linux-kernel - Re: [PATCH v2] clocksource/drivers/arm_global_timer: Simplify prescaler register access

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Message-ID: <5d3b6c7c-b526-45e0-95fd-deedad2411f6@linaro.org>
Date: Sun, 25 Feb 2024 13:37:18 +0100
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc: tglx@...utronix.de, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, patrice.chotard@...s.st.com,
 linux-amlogic@...ts.infradead.org
Subject: Re: [PATCH v2] clocksource/drivers/arm_global_timer: Simplify
 prescaler register access

On 24/02/2024 23:13, Martin Blumenstingl wrote:
> On Sat, Feb 24, 2024 at 10:55 PM Daniel Lezcano
> <daniel.lezcano@...aro.org> wrote:
> [...]
>>> @@ -301,7 +298,7 @@ static int gt_clk_rate_change_cb(struct notifier_block *nb,
>>>                psv--;
>>>
>>>                /* prescaler within legal range? */
>>> -             if (psv < 0 || psv > GT_CONTROL_PRESCALER_MAX)
>>> +             if (psv < 0 || !FIELD_FIT(GT_CONTROL_PRESCALER_MASK, psv))
>>>                        return NOTIFY_BAD;
>>
>> Won't FIELD_FIT cover psv < 0 also ?
> Hmm, I wanted to reply that it doesn't because internally FIELD_FIT()
> uses a cast:
> ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask)
> My original thought was that the cast would clear the sign bit when in
> fact (I think) it will not - it will result in the signed number and
> BIT(31) set.
> So I think you're right, FIELD_FIT() does cover it.
> 
> However, there's something else odd with this code:
> We're dividing two frequencies (using DIV_ROUND_CLOSEST) which are two
> unsigned values. So the result of the division can never be negative:
>    psv = DIV_ROUND_CLOSEST(ndata->new_rate, gt_target_rate);
> However, we're additionally decrementing psv by one:
>    psv--;
> So in reality it can only ever be negative if the result of the
> division was zero (for example if new_rate is smaller than
> gt_target_rate).
> However, in that case we would have crashed - with a division by zero
> - in the statement right in the middle of the two mentioned above:
>    if (abs(gt_target_rate - (ndata->new_rate / psv)) > MAX_F_ERR)
> 
> So I think we need another patch (it's best to order that before this
> one): make psv an unsigned int and error out before trying to divide
> by zero.
> If you have any objections: let me know, otherwise I'll prepare a
> patch tomorrow.

I think it makes perfectly sense, thanks for making the code nicer

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