Message-ID: <9dd40847-a2c7-d30c-0af4-07ff606a61f8@roeck-us.net>
Date:   Fri, 10 Apr 2020 09:21:35 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Sergey Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] watchdog: dw_wdt: Support devices with non-fixed TOP
 values

On 4/10/20 5:59 AM, Sergey Semin wrote:
> On Sun, Mar 15, 2020 at 07:12:38AM -0700, Guenter Roeck wrote:
>> On Fri, Mar 06, 2020 at 04:27:44PM +0300, Sergey.Semin@...kalelectronics.ru wrote:
>>> From: Serge Semin <Sergey.Semin@...kalelectronics.ru>
>>>
>>> In case if the DW Watchdog IP core is synthesised with
>>> WDT_USE_FIX_TOP == false, the TOP interval indexes make the device
>>> to load a custom periods to the counter. These periods are hardwired
>>> at the synthesis stage and can be within [2^8, 2^(WDT_CNT_WIDTH - 1)].
>>> Alas their values can't be detected at runtime and must be somehow
>>> supplied to the driver so one could properly determine the watchdog
>>> timeout intervals. For this purpose we suggest to have a vendor-
>>> specific dts property "snps,watchdog-tops" utilized, which would
>>> provide an array of sixteen counter values. At device probe stage they
>>> will be used to initialize the watchdog device timeouts determined
>>> from the array values and current clocks source rate.
>>>
>>> In order to have custom TOP values supported the driver must be
>>> altered in the following way. First of all the fixed-top values
>>> ready-to-use array must be determined for compatibility with currently
>>> supported devices, which were synthesised with WDT_USE_FIX_TOP == true.
>>> Secondly we must redefine the timer period search functions. For
>>> generality they are redesigned in a way to support the TOP array with
>>> no limitations on the items order or value. Finally an array with
>>> pre-defined timeouts must be calculated at probe stage from either
>>> the custom or fixed TOP values depending on the DW watchdog component
>>> parameter WDT_USE_FIX_TOP value.
>>>
>>> Signed-off-by: Serge Semin <Sergey.Semin@...kalelectronics.ru>
>>> Signed-off-by: Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>
>>> Cc: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
>>> Cc: Paul Burton <paulburton@...nel.org>
>>> Cc: Ralf Baechle <ralf@...ux-mips.org>
>>> ---
>>>  drivers/watchdog/dw_wdt.c | 145 +++++++++++++++++++++++++++++++-------
>>>  1 file changed, 119 insertions(+), 26 deletions(-)
>>>
>>> diff --git a/drivers/watchdog/dw_wdt.c b/drivers/watchdog/dw_wdt.c
>>> index fba21de2bbad..4a57b7d777dc 100644
>>> --- a/drivers/watchdog/dw_wdt.c
>>> +++ b/drivers/watchdog/dw_wdt.c
>>> @@ -13,6 +13,7 @@
>>>   */
>>>  
>>>  #include <linux/bitops.h>
>>> +#include <linux/limits.h>
>>>  #include <linux/clk.h>
>>>  #include <linux/delay.h>
>>>  #include <linux/err.h>
>>> @@ -34,12 +35,24 @@
>>>  #define WDOG_CURRENT_COUNT_REG_OFFSET	    0x08
