Subject: [PATCHv7 8/8] watchdog: omap_wdt: Convert to use new core extensions

From: [email-masked] (Timo Kokkonen)
Date: Mon, 04 May 2015 13:06:20 +0300

On 04.05.2015 10:04, Uwe Kleine-K?nig wrote:
> Hello Timo,
>
> On Mon, May 04, 2015 at 08:59:03AM +0300, Timo Kokkonen wrote:
>> Hi, 03.05.2015 21:56, Uwe Kleine-K?nig wrote:
>>> On Wed, Apr 22, 2015 at 02:11:42PM +0300, Timo Kokkonen wrote:
>>>> +static int omap_wdt_is_running(struct omap_wdt_dev *wdev)
>>>> +{
>>>> +	void __iomem *base = wdev->base;
>>>> +
>>>> +	return readl_relaxed(base + OMAP_WATCHDOG_SPR) == 0x4444;
>>>> +}
>>> This isn't reliable. The sequence needed to enable the watchdog is
>>> 	writel(0xbbbb, base + OMAP_WATCHDOG_SPR);
>>> 	writel(0x4444, base + OMAP_WATCHDOG_SPR);
>>>
>>> The sequence to stop is:
>>> 	writel(0xaaaa, base + OMAP_WATCHDOG_SPR);
>>> 	writel(0x5555, base + OMAP_WATCHDOG_SPR);
>>>
>>> But:
>>>
>>> barebox at TI AM335x BeagleBone black:/ md 0x44e35048+4
>>> 44e35048: 00005555                                           UU..
>>> barebox at TI AM335x BeagleBone black:/ mw 0x44e35048 0x4444
>>> barebox at TI AM335x BeagleBone black:/ md 0x44e35048+4
>>> 44e35048: 00004444                                           DD..
>>>
>>> So the register contains 0x4444 but the timer doesn't run. So at best
>>> testing for 0x4444 is a good heuristic.
>>
>> Yeah.. I don't think we can get any better than that. Unless we
>> start checking the counter register and see whether it really counts
>> or not, and I think that's a bit overkill.. So I'd say we should be
>> safe when assuming bootloader is doing things correctly. Although,
>> we could add a comment to the code that the test may not be 100%
>> reliable in case the start sequence have not been issued properly.
>>
>> Thanks for pointing this out!
> It doesn't seem to much overhead to do:
>
> 	/*
> 	 * There is no register that tells us if the timer is running,
> 	 * so we have to resort to sample twice. The minimal frequency
> 	 * is 256 Hz (32768 Hz prescaled with 2**7).
> 	 */
> 	counter1 = readl(base + OMAP_WATCHDOG_CCR);
> 	mdelay(4);
> 	counter2 = readl(base + OMAP_WATCHDOG_CCR);
> 	return counter1 != counter2;
>
> I'd say it's even worth to do:
>
> 	cntrl = readl(base + OMAP_WATCHDOG_CNTRL);
> 	if (cntrl & (1 << 5))
> 		shift = (cntrl >> 2) & 0x7;
> 	else
> 		shift = 0;
> 	counter1 = readl(base + OMAP_WATCHDOG_CCR);
> 	udelay(31 << shift);
> 	counter2 = readl(base + OMAP_WATCHDOG_CCR);
> 	return counter1 != counter2;
>

I think it is very rare that the watchdog is running in any other than 
32kHz clock, so the above should be sufficient in any case. The worst 
case delay of 3968us is a bit long, but even that is happening only 
during probe. And vast majority of user are going to have the 31us 
delay, so that should be fine.

Thanks,
-Timo

> For some bonus points add some defines for the magic constants.
>
> This is save as the OMAP_WATCHDOG_CNTRL doesn't seem to accept reads
> while the counter is running. Maybe even this could be used to detect a
> running timer?:
>
>   - enable timer:
> 	barebox at TI AM335x BeagleBone black:/ mw 0x44e35048 0xbbbb
> 	barebox at TI AM335x BeagleBone black:/ mw 0x44e35048 0x4444
>
>   - read out WCLR
> 	barebox at TI AM335x BeagleBone black:/ md 0x44e35024+4
> 	44e35024: 00000020                                            ...
>
>   - write to WCLR
> 	barebox at TI AM335x BeagleBone black:/ mw 0x44e35024 0x0
>
>   - check result; didn't work
> 	barebox at TI AM335x BeagleBone black:/ md 0x44e35024+4
> 	44e35024: 00000020                                            ...
>
>   - stop timer
> 	barebox at TI AM335x BeagleBone black:/ mw 0x44e35048 0xaaaa
> 	barebox at TI AM335x BeagleBone black:/ mw 0x44e35048 0x5555
>
>   - recheck WCLR
> 	barebox at TI AM335x BeagleBone black:/ md 0x44e35024+4
> 	44e35024: 00000020                                            ...
>
>   - write to WCLR
> 	barebox at TI AM335x BeagleBone black:/ mw 0x44e35024 0x0
>
>   - check result; write succeeded
> 	barebox at TI AM335x BeagleBone black:/ md 0x44e35024+4
> 	44e35024: 00000000                                           ....
>
> (This is was tested on an AM335x.)
>
> Best regards
> Uwe
>