Re: [PATCH v9 0/5] Re-introduce TX FIFO resize for larger EP bursting

[Andy Shevchenko <andy.shevchenko@gmail.com>]

On Fri, Jun 11, 2021 at 4:14 PM Heikki Krogerus
<heikki.krogerus@linux.intel.com> wrote:
>
> On Fri, Jun 11, 2021 at 04:00:38PM +0300, Felipe Balbi wrote:
> >
> > Hi,
> >
> > Wesley Cheng <wcheng@codeaurora.org> writes:
> > >>>>>>> to be honest, I don't think these should go in (apart from the build
> > >>>>>>> failure) because it's likely to break instantiations of the core with
> > >>>>>>> differing FIFO sizes. Some instantiations even have some endpoints with
> > >>>>>>> dedicated functionality that requires the default FIFO size configured
> > >>>>>>> during coreConsultant instantiation. I know of at OMAP5 and some Intel
> > >>>>>>> implementations which have dedicated endpoints for processor tracing.
> > >>>>>>>
> > >>>>>>> With OMAP5, these endpoints are configured at the top of the available
> > >>>>>>> endpoints, which means that if a gadget driver gets loaded and takes
> > >>>>>>> over most of the FIFO space because of this resizing, processor tracing
> > >>>>>>> will have a hard time running. That being said, processor tracing isn't
> > >>>>>>> supported in upstream at this moment.
> > >>>>>>>
> > >>>>>
> > >>>>> I agree that the application of this logic may differ between vendors,
> > >>>>> hence why I wanted to keep this controllable by the DT property, so that
> > >>>>> for those which do not support this use case can leave it disabled.  The
> > >>>>> logic is there to ensure that for a given USB configuration, for each EP
> > >>>>> it would have at least 1 TX FIFO.  For USB configurations which don't
> > >>>>> utilize all available IN EPs, it would allow re-allocation of internal
> > >>>>> memory to EPs which will actually be in use.
> > >>>>
> > >>>> The feature ends up being all-or-nothing, then :-) It sounds like we can
> > >>>> be a little nicer in this regard.
> > >>>>
> > >>>
> > >>> Don't get me wrong, I think once those features become available
> > >>> upstream, we can improve the logic.  From what I remember when looking
> > >>
> > >> sure, I support that. But I want to make sure the first cut isn't likely
> > >> to break things left and right :)
> > >>
> > >> Hence, let's at least get more testing.
> > >>
> > >
> > > Sure, I'd hope that the other users of DWC3 will also see some pretty
> > > big improvements on the TX path with this.
> >
> > fingers crossed
> >
> > >>> at Andy Shevchenko's Github, the Intel tracer downstream changes were
> > >>> just to remove physical EP1 and 2 from the DWC3 endpoint list.  If that
> > >>
> > >> right, that's the reason why we introduced the endpoint feature
> > >> flags. The end goal was that the UDC would be able to have custom
> > >> feature flags paired with ->validate_endpoint() or whatever before
> > >> allowing it to be enabled. Then the UDC driver could tell UDC core to
> > >> skip that endpoint on that particular platform without interefering with
> > >> everything else.
> > >>
> > >> Of course, we still need to figure out a way to abstract the different
> > >> dwc3 instantiations.
> > >>
> > >>> was the change which ended up upstream for the Intel tracer then we
> > >>> could improve the logic to avoid re-sizing those particular EPs.
> > >>
> > >> The problem then, just as I mentioned in the previous paragraph, will be
> > >> coming up with a solution that's elegant and works for all different
> > >> instantiations of dwc3 (or musb, cdns3, etc).
> > >>
> > >
> > > Well, at least for the TX FIFO resizing logic, we'd only be needing to
> > > focus on the DWC3 implementation.
> > >
> > > You bring up another good topic that I'll eventually needing to be
> > > taking a look at, which is a nice way we can handle vendor specific
> > > endpoints and how they can co-exist with other "normal" endpoints.  We
> > > have a few special HW eps as well, which we try to maintain separately
> > > in our DWC3 vendor driver, but it isn't the most convenient, or most
> > > pretty method :).
> >
> > Awesome, as mentioned, the endpoint feature flags were added exactly to
> > allow for these vendor-specific features :-)
> >
> > I'm more than happy to help testing now that I finally got our SM8150
> > Surface Duo device tree accepted by Bjorn ;-)
