pawel, I think we could try Krzysztof's suggestion and differentiating USBSS IP and
> >USBSSP IP at runtime, we could use DID register (cdns->version) to do that and
> >avoid adding new IP general binding doc. What do you think so?
> >
> >> Again, driver stuff.
> >>
> >> >
> >> >   if (device_get_match_data(dev) == &cdnsp_plat)
> >> >       cdns->gadget_init = cdnsp_gadget_init;
> >> >   else
> >> >       cdns->gadget_init = cdns3_gadget_init;
> >> >
> >> > Without a distinct compatible, the driver cannot know which gadget
> >> > controller is present. This is a Cadence IP-level distinction (not
> >> > SoC-specific), so a generic compatible seems appropriate here. But
> >> > please let me know if you'd prefer a different approach.
> >>
> >> Generic compatibles are almost never appropriate and driver code
> >> rarely convinces.
> >>
> >> Best regards,
> >> Krzysztof
> 
> The Device (Peripheral) controller in USBSSP is not just an evolution
> of USBSS; it is a completely different IP core with a distinct register
> map.
> While the Host (XHCI) and OTG/DRD portions overlap significantly, the
> Device register space in USBSSP is architecturally incompatible with USBSS.
> They are different IP cores integrated into the same subsystem:
> 
> The register map for the Device controller in USBSSP is entirely
> different. Offsets that are valid for one controller point to different
> functions or reserved space in the other. Using a single compatible string
> would imply register-level compatibility that does not exist.
> USBSS uses a custom DMA-based peripheral controller, whereas USBSSP uses
> an entirely different XHCI-based architecture for the device side.
> 
> These two controllers do not provide a reliable way (e.g., via registers)
> to distinguish between them in every case.
> We should treat them as two separate controllers that happen to share
> some common code, rather than viewing USBSSP simply as a newer
> version of USBSS.
> 
> I believe Peter's intention was to use a common DT binding and a single
> platform driver, as the resource model and parts of the code are nearly
> identical.
> However, given the architectural differences on the device side,
> I believe having distinct compatible strings is the most accurate way to
> describe the hardware. It prevents the driver from making "blind" accesses
> to registers that have different meanings depending on the underlying IP.
> 
> Maybe to resolve this, we should have separate DT binding documentation
> and potentially separate platform drivers to reflect this architectural
> split?
> 

Pawel, currently, the OTG and host share the same code between USBSS
and USBSSP, the execution flow is decided by runtime already.
And for device mode, there are two different drivers for them, we could
decide which driver's initialization during the runtime, after the
initialization, the device driver (UDC)'s behaviour has already
decided per IP (USBSS vs USBSSP). Are there any examples the controller
would visit wrong register, eg, USBSS IP visits USBSSP registers?

Peter


-- 

Best regards,
Peter