Re: Porting barebox (devicetree) to Variscite iMX6 SOM [USB FIX]

[Sascha Hauer <s.hauer@pengutronix.de>]

On Thu, Feb 06, 2014 at 10:24:43PM -0500, Michael D. Burkey wrote:
> Sascha,
> 
> Well, a bit of good news -- I know what the problem is, and it is
> probably one that has actually caused headaches for other people over
> the years (probably in the Linux kernel as well).
> 
> The problem is one that only shows up with a powered hub which uses
> external power switches to feed the ports. The hub reports back a
> PowerOn2PowerGood signal of 100ms. Unfortunately, standard MOSFET
> switches such as the MIC2076 add up to another 10ms of delay to the
> time it takes for the power lines to the ports to become stable.
> 
> This would also explain a few problems that I have seen people talking
> about online with BeagleBoards, etc under Linux where a USB device would
> work fine if hot-plugged later on, but would not work if it was plugged
> in at startup.
> 
> Also, looking at the more recent Linux kernels, they have hardcoded
> the value for the delay in usb_hub_power_on() to a minimum of 100ms,
> rather than just using the PwrOn2PwrGood value -- so it probably makes
> sense to do the same in barebox, except that we really should make the
> delay timing for both about 110ms or PwrOn2PwrGood+10ms.
> 
> I've (finally) gotten all my Variscite patches rolled together and
> merged on my end with all the DTS changes that got checked into the
> mainline over the last few days (you've been WAY too busy!!!) and I
> can post a real patch file including the USB update shortly.
> 
> That said, the change is to /drivers/usb/core/usb.c and the only
> function changed is:
> 
> static void usb_hub_power_on(struct usb_hub_device *hub)
> {
>         int i;
>         unsigned delay;
>         struct usb_device *dev;
> 
>         dev = hub->pusb_dev;
>         /* Enable power to the ports */
>         USB_HUB_PRINTF("enabling power on all ports\n");
>         for (i = 0; i < dev->maxchild; i++) {
>                 usb_set_port_feature(dev, i + 1, USB_PORT_FEAT_POWER);
>                 USB_HUB_PRINTF("port %d returns %lX\n", i + 1, dev->status);
>         }
> 
>         /* Wait at least 110 msec for power to become stable. */
>         /* PowerOn2PowerGood is encoded in 2ms increments -> times 2
> for the actual delay */
>         /* The additional 10ms is to allow for the turn-on/rise-time
> delay often induced  */
>         /* by MOSFET power distribution switches such as the Micrel
> MIC2076.              */
>         delay = max((unsigned)(hub->desc.bPwrOn2PwrGood*2), (unsigned) 100);
>         mdelay(delay+10);
> }
> 
> 
> This gets USB detection working properly on the Variscite -- and
> probably helps a few other boards too!

Indeed this function caused problems earlier, also on U-Boot. I thought
the way we do it is fine now. It might be worth running a git blame/git
log on this function in both barebox and U-Boot to get some background.

Sascha

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