> >
> > >>> However, I'm not sure how the changes would look like in the end, so I
> > >>> would like to wait later down the line to include that :).
> > >>
> > >> Fair enough, I agree. Can we get some more testing of $subject, though?
> > >> Did you test $subject with upstream too? Which gadget drivers did you
> > >> use? How did you test
> > >>
> > >
> > > The results that I included in the cover page was tested with the pure
> > > upstream kernel on our device.  Below was using the ConfigFS gadget w/ a
> > > mass storage only composition.
> > >
> > > Test Parameters:
> > >  - Platform: Qualcomm SM8150
> > >  - bMaxBurst = 6
> > >  - USB req size = 256kB
> > >  - Num of USB reqs = 16
> >
> > do you mind testing with the regular request size (16KiB) and 250
> > requests? I think we can even do 15 bursts in that case.
> >
> > >  - USB Speed = Super-Speed
> > >  - Function Driver: Mass Storage (w/ ramdisk)
> > >  - Test Application: CrystalDiskMark
> > >
> > > Results:
> > >
> > > TXFIFO Depth = 3 max packets
> > >
> > > Test Case | Data Size | AVG tput (in MB/s)
> > > -------------------------------------------
> > > Sequential|1 GB x     |
> > > Read      |9 loops    | 193.60
> > >           |           | 195.86
> > >           |           | 184.77
> > >           |           | 193.60
> > > -------------------------------------------
> > >
> > > TXFIFO Depth = 6 max packets
> > >
> > > Test Case | Data Size | AVG tput (in MB/s)
> > > -------------------------------------------
> > > Sequential|1 GB x     |
> > > Read      |9 loops    | 287.35
> > >         |           | 304.94
> > >           |           | 289.64
> > >           |           | 293.61
> >
> > I remember getting close to 400MiB/sec with Intel platforms without
> > resizing FIFOs and I'm sure the FIFO size was set to 2x1024, though my
> > memory could be failing.
> >
> > Then again, I never ran with CrystalDiskMark, I was using my own tool
> > (it's somewhere in github. If you care, I can look up the URL).
> >
> > > We also have internal numbers which have shown similar improvements as
> > > well.  Those are over networking/tethering interfaces, so testing IPERF
> > > loopback over TCP/UDP.
> >
> > loopback iperf? That would skip the wire, no?
> >
> > >>> size of 2 and TX threshold of 1, this would really be not beneficial to
> > >>> us, because we can only change the TX threshold to 2 at max, and at
> > >>> least in my observations, once we have to go out to system memory to
> > >>> fetch the next data packet, that latency takes enough time for the
> > >>> controller to end the current burst.
> > >>
> > >> What I noticed with g_mass_storage is that we can amortize the cost of
> > >> fetching data from memory, with a deeper request queue. Whenever I
> > >> test(ed) g_mass_storage, I was doing so with 250 requests. And that was
> > >> enough to give me very good performance. Never had to poke at TX FIFO
> > >> resizing. Did you try something like this too?
> > >>
> > >> I feel that allocating more requests is a far simpler and more generic
> > >> method that changing FIFO sizes :)
> > >>
> > >
> > > I wish I had a USB bus trace handy to show you, which would make it very
> > > clear how the USB bus is currently utilized with TXFIFO size 2 vs 6.  So
> > > by increasing the number of USB requests, that will help if there was a
> > > bottleneck at the SW level where the application/function driver
> > > utilizing the DWC3 was submitting data much faster than the HW was
> > > processing them.
> > >
> > > So yes, this method of increasing the # of USB reqs will definitely help
> > > with situations such as HSUSB or in SSUSB when EP bursting isn't used.
> > > The TXFIFO resize comes into play for SSUSB, which utilizes endpoint
> > > bursting.
> >
> > Hmm, that's not what I remember. Perhaps the TRB cache size plays a role
> > here too. I have clear memories of testing this very scenario of
> > bursting (using g_mass_storage at the time) because I was curious about
> > it. Back then, my tests showed no difference in behavior.
> >
> > It could be nice if Heikki could test Intel parts with and without your
> > changes on g_mass_storage with 250 requests.
>
> Andy, you have a system at hand that has the DWC3 block enabled,
> right? Can you help out here?