>>>  #define WDOG_COUNTER_RESTART_REG_OFFSET     0x0c
>>>  #define WDOG_COUNTER_RESTART_KICK_VALUE	    0x76
>>> +#define WDOG_COMP_PARAMS_1_REG_OFFSET       0xf4
>>> +#define WDOG_COMP_PARAMS_1_USE_FIX_TOP      BIT(6)
>>>  
>>> -/* The maximum TOP (timeout period) value that can be set in the watchdog. */
>>> -#define DW_WDT_MAX_TOP		15
>>> +/* There are sixteen TOPs (timeout periods) that can be set in the watchdog. */
>>> +#define DW_WDT_NUM_TOPS		16
>>> +#define DW_WDT_FIX_TOP(_idx)	(1U << (16 + _idx))
>>>  
>>>  #define DW_WDT_DEFAULT_SECONDS	30
>>>  
>>> +static const u32 dw_wdt_fix_tops[DW_WDT_NUM_TOPS] = {
>>> +	DW_WDT_FIX_TOP(0), DW_WDT_FIX_TOP(1), DW_WDT_FIX_TOP(2),
>>> +	DW_WDT_FIX_TOP(3), DW_WDT_FIX_TOP(4), DW_WDT_FIX_TOP(5),
>>> +	DW_WDT_FIX_TOP(6), DW_WDT_FIX_TOP(7), DW_WDT_FIX_TOP(8),
>>> +	DW_WDT_FIX_TOP(9), DW_WDT_FIX_TOP(10), DW_WDT_FIX_TOP(11),
>>> +	DW_WDT_FIX_TOP(12), DW_WDT_FIX_TOP(13), DW_WDT_FIX_TOP(14),
>>> +	DW_WDT_FIX_TOP(15)
>>> +};
>>> +
>>>  static bool nowayout = WATCHDOG_NOWAYOUT;
>>>  module_param(nowayout, bool, 0);
>>>  MODULE_PARM_DESC(nowayout, "Watchdog cannot be stopped once started "
>>> @@ -49,6 +62,8 @@ struct dw_wdt {
>>>  	void __iomem		*regs;
>>>  	struct clk		*clk;
>>>  	unsigned long		rate;
>>> +	unsigned int		max_top;
>>> +	unsigned int		timeouts[DW_WDT_NUM_TOPS];
>>>  	struct watchdog_device	wdd;
>>>  	struct reset_control	*rst;
>>>  	/* Save/restore */
>>> @@ -64,20 +79,68 @@ static inline int dw_wdt_is_enabled(struct dw_wdt *dw_wdt)
>>>  		WDOG_CONTROL_REG_WDT_EN_MASK;
>>>  }
>>>  
>>> -static inline int dw_wdt_top_in_seconds(struct dw_wdt *dw_wdt, unsigned top)
>>> +static unsigned int dw_wdt_find_best_top(struct dw_wdt *dw_wdt,
>>> +					 unsigned int timeout, u32 *top)
>>>  {
>>> +	u32 diff = UINT_MAX, tmp;
>>> +	int idx;
>>> +
>>>  	/*
>>> -	 * There are 16 possible timeout values in 0..15 where the number of
>>> -	 * cycles is 2 ^ (16 + i) and the watchdog counts down.
>>> +	 * In general case of non-fixed timeout values they can be arranged in
>>> +	 * any order so we have to traverse all the array values. We also try
>>> +	 * to find a closest timeout number and make sure its value is greater
>>> +	 * than the requested timeout. Note we'll return a maximum timeout
>>> +	 * if reachable value couldn't be found.
>>>  	 */
>>> -	return (1U << (16 + top)) / dw_wdt->rate;
>>> +	for (*top = dw_wdt->max_top, idx = 0; idx < DW_WDT_NUM_TOPS; ++idx) {
>>> +		if (dw_wdt->timeouts[idx] < timeout)
>>> +			continue;
>>> +
>>> +		tmp = dw_wdt->timeouts[idx] - timeout;
>>> +		if (tmp < diff) {
>>> +			diff = tmp;
>>> +			*top = idx;
>>> +		}
>>> +	}
>>> +
>>> +	return dw_wdt->timeouts[*top];
>>> +}
>>> +
>>> +static unsigned int dw_wdt_find_min_timeout(struct dw_wdt *dw_wdt)
>>
>> I would appreciate if the names of functions returning ms end with _ms
>> to avoid confusion.
