linux-kernel - Re: [PATCH RFC] [media] s5k6aa: set usleep

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Message-ID: <ce9f2ee0-c0d3-2eb4-a733-b108d12b43fb@gmail.com>
Date:   Tue, 13 Dec 2016 10:10:51 +0000
From:   Ian Arkver <ian.arkver.dev@...il.com>
To:     Sakari Ailus <sakari.ailus@....fi>,
        Nicholas Mc Guire <hofrat@...dl.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sylwester Nawrocki <s.nawrocki@...sung.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Hans Verkuil <hans.verkuil@...co.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] [media] s5k6aa: set usleep_range greater 0

On 13/12/16 09:43, Sakari Ailus wrote:
> Hi Nicholas,
>
> On Tue, Dec 13, 2016 at 02:58:02AM +0100, Nicholas Mc Guire wrote:
>> As this is not in atomic context and it does not seem like a critical
>> timing setting a range of 1ms allows the timer subsystem to optimize
>> the hrtimer here.
> I'd suggest not to. These delays are often directly visible to the user in
> use cases where attention is indeed paid to milliseconds.
>
> The same applies to register accesses. An delay of 0 to 100 µs isn't much as
> such, but when you multiply that with the number of accesses it begins to
> add up.
>
Data sheet for this device [1] says STBYN deassertion to RSTN 
deassertion should be >50us, though this is actually referenced to MCLK 
startup. See Figure 36, Power-Up Sequence, page 42.

I think the usleep range here could be greatly reduced and opened up to 
allow hr timer tweaks if desired.

[1] http://www.bdtic.com/DataSheet/SAMSUNG/S5K6AAFX13.pdf

Regards,
Ian