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Message-ID: <29495f1d0511210708t380b7ff7ic6759d59927c12d1@mail.gmail.com>
Date: Mon, 21 Nov 2005 07:08:01 -0800
From: Nish Aravamudan <nish.aravamudan@gmail.com>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Subject: Re: I made a patch and would like feedback/testers (drivers/cdrom/aztcd.c)
Cc: Con Kolivas <kernel@kolivas.org>,
       =?ISO-8859-1?Q?Daniel_Marjam=E4ki?= <daniel.marjamaki@comhem.se>,
       linux-kernel@vger.kernel.org
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On 11/21/05, Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> Con Kolivas wrote:
> > On Mon, 21 Nov 2005 19:42, Daniel Marjam?ki wrote:
>
> >>Hmm.. The minimum value should be 2, right?
> >>Otherwise the loop could time out after only a few nanoseconds.. since the
> >>loop will then timeout immediately on a clock tick. Or am I wrong?
> >
> >
> >       aztTimeOut =  HZ / 500 ? : 1;
> > Would give you a 2ms timeout on 1000Hz and 500Hz
> > It would give you 5ms on 250Hz and 10ms on 100Hz
> >
> > ie the absolute minimum it would be would be 2ms, but it would always be at
> > least one timer tick which is longer than 2ms at low HZ values.
> >
>
> Not true. From 'now', the next timer interrupt is somewhere
> between epsilon and 1/HZ seconds away.
>
> Luckily, time_after is < rather than <=, so your aztTimeOut would
> actually make Daniel's code wait until a minimum of *two* timer
> ticks have elapsed since reading jiffies. So it would manage to
> scrape by the values of HZ you quoted.
>
> OTOH, if HZ were between 500 and 1000, it would again be too short
> due to truncation.
>
> Better I think would be to use the proper interfaces for converting
> msecs to jiffies:
>
>    aztTimeOut = jiffies + msecs_to_jiffies(2);

That's what I get for sleeping. Yes, Nick is right, don't do this
conversion yourself, please, use the interfaces designed exactly to
avoid any confusion / HZ-issues (and if those interfaces have
limitations, as Willy discovered earlier, please fix them).

Thanks,
Nish
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