linux-kernel - Re: [PATCH] hpet: Optimize local variable data type in hpet

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Message-Id: <fac1e788-f05e-4885-802c-ec22daeeeee5@app.fastmail.com>
Date: Thu, 11 Jul 2024 08:59:09 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Thorsten Blum" <thorsten.blum@...lux.com>,
 "Clemens Ladisch" <clemens@...isch.de>,
 "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hpet: Optimize local variable data type in hpet_alloc()

On Wed, Jul 10, 2024, at 23:52, Thorsten Blum wrote:
> The local variable period uses at most 32 bits and can be a u32 instead
> of unsigned long. The upper 32 bits are all 0 after masking and right
> shifting cap by HPET_COUNTER_CLK_PERIOD_SHIFT and can be discarded.
>
> Since do_div() casts the divisor to u32 anyway, changing the data type
> of period to u32 also removes the following Coccinelle/coccicheck
> warning reported by do_div.cocci:
>
>   WARNING: do_div() does a 64-by-32 division, please consider using 
> div64_ul instead
>
> Signed-off-by: Thorsten Blum <thorsten.blum@...lux.com>

Acked-by: Arnd Bergmann <arnd@...db.de>

> ---
>  drivers/char/hpet.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
> index d51fc8321d41..bd68afa22cff 100644
> --- a/drivers/char/hpet.c
> +++ b/drivers/char/hpet.c
> @@ -784,7 +784,7 @@ int hpet_alloc(struct hpet_data *hdp)
>  	struct hpets *hpetp;
>  	struct hpet __iomem *hpet;
>  	static struct hpets *last;
> -	unsigned long period;
> +	u32 period;
>  	unsigned long long temp;

The change is sufficient to address the issue. I had another
look at this and came up with a different approach that
would also help readability:

--- a/drivers/char/hpet.c
+++ b/drivers/char/hpet.c
@@ -855,10 +855,9 @@ int hpet_alloc(struct hpet_data *hdp)
 
        period = (cap & HPET_COUNTER_CLK_PERIOD_MASK) >>
                HPET_COUNTER_CLK_PERIOD_SHIFT; /* fs, 10^-15 */
-       temp = 1000000000000000uLL; /* 10^15 femtoseconds per second */
-       temp += period >> 1; /* round */
-       do_div(temp, period);
-       hpetp->hp_tick_freq = temp; /* ticks per second */
+
+       /* femtosecond period to ticks per second */
+       hpetp->hp_tick_freq = div64_round(1000000000000000uLL, period);
 
        printk(KERN_INFO "hpet%d: at MMIO 0x%lx, IRQ%s",
                hpetp->hp_which, hdp->hd_phys_address,

This would also work by itself, or we could do both.
Clemens, any opinion?

     Arnd