> 
> Ok. I'll also modify the functions a bit, so only the
> dw_wdt_find_best_top_ms() and dw_wdt_find_max_top_ms() methods would
> return timeouts in milliseconds. Though if you insist in keeping seconds
> in the timeouts array (see the comment after the next one), it'll be
> dw_wdt_find_max_top_ms() only.
> 
>>
>>> +{
>>> +	u32 min_timeout = UINT_MAX, top;
>>> +	int idx;
>>> +
>>> +	for (top = 0, idx = 0; idx < DW_WDT_NUM_TOPS; ++idx) {
>>> +		if (dw_wdt->timeouts[idx] <= min_timeout) {
>>> +			min_timeout = dw_wdt->timeouts[idx];
>>> +			top = idx;
>>> +		}
>>> +	}
>>> +
>>> +	return dw_wdt->timeouts[top];
>>> +}
>>> +
>>> +static unsigned int dw_wdt_find_max_top(struct dw_wdt *dw_wdt, u32 *top)
>>> +{
>>> +	u32 max_timeout = 0;
>>> +	int idx;
>>> +
>>> +	for (*top = 0, idx = 0; idx < DW_WDT_NUM_TOPS; ++idx) {
>>> +		if (dw_wdt->timeouts[idx] >= max_timeout) {
>>> +			max_timeout = dw_wdt->timeouts[idx];
>>> +			*top = idx;
>>> +		}
>>> +	}
>>> +
>>> +	return dw_wdt->timeouts[*top];
>>>  }
>>>  
>>> -static int dw_wdt_get_top(struct dw_wdt *dw_wdt)
>>> +static unsigned int dw_wdt_get_timeout(struct dw_wdt *dw_wdt)
>>>  {
>>>  	int top = readl(dw_wdt->regs + WDOG_TIMEOUT_RANGE_REG_OFFSET) & 0xF;
>>>  
>>> -	return dw_wdt_top_in_seconds(dw_wdt, top);
>>> +	return dw_wdt->timeouts[top];
>>>  }
>>>  
>>>  static int dw_wdt_ping(struct watchdog_device *wdd)
>>> @@ -90,20 +153,13 @@ static int dw_wdt_ping(struct watchdog_device *wdd)
>>>  	return 0;
>>>  }
>>>  
>>> -static int dw_wdt_set_timeout(struct watchdog_device *wdd, unsigned int top_s)
>>> +static int dw_wdt_set_timeout(struct watchdog_device *wdd, unsigned int req)
>>>  {
>>>  	struct dw_wdt *dw_wdt = to_dw_wdt(wdd);
>>> -	int i, top_val = DW_WDT_MAX_TOP;
>>> +	unsigned int timeout;
>>> +	u32 top;
>>>  
>>> -	/*
>>> -	 * Iterate over the timeout values until we find the closest match. We
>>> -	 * always look for >=.
>>> -	 */
>>> -	for (i = 0; i <= DW_WDT_MAX_TOP; ++i)
>>> -		if (dw_wdt_top_in_seconds(dw_wdt, i) >= top_s) {
>>> -			top_val = i;
>>> -			break;
>>> -		}
>>> +	timeout = dw_wdt_find_best_top(dw_wdt, req * MSEC_PER_SEC, &top);
>>>  
>>>  	/*
>>>  	 * Set the new value in the watchdog.  Some versions of dw_wdt
>>> @@ -111,7 +167,7 @@ static int dw_wdt_set_timeout(struct watchdog_device *wdd, unsigned int top_s)
>>>  	 * CP_WDT_DUAL_TOP in WDT_COMP_PARAMS_1).  On those we
>>>  	 * effectively get a pat of the watchdog right here.
