linux-kernel - Re: [PATCH 1/3] usb: dwc3: ulpi: Use VStsDone to detect PHY regs access completion

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Date:   Tue, 27 Oct 2020 23:13:15 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Felipe Balbi <balbi@...nel.org>
CC:     Serge Semin <fancer.lancer@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        John Youn <John.Youn@...opsys.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] usb: dwc3: ulpi: Use VStsDone to detect PHY regs
 access completion

On Tue, Oct 27, 2020 at 11:15:24AM +0200, Felipe Balbi wrote:
> 
> Hi,
> 
> Serge Semin <Sergey.Semin@...kalelectronics.ru> writes:
> 
> > In accordance with [1] the DWC_usb3 core sets the GUSB2PHYACCn.VStsDone
> > bit when the PHY vendor control access is done and clears it when the
> > application initiates a new transaction. The doc doesn't say anything
> > about the GUSB2PHYACCn.VStsBsy flag serving for the same purpose. Moreover
> > we've discovered that the VStsBsy flag can be cleared before the VStsDone
> > bit. So using the former as a signal of the PHY control registers
> > completion might be dangerous. Let's have the VStsDone flag utilized
> > instead then.
> >
> > [1] Synopsys DesignWare Cores SuperSpeed USB 3.0 xHCI Host Controller
> >     Databook, 2.70a, December 2013, p.388
> >
> > Signed-off-by: Serge Semin <Sergey.Semin@...kalelectronics.ru>
> > ---
> >  drivers/usb/dwc3/core.h | 1 +
> >  drivers/usb/dwc3/ulpi.c | 2 +-
> >  2 files changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/usb/dwc3/core.h b/drivers/usb/dwc3/core.h
> > index 2f04b3e42bf1..8d5e5bba1bc2 100644
> > --- a/drivers/usb/dwc3/core.h
> > +++ b/drivers/usb/dwc3/core.h
> > @@ -284,6 +284,7 @@
> >  
> >  /* Global USB2 PHY Vendor Control Register */
> >  #define DWC3_GUSB2PHYACC_NEWREGREQ	BIT(25)
> > +#define DWC3_GUSB2PHYACC_DONE		BIT(24)
> >  #define DWC3_GUSB2PHYACC_BUSY		BIT(23)
> >  #define DWC3_GUSB2PHYACC_WRITE		BIT(22)
> >  #define DWC3_GUSB2PHYACC_ADDR(n)	(n << 16)
> > diff --git a/drivers/usb/dwc3/ulpi.c b/drivers/usb/dwc3/ulpi.c
> > index e6e6176386a4..20f5d9aba317 100644
> > --- a/drivers/usb/dwc3/ulpi.c
> > +++ b/drivers/usb/dwc3/ulpi.c
> > @@ -24,7 +24,7 @@ static int dwc3_ulpi_busyloop(struct dwc3 *dwc)
> >  
> >  	while (count--) {
> >  		reg = dwc3_readl(dwc->regs, DWC3_GUSB2PHYACC(0));
> > -		if (!(reg & DWC3_GUSB2PHYACC_BUSY))
> > +		if (reg & DWC3_GUSB2PHYACC_DONE)
> 

> are you sure this works in all supported versions of the core?

I can't be sure about that since I've got only the 2.70a version of the
core. But as I said in the patch log it was a bit incorrect to use the
BUSY flag here in the first place. So if there is no IP core peculiarity
here which had been workarounded by polling the BUSY-flag, then I'd stick
with the DONE-flag in the busy-loop. In the former case I'd suggest to add
a useful comment why the BUSY-flag is required to be polled though...

-Sergey

> 
> John, could you confirm this for us?
> 
> thanks
> 
> -- 
> balbi