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

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Message-ID: <20161213105406.GY16630@valkosipuli.retiisi.org.uk>
Date:   Tue, 13 Dec 2016 12:54:07 +0200
From:   Sakari Ailus <sakari.ailus@....fi>
To:     Ian Arkver <ian.arkver.dev@...il.com>
Cc:     Nicholas Mc Guire <hofrat@...dl.org>,
        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 Tue, Dec 13, 2016 at 10:10:51AM +0000, Ian Arkver wrote:
> 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

Good point. Datasheets do not always tell everything though; it'd be good to
get a comment from the original driver authors on why they've used the value
which can now be found in the driver.

-- 
Sakari Ailus
e-mail: sakari.ailus@....fi	XMPP: sailus@...iisi.org.uk