Messages in this thread | | | From | Martin Blumenstingl <> | Date | Sat, 24 Feb 2024 23:13:58 +0100 | Subject | Re: [PATCH v2] clocksource/drivers/arm_global_timer: Simplify prescaler register access |
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On Sat, Feb 24, 2024 at 10:55 PM Daniel Lezcano <daniel.lezcano@linaro.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.
Best regards, Martin
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