-----Original Message----- From: Andy Fleming [mailto:afleming@gmail.com] Sent: Saturday, October 20, 2012 5:22 AM To: Marek Vasut Cc: Liu Shengzhou-B36685; u-boot@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@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.