linux-kernel - Re: [RFC PATCH v1 13/50] Avoid some useless msecs/jiffies conversions

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Date:   Sun, 29 Mar 2020 17:50:32 +0000
From:   George Spelvin <lkml@....ORG>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     linux-kernel@...r.kernel.org, Hannes Reinecke <hare@...e.de>,
        linux-scsi@...r.kernel.org,
        Marek Lindner <mareklindner@...mailbox.ch>,
        Simon Wunderlich <sw@...onwunderlich.de>,
        Antonio Quartulli <a@...table.cc>,
        Sven Eckelmann <sven@...fation.org>,
        b.a.t.m.a.n@...tynna.open-mesh.org,
        Johannes Berg <johannes@...solutions.net>,
        linux-wireless@...r.kernel.org, Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
        lkml@....org
Subject: Re: [RFC PATCH v1 13/50] Avoid some useless msecs/jiffies conversions

On Sun, Mar 29, 2020 at 07:13:33PM +0200, Takashi Iwai wrote:
> On Sun, 29 Mar 2020 14:11:29 +0200, George Spelvin wrote:
>> On Sun, Mar 29, 2020 at 09:52:23AM +0200, Takashi Iwai wrote:
>>> I thought the compiler already optimizes to the constant calculation
>>> for the above case?
>> 
>> It optimizes that if the entire argument, including "seconds", is
>> a compile-time constant.
>> 
>> However, given "msecs_to_jiffies(hdev->rpa_timeout * 1000);",
>> the computatin is non-trivial.
> 
> Fair enough.  But it's still a question whether an open code X * HZ is
> good at all...

I'm sorry, I don't understand what you mean by "good at all" here.
The value computed is exactly the same.

msecs_to_jiffies(x) is basically (x * HZ + 999) / 1000, so
  msecs_to_jiffies(s * 1000)
= (s * 1000 * HZ + 999) / 1000
= s * HZ + 999/1000
= s * HZ.