linux-kernel - Re: [PATCH v2 3/3] power: wm831x_power: Support USB charger current limit management

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Date:	Wed, 19 Aug 2015 08:02:37 +0800
From:	Peter Chen <peter.chen@...escale.com>
To:	Mark Brown <broonie@...nel.org>
CC:	Baolin Wang <baolin.wang@...aro.org>, <balbi@...com>,
	<linus.walleij@...aro.org>, <linux-kernel@...r.kernel.org>,
	<gregkh@...uxfoundation.org>, <sojka@...ica.cz>,
	<stern@...land.harvard.edu>, <r.baldyga@...sung.com>,
	<yoshihiro.shimoda.uh@...esas.com>, <linux-usb@...r.kernel.org>,
	<device-mainlining@...ts.linuxfoundation.org>, <sre@...nel.org>,
	<dbaryshkov@...il.com>, <dwmw2@...radead.org>,
	<sameo@...ux.intel.com>, <lee.jones@...aro.org>,
	<patches@...nsource.wolfsonmicro.com>, <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] power: wm831x_power: Support USB charger current
 limit management

On Tue, Aug 18, 2015 at 09:12:00AM -0700, Mark Brown wrote:
> On Tue, Aug 18, 2015 at 01:20:12PM +0800, Peter Chen wrote:
> 
> > ok, I just had suspected below function's correctness, after looking
> > it again, it always set 1800 as charging limit, does it be expected?
> 
> > +       /* Find the highest supported limit */
> > +       best = 0;
> > +       for (i = 0; i < ARRAY_SIZE(wm831x_usb_limits); i++) {
> > +               if (limit < wm831x_usb_limits[i] &&
> 

+/* In miliamps */
+static unsigned int wm831x_usb_limits[] = {
+       0,
+       2,
+       100,
+       500,
+       900,
+       1500,
+       1800,
+       550,
+};
+
+static int wm831x_usb_limit_change(struct notifier_block *nb,
+                                  unsigned long limit, void *data)
+{
+       struct wm831x_power *wm831x_power = container_of(nb,
+                                                        struct wm831x_power,
+                                                        usb_notify);
+       int i, best;
+
+       /* Find the highest supported limit */
+       best = 0;
+       for (i = 0; i < ARRAY_SIZE(wm831x_usb_limits); i++) {
+               if (limit < wm831x_usb_limits[i] &&
+                   wm831x_usb_limits[best] < wm831x_usb_limits[i])
+                       best = i;
+       }
+
+       dev_dbg(wm831x_power->wm831x->dev,
+               "Limiting USB current to %dmA", wm831x_usb_limits[best]);
+
+       wm831x_set_bits(wm831x_power->wm831x, WM831X_POWER_STATE,
+                       WM831X_USB_ILIM_MASK, best);
+
+       return 0;
+}
+

> The above check is intended to ensure that we don't go over the limit
> that was passed in in the callback.  The goal is to select the highest
> option that is less than the limit passed in.

Please correct me if I was wrong, If there is SDP (PC), the current
limit is 500mA, then when the i = 4, (500 < 900) && (0 < 900) is true,
the best is 4. When the i goes to 5, (500 < 1500) && (900 < 1500) is
true, the best is 5. At last, the best will be 1800, which is not our
expectation.

Below code may be correct for the goal you expressed.

       for (i = 0; i < ARRAY_SIZE(wm831x_usb_limits); i++) {
               if (limit >= wm831x_usb_limits[i] &&
                   wm831x_usb_limits[best] < wm831x_usb_limits[i])
                       best = i;
       }

-- 

Best Regards,
Peter Chen
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