linux-kernel - Re: [PATCH v3 3/3] gnss: ubx: add support for the reset gpio

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Message-ID: <ZUj16pzYG_6WR5Jc@hovoldconsulting.com