linux-kernel - Re: [PATCH v1 0/3] usb: dwc3: Avoid using reserved EPs

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Message-ID: <20250121234313.2xiixrqru5m35dyp@synopsys.com>
Date: Tue, 21 Jan 2025 23:43:21 +0000
From: Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
CC: Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Felipe Balbi <balbi@...nel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>, Ferry Toth <fntoth@...il.com>
Subject: Re: [PATCH v1 0/3] usb: dwc3: Avoid using reserved EPs

On Mon, Jan 20, 2025, Andy Shevchenko wrote:
> On Fri, Jan 17, 2025 at 03:38:46PM +0200, Andy Shevchenko wrote:
> > On Thu, Jan 16, 2025 at 11:18:45PM +0000, Thinh Nguyen wrote:
> > > On Thu, Jan 16, 2025, Andy Shevchenko wrote:
> > > > On some platforms (Intel-based and AFAIK ARM-based) the EPs in the gadget
> > > > (USB Device Controller mode) may be reserved for some special means, such as
> > > > tracing. This series extends DT schema and driver to avoid using those.
> > > > Without this the USB gadget mode won't work properly (those devices that
> > > > "luckily" allocated the reserved EPs).
> > > > 
> > > > Ferry already tested the privately sent PoC of this, but I ask him again to
> > > > re-test this version which is slightly different.
> 
> ...
> 
> > > I'm not entirely clear on the reason for this change yet.
> > > 
> > > How would this even work without dwc3 managing these endpoints (all the
> > > init/teardown/fifo allocation/start/stop flow).
> > 
> > You perhaps know much better how it can be done, I have access to a limited
> > documentation and in practice if those endpoints are not skipped any gadget
> > that allocates them simply won't work, and IIRC the entire USB transfers are
> > stuck.
> > 
> > > Can you provide more info on this hardware?
> > 
> > I am afraid I can't provide more, sorry. I can look for some specifics,
> > but I'm not that guy who know anything about in-SoC tracing.
> 
> So, here is what I found:
> 
> ---8<---
> 
> However the endpoints allocated for STM and EXI debug traffic cannot be re-allocated
> if being used because the sideband flow control signals are hard wired to certain
> endpoints:
> • 1 High BW Bulk IN (IN#1) (RTIT)
> • 1 1KB BW Bulk IN (IN#8) + 1 1KB BW Bulk OUT (Run Control) (OUT#8)
> 
> In device mode, since RTIT (EP#1) and EXI/RunControl (EP#8) uses External Buffer
> Control (EBC) mode, these endpoints are to be mapped to EBC mode (to be done by
> EXI target driver). Additionally TRB for RTIT and EXI are maintained in STM (System
> Trace Module) unit and the EXI target driver will as well configure the TRB location for
> EP #1 IN and EP#8 (IN and OUT). Since STM/PTI and EXI hardware blocks manage
> these endpoints and interface to OTG3 controller through EBC interface, there is no
> need to enable any events (such as XferComplete etc) for these end points.
> 
> Does it help you to understand the required quirk better?
> 

Thanks for looking up the info. This makes more sense now. So these
endpoints use EBC. Can you also provide this info to the commit?

BR,
Thinh