>>>  	 */
>>> -	writel(top_val | top_val << WDOG_TIMEOUT_RANGE_TOPINIT_SHIFT,
>>> +	writel(top | top << WDOG_TIMEOUT_RANGE_TOPINIT_SHIFT,
>>>  	       dw_wdt->regs + WDOG_TIMEOUT_RANGE_REG_OFFSET);
>>>  
>>>  	/*
>>> @@ -119,10 +175,10 @@ static int dw_wdt_set_timeout(struct watchdog_device *wdd, unsigned int top_s)
>>>  	 * kernel(watchdog_dev.c) helps to feed watchdog before
>>>  	 * wdd->max_hw_heartbeat_ms
>>>  	 */
>>> -	if (top_s * 1000 <= wdd->max_hw_heartbeat_ms)
>>> -		wdd->timeout = dw_wdt_top_in_seconds(dw_wdt, top_val);
>>> +	if (req * MSEC_PER_SEC > wdd->max_hw_heartbeat_ms)
>>> +		wdd->timeout = req;
>>>  	else
>>> -		wdd->timeout = top_s;
>>> +		wdd->timeout = timeout / MSEC_PER_SEC;
>>>  
>>>  	return 0;
>>>  }
>>> @@ -238,6 +294,41 @@ static int dw_wdt_resume(struct device *dev)
>>>  
>>>  static SIMPLE_DEV_PM_OPS(dw_wdt_pm_ops, dw_wdt_suspend, dw_wdt_resume);
>>>  
>>> +static void dw_wdt_init_timeouts(struct dw_wdt *dw_wdt, struct device *dev)
>>> +{
>>> +	u32 data, of_tops[DW_WDT_NUM_TOPS];
>>> +	const u32 *tops;
>>> +	int ret, idx;
>>> +
>>> +	/*
>>> +	 * Retrieve custom or fixed counter values depending on the
>>> +	 * WDT_USE_FIX_TOP flag found in the component specific parameters
>>> +	 * #1 register.
>>> +	 */
>>> +	data = readl(dw_wdt->regs + WDOG_COMP_PARAMS_1_REG_OFFSET);
>>> +	if (data & WDOG_COMP_PARAMS_1_USE_FIX_TOP) {
>>> +		tops = dw_wdt_fix_tops;
>>> +	} else {
>>> +		ret = of_property_read_variable_u32_array(dev_of_node(dev),
>>> +			"snps,watchdog-tops", of_tops, DW_WDT_NUM_TOPS,
>>> +			DW_WDT_NUM_TOPS);
>>> +		if (ret < 0) {
>>> +			dev_warn(dev, "No valid TOPs array specified\n");
>>> +			tops = dw_wdt_fix_tops;
>>> +		} else {
>>> +			tops = of_tops;
>>> +		}
>>> +	}
>>> +
>>> +	/*
>>> +	 * We'll keep the timeout values in ms to approximate requested
>>> +	 * timeouts with better accuracy.
>>> +	 */
>>> +	for (idx = 0; idx < DW_WDT_NUM_TOPS; ++idx)
>>> +		dw_wdt->timeouts[idx] =
>>> +			mult_frac(tops[idx], MSEC_PER_SEC, dw_wdt->rate);
>>
>> tops[idx] type is u32. Its value can be up to 0xffffffff. That means
>> dw_wdt->rate must be >= 1000 to avoid overflow, which you should check.
> 
> Right. I don't think that TOPs with timeouts bigger than
> 0xffffffff milliseconds have any valuable usecases, so I'll just round
> the overflows down to FFs.
> 

Neither do unsorted random timeouts milli-seconds apart. You see the need
to address one, so addressing other weaknesses is appropriate.

>>
>> Note that I don't see the point of keeping the timeout values in ms.
>> The code selects a larger value (in seconds) anyway. All this does is
>> to add a lot of multiply and divide operations, plus a source of bugs
>> and confusion, for little if any gain.
