linux-kernel - Re: [PATCH] counter: stm32-timer-cnt: fix ceiling write max value

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Message-ID: <e54d1446-b583-9625-1ab3-09e54d6a7456@foss.st.com>
Date:   Tue, 2 Mar 2021 18:03:25 +0100
From:   Fabrice Gasnier <fabrice.gasnier@...s.st.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
CC:     <jic23@...nel.org>, <david@...hnology.com>,
        <alexandre.torgue@...s.st.com>, <mcoquelin.stm32@...il.com>,
        <olivier.moysan@...s.st.com>, <linux-iio@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] counter: stm32-timer-cnt: fix ceiling write max value

On 3/2/21 3:56 PM, William Breathitt Gray wrote:
> On Tue, Mar 02, 2021 at 03:43:55PM +0100, Fabrice Gasnier wrote:
>> The ceiling value isn't checked before writing it into registers. The user
>> could write a value higher than the counter resolution (e.g. 16 or 32 bits
>> indicated by max_arr). This makes most significant bits to be truncated.
>> Fix it by checking the max_arr to report a range error [1] to the user.
>>
>> Fixes: ad29937e206f ("counter: Add STM32 Timer quadrature encoder")
>>
>> [1] https://lkml.org/lkml/2021/2/12/358
>>
>> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@...s.st.com>
> 
> Acked-by: William Breathitt Gray <vilhelm.gray@...il.com>
> 
> Side question: if priv->ceiling is tracking the current ceiling
> configuration, would it make sense to change stm32_count_ceiling_read()
> to print the value of priv->ceiling instead of doing a regmap_read()
> call?

Hi William,

Thanks for reviewing.

I'd be fine either way. So no objection to move to the priv->ceiling
(cached) value. It could also here here.
By looking at this, I figured out there's probably another thing to fix
here, for initial conditions.

At probe time priv->ceiling is initialized to max value (ex 65535 for a
16 bits counter). But the register content is 0 (clear by mfd driver at
probe time).

- So, reading ceiling from sysfs currently reports 0 (regmap_read())
after booting and probing.

I see two cases at this point:
- In case the counter gets enabled without any prior configuration, it
won't count: ceiling value (e.g. 65535) should be written to register
before it is enabled, so the counter will actually count. So there's
room for a fix here.

- In case function gets set (ex: quadrature x4), priv->ceiling (e.g.
65535) gets written to the register (although it's been read earlier as
0 from sysfs).
This could be fixed by reading the priv->ceiling in
stm32_count_ceiling_read() as you're asking (provided 1st case has been
fixed as well)

I'll probably prepare one or two patches for the above cases, if you agree ?

Best Regards,
Fabrice

> 
>> ---
>>  drivers/counter/stm32-timer-cnt.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/drivers/counter/stm32-timer-cnt.c b/drivers/counter/stm32-timer-cnt.c
>> index ef2a974..2cf0c05 100644
>> --- a/drivers/counter/stm32-timer-cnt.c
>> +++ b/drivers/counter/stm32-timer-cnt.c
>> @@ -32,6 +32,7 @@ struct stm32_timer_cnt {
>>  	struct regmap *regmap;
>>  	struct clk *clk;
>>  	u32 ceiling;
>> +	u32 max_arr;
>>  	bool enabled;
>>  	struct stm32_timer_regs bak;
>>  };
>> @@ -185,6 +186,9 @@ static ssize_t stm32_count_ceiling_write(struct counter_device *counter,
>>  	if (ret)
>>  		return ret;
>>  
>> +	if (ceiling > priv->max_arr)
>> +		return -ERANGE;
>> +
>>  	/* TIMx_ARR register shouldn't be buffered (ARPE=0) */
>>  	regmap_update_bits(priv->regmap, TIM_CR1, TIM_CR1_ARPE, 0);
>>  	regmap_write(priv->regmap, TIM_ARR, ceiling);
>> @@ -360,6 +364,7 @@ static int stm32_timer_cnt_probe(struct platform_device *pdev)
>>  	priv->regmap = ddata->regmap;
>>  	priv->clk = ddata->clk;
>>  	priv->ceiling = ddata->max_arr;
>> +	priv->max_arr = ddata->max_arr;
>>  
>>  	priv->counter.name = dev_name(dev);
>>  	priv->counter.parent = dev;
>> -- 
>> 2.7.4
>>