RE: [PATCH] soc: qcom: stats: Fix division issue on 32-bit platforms

[David Laight <David.Laight@ACULAB.COM>]

From: Bjorn Andersson
> Sent: 06 December 2023 00:44
> 
> commit 'e84e61bdb97c ("soc: qcom: stats: Add DDR sleep stats")' made it
> in with a mult_frac() which causes link errors on Arm and PowerPC
> builds:
> 
>   ERROR: modpost: "__aeabi_uldivmod" [drivers/soc/qcom/qcom_stats.ko] undefined!
> 
> Expand the mult_frac() to avoid this problem.
> 
> Fixes: e84e61bdb97c ("soc: qcom: stats: Add DDR sleep stats")
> Reported-by: Randy Dunlap <rdunlap@infradead.org>
> Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
> ---
>  drivers/soc/qcom/qcom_stats.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/soc/qcom/qcom_stats.c b/drivers/soc/qcom/qcom_stats.c
> index 4763d62a8cb0..5ba61232313e 100644
> --- a/drivers/soc/qcom/qcom_stats.c
> +++ b/drivers/soc/qcom/qcom_stats.c
> @@ -221,7 +221,8 @@ static int qcom_ddr_stats_show(struct seq_file *s, void *unused)
> 
>  	for (i = 0; i < ddr.entry_count; i++) {
>  		/* Convert the period to ms */
> -		entry[i].dur = mult_frac(MSEC_PER_SEC, entry[i].dur, ARCH_TIMER_FREQ);
> +		entry[i].dur *= MSEC_PER_SEC;
> +		entry[i].dur = div_u64(entry[i].dur, ARCH_TIMER_FREQ);

Is that right?
At a guess mult_frac(a, b, c) is doing a 32x32 multiply and then a 64x32
divide to generate a 32bit result.
So I'd guess entry[i].dur is 32bit? (this code isn't in -rc4 ...).
Which means you are now discarding the high bits.

You've also added a very slow 64bit divide.
A multiple by reciprocal calculation will be much better.
Since absolute accuracy almost certainly doesn't matter here convert:
	dur * 1000 / FREQ
to
	(dur * (u32)(1000ull << 32 / FREQ)) >> 32
which will be fine provided FREQ >= 1000

	David

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