[U-Boot] [PATCH] powerpc/usb: fix bug of CPU hang when missing USB PHY clock

[Liu Shengzhou-B36685]

> -----Original Message-----
> From: Andy Fleming [mailto:afleming at gmail.com]
> Sent: Saturday, October 20, 2012 5:22 AM
> To: Marek Vasut
> Cc: Liu Shengzhou-B36685; u-boot at lists.denx.de; Stefan Roese
> Subject: Re: [U-Boot] [PATCH] powerpc/usb: fix bug of CPU hang when
> missing USB PHY clock
> 
> On Thu, Oct 18, 2012 at 4:04 AM, Marek Vasut <marex at denx.de> wrote:
> > Dear Liu Shengzhou-B36685,
> 
> >> > > >
> >> > > > > +/* Check USB PHY clock valid */ static int
> >> > > > > +usb_phy_clk_valid(struct usb_ehci *ehci) {
> >> > > > > +     if ((!(in_be32(&ehci->control) & PHY_CLK_VALID)) &&
> >> > > > > +                     (!in_be32(&ehci->prictrl))) {
> >> > > >
> >> > > > (!A && !B) condition can certainly be done without the double
> >> > > > negation ;-)
> >> > >
> >> > > [Shengzhou] Yes, using !(A||B) is also okay:)
> >> >
> >> > Good, you did your logic homework well. Now go one step further:
> >> >
> >> > if (a || b)
> >> >
> >> >  return 1;
> >>
> >> [Shengzhou] No, this doesn't work, b is 0 at initial time, but b is 1
> >> at the second time, a is depend on the register PHY_CLK_VALID bit, We
> >> just want to check it at the first time and then think it is always
> >> valid after that, it's using a trick:)
> >
> > Good point, I was just testing you of course ;-)
> 
> 
> I may just be dim. Why is this a good point? If
> in_be32(&ehci->prictrl) is non-zero, then this function will return '1'.
> If (in_be32(&ehci->control) & PHY_CLK_VALID) is non-zero, this function
> will return 1.
> 
> What am I missing?
> 
> Andy
[Shengzhou] Because the indication of PHY_CLK_VALID is time-sensitive, we just think the value read at the first time is reliable, then PHY_CLK_VALID will be zero after that though actually PHY clock is still valid.