I'm not sure if i will have time soon, I Cc'ed to Ferry who has a few
more test cases (I have only one or two) and maybe can help. But I'll
keep this in mind.

> > > Now with endpoint bursting, if the function notifies the host that
> > > bursting is supported, when the host sends the ACK for the Data Packet,
> > > it should have a NumP value equal to the bMaxBurst reported in the EP
> >
> > Yes and no. Looking back at the history, we used to configure NUMP based
> > on bMaxBurst, but it was changed later in commit
> > 4e99472bc10bda9906526d725ff6d5f27b4ddca1 by yours truly because of a
> > problem reported by John Youn.
> >
> > And now we've come full circle. Because even if I believe more requests
> > are enough for bursting, NUMP is limited by the RxFIFO size. This ends
> > up supporting your claim that we need RxFIFO resizing if we want to
> > squeeze more throughput out of the controller.
> >
> > However, note that this is about RxFIFO size, not TxFIFO size. In fact,
> > looking at Table 8-13 of USB 3.1 r1.0, we read the following about NumP
> > (emphasis is mine):
> >
> >       "Number of Packets (NumP). This field is used to indicate the
> >       number of Data Packet buffers that the **receiver** can
> >       accept. The value in this field shall be less than or equal to
> >       the maximum burst size supported by the endpoint as determined
> >       by the value in the bMaxBurst field in the Endpoint Companion
> >       Descriptor (refer to Section 9.6.7)."
> >
> > So, NumP is for the receiver, not the transmitter. Could you clarify
> > what you mean here?
> >
> > /me keeps reading
> >
> > Hmm, table 8-15 tries to clarify:
> >
> >       "Number of Packets (NumP).
> >
> >       For an OUT endpoint, refer to Table 8-13 for the description of
> >       this field.
> >
> >       For an IN endpoint this field is set by the endpoint to the
> >       number of packets it can transmit when the host resumes
> >       transactions to it. This field shall not have a value greater
> >       than the maximum burst size supported by the endpoint as
> >       indicated by the value in the bMaxBurst field in the Endpoint
> >       Companion Descriptor. Note that the value reported in this field
> >       may be treated by the host as informative only."
> >
> > However, if I remember correctly (please verify dwc3 databook), NUMP in
> > DCFG was only for receive buffers. Thin, John, how does dwc3 compute
> > NumP for TX/IN endpoints? Is that computed as a function of DCFG.NUMP or
> > TxFIFO size?
> >
> > > desc.  If we have a TXFIFO size of 2, then normally what I have seen is
> > > that after 2 data packets, the device issues a NRDY.  So then we'd need
> > > to send an ERDY once data is available within the FIFO, and the same
> > > sequence happens until the USB request is complete.  With this constant
> > > NRDY/ERDY handshake going on, you actually see that the bus is under
> > > utilized.  When we increase an EP's FIFO size, then you'll see constant
> > > bursts for a request, until the request is done, or if the host runs out
> > > of RXFIFO. (ie no interruption [on the USB protocol level] during USB
> > > request data transfer)
> >
> > Unfortunately I don't have access to a USB sniffer anymore :-(
> >
> > >>>>>> Good points.
> > >>>>>>
> > >>>>>> Wesley, what kind of testing have you done on this on different devices?
> > >>>>>>
> > >>>>>
> > >>>>> As mentioned above, these changes are currently present on end user
> > >>>>> devices for the past few years, so its been through a lot of testing :).
> > >>>>
> > >>>> all with the same gadget driver. Also, who uses USB on android devices
> > >>>> these days? Most of the data transfer goes via WiFi or Bluetooth, anyway
> > >>>> :-)
> > >>>>
> > >>>> I guess only developers are using USB during development to flash dev
> > >>>> images heh.
> > >>>>
> > >>>
> > >>> I used to be a customer facing engineer, so honestly I did see some
> > >>> really interesting and crazy designs.  Again, we do have non-Android
> > >>> products that use the same code, and it has been working in there for a
> > >>> few years as well.  The TXFIFO sizing really has helped with multimedia
> > >>> use cases, which use isoc endpoints, since esp. in those lower end CPU
> > >>> chips where latencies across the system are much larger, and a missed
> > >>> ISOC interval leads to a pop in your ear.
> > >>
> > >> This is good background information. Thanks for bringing this
> > >> up. Admitedly, we still have ISOC issues with dwc3. I'm interested in
> > >> knowing if a deeper request queue would also help here.
> > >>
> > >> Remember dwc3 can accomodate 255 requests + link for each endpoint. If
> > >> our gadget driver uses a low number of requests, we're never really
> > >> using the TRB ring in our benefit.
> > >>
> > >
> > > We're actually using both a deeper USB request queue + TX fifo resizing. :).
> >
> > okay, great. Let's see what John and/or Thinh respond WRT dwc3 TX Burst
> > behavior.
>
> --
> heikki



-- 
With Best Regards,
Andy Shevchenko