linux-kernel - Re: [v10 3/4] media: vidtv: add a bridge driver

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Date:   Wed, 16 Sep 2020 09:09:30 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
Cc:     "Daniel W. S. Almeida" <dwlsalmeida@...il.com>,
        "r.verdejo@...sung.com" <r.verdejo@...sung.com>,
        "nicolas@...fresne.ca" <nicolas@...fresne.ca>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "skhan@...uxfoundation.org" <skhan@...uxfoundation.org>,
        "linux-kernel-mentees@...ts.linuxfoundation.org
        <linux-kernel-mentees@...ts.linuxfoundation.org>,
        linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [v10 3/4] media: vidtv: add a bridge driver

Hi Mauro,

On Wed, Sep 16, 2020 at 9:01 AM Mauro Carvalho Chehab
<mchehab+huawei@...nel.org> wrote:
> Em Tue, 15 Sep 2020 15:35:00 +0200
> Geert Uytterhoeven <geert@...ux-m68k.org> escreveu:
> > On Tue, Sep 15, 2020 at 3:26 PM Daniel W. S. Almeida
> > <dwlsalmeida@...il.com> wrote:
> > > >> +    u32 nbytes = 0;  /* the number of bytes written by this function */
> > > >> +
> > > >> +    u64 nbytes_expected; /* the number of bytes we should have written */
> > > >> +    u64 nbytes_streamed; /* the number of bytes we actually wrote */
> > > >> +    u32 num_null_pkts; /* number of null packets to bridge the gap */
> > > >> +
> > > >> +    u64 elapsed_time_msecs = jiffies_to_usecs(m->timing.current_jiffies -
> > > >> +                                              m->timing.past_jiffies);
> > > >> +
> > > >> +    elapsed_time_msecs = min(elapsed_time_msecs,
> > > >> (u64)VIDTV_MAX_SLEEP_USECS / 1000);
> > > >> +    nbytes_expected = div64_u64(m->mux_rate_kbytes_sec * 1000, MSEC_PER_SEC);
> > > >
> > > > Seriously?!?
> > > >
> > > > You multiply by 1000 first, followed by a division by 1000 using an
> > > > expensive 64-by-64 division?
> > >
> > > This entire function is broken and needs a do-over :)
> > >
> > > > using an expensive 64-by-64 division?
> > >
> > > I am new to kernel development. I wasn't even aware that this was
> > > expensive, to be honest.
> >
> > All divisions involving 64-bit data are expensive, especially on 32-bit
> > platforms.  That's why we have the helpers in <linux/math.h>.  Most
> > of them implement simplified variants, which are less expensive.
>
> I agree that 64-bit math is something that should be used with some
> care. However, it is almost unavoidable do to 64-bit divisions for
> digital TV.

Sure. If 64-bit math is needed, it should be used.

The macros (and the link failure on 32-bit) exist to (a) make sure
people think twice before using 64-math, and (b) let people pick a
less-expensive variant if that is sufficient.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds