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SubjectRe: [PATCH v5 3/3] media: i2c: imx274: Add IMX274 power on and off sequence

On Fri, Sep 04, 2020 at 10:04:10AM -0700, Sowjanya Komatineni wrote:
> On 9/4/20 1:55 AM, Jacopo Mondi wrote:
> > > > usleep_range() allows you to provide an interval in which your timeout
> > > > can be coalesced with others. Giving a [1usec, 2usec] range kind of
> > > > defeat the purpose. And most than everything, does sleeping for 2usec
> > > > serve any real purpose ?
> > > Following delay recommendation from DS for power on sequence.
> > >
> > 2 useconds ? Seems very short:)
> >
> As per IMX274 datasheet for power on sequence, 100ns is the min wait time
> after the last power supply of 1v8/1v2/2v8 is ON before releasing RESET
> high.

ook.. well, it's actually reasonable, it's just the time for the
regulators to ramp up, I initially thought it was the time for the
chip to exit reset.

Let me be a bit more picky and ask if you have considered busy waiting
on such a small sleep interval by using udelay. Again, as this happens
at chip power on only, the impact on the system of mis-using
usleep_range() is negligible, but according to documentation:

SLEEPING FOR "A FEW" USECS ( < ~10us? ):
* Use udelay

- Why not usleep?
On slower systems, (embedded, OR perhaps a speed-
stepped PC!) the overhead of setting up the hrtimers
for usleep *may* not be worth it. Such an evaluation
will obviously depend on your specific situation, but
it is something to be aware of.

Up to you, really!


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