> 
> As I said in the comment to the code the idea of keeping values in ms was
> to better approximate the requested timeouts. This is necessary since
> unlike the fixed TOPs case in which each next timeout is doubled with
> respect to a previous one (65536, 131072, etc), the unfixed TOPs set may
> have any values within [2^8; (2^WDT_CNT_WIDTH - 1)]. Actual numerical
> values of the set are defined by a SoC engineer at the moment of the
> IP-core synthesis. So in general they can be unordered and can differ one
> from another in very small time deltas, like ms and even us. It depends
> on the ways the watchdog was supposed to be utilized in accordance with
> the system requirements and the reference clock rate. In this case how to
> distinguish the values if we had only seconds array? In this patch I
> suggest an approach to at least cover the case of the TOPs with
> milliseconds granularity.
> 
> I don't deny that this might be not that much gain seeing the watchdog
> core supports the timeouts in seconds only, but at least it provides a
> way to distinguish one TOP from another instead of picking a first found
> one. As I see it there aren't that much multiplication and division
> caused by such solution and it's a small prices with respect to an
> ability to find a better timeout approximation. A few tens nsecs of
> the code execution is much smaller than milliseconds accuracy of the
> watchdog timeout. Though OS-wise such accuracy might be redundant.
> 
> Regarding bugs and confusion. Well I find confusing a numerical literals
> usage while there are self-documented macros in the kernel available,
> which would better represent the code context and would point out what
> they are used for.) That's why I've sent the previous patch in the first
> place. But as I said in the response to your review comments there, while
> it doesn't contradict to the kernel requirements with ceteris paribus
> it's up to you which approach to choose since you are the subsystem
> maintainer and it will be your duty to continue the code maintenance in
> future. The same thing is here. It's up to you what approach to choose.
> So if my reasoning above didn't make you to change your mind, could you
> please explicitly respond to this message that you'd better see the
> timeouts array having seconds instead of milliseconds?
> 
Please include a summary of the above in the driver to explain the need
for the complexity for others in the future.

Guenter

>>
>>> +}
>>> +
>>>  static int dw_wdt_drv_probe(struct platform_device *pdev)
>>>  {
>>>  	struct device *dev = &pdev->dev;
>>> @@ -275,12 +366,14 @@ static int dw_wdt_drv_probe(struct platform_device *pdev)
>>>  
>>>  	reset_control_deassert(dw_wdt->rst);
>>>  
>>> +	dw_wdt_init_timeouts(dw_wdt, dev);
>>> +
>>>  	wdd = &dw_wdt->wdd;
>>>  	wdd->info = &dw_wdt_ident;
>>>  	wdd->ops = &dw_wdt_ops;
>>> -	wdd->min_timeout = 1;
>>> +	wdd->min_timeout = dw_wdt_find_min_timeout(dw_wdt) / MSEC_PER_SEC;
>>
>> dw_wdt_find_min_timeout can return a value < 1000. In that case min_timeout
>> would be 0, ie unspecified.
> 
> Ok. I'll implement this limitation in the dw_wdt_find_min_timeout()
> method. It will return seconds in v2.
> 
> Regards,
> -Sergey
> 
>>
>>>  	wdd->max_hw_heartbeat_ms =
>>> -		dw_wdt_top_in_seconds(dw_wdt, DW_WDT_MAX_TOP) * 1000;
>>> +		dw_wdt_find_max_top(dw_wdt, &dw_wdt->max_top);
>>>  	wdd->parent = dev;
>>>  
>>>  	watchdog_set_drvdata(wdd, dw_wdt);
>>> @@ -293,7 +386,7 @@ static int dw_wdt_drv_probe(struct platform_device *pdev)
>>>  	 * devicetree.
>>>  	 */
>>>  	if (dw_wdt_is_enabled(dw_wdt)) {
>>> -		wdd->timeout = dw_wdt_get_top(dw_wdt);
>>> +		wdd->timeout = dw_wdt_get_timeout(dw_wdt) / MSEC_PER_SEC;
>>>  		set_bit(WDOG_HW_RUNNING, &wdd->status);
>>>  	} else {
>>>  		wdd->timeout = DW_WDT_DEFAULT_SECONDS;

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