linux-kernel - Re: [PATCH v4] media: vivid: Improve timestamping

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Date:   Mon, 3 Dec 2018 10:15:14 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Arnd Bergmann <arnd@...db.de>, gfmandaji@...il.com
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        lkcamp@...ts.libreplanetbr.org
Subject: Re: [PATCH v4] media: vivid: Improve timestamping

On 12/02/2018 09:43 PM, Arnd Bergmann wrote:
> On Sun, Dec 2, 2018 at 2:47 PM Gabriel Francisco Mandaji
> <gfmandaji@...il.com> wrote:
> 
>> @@ -667,10 +653,28 @@ static void vivid_overlay(struct vivid_dev *dev, struct vivid_buffer *buf)
>>         }
>>  }
>>
>> +static void vivid_cap_update_frame_period(struct vivid_dev *dev)
>> +{
>> +       u64 f_period;
>> +
>> +       f_period = (u64)dev->timeperframe_vid_cap.numerator * 1000000000;
>> +       do_div(f_period, dev->timeperframe_vid_cap.denominator);
>> +       if (dev->field_cap == V4L2_FIELD_ALTERNATE)
>> +               do_div(f_period, 2);
>> +       /*
>> +        * If "End of Frame", then offset the exposure time by 0.9
>> +        * of the frame period.
>> +        */
>> +       dev->cap_frame_eof_offset = f_period * 9;
>> +       do_div(dev->cap_frame_eof_offset, 10);
>> +       dev->cap_frame_period = f_period;
>> +}
> 
> Doing two or three do_div() operations is going to make this rather
> expensive on 32-bit architectures, and it looks like this happens for
> each frame?
> 
> Since each one is a multiplication followed by a division, could this
> be changed to using a different factor followed by a bit shift?

The division by 2 can obviously be replaced by a shift, and the
'End of Frame' calculation can be simplified as well by multiplying by
7 and dividing by 8 (again a simple shift): this equals 0.875 which is
close enough to 0.9 (so update the comment as well).

It's all a bit overkill since this function isn't called very often,
but these are easy changes to make.

Regards,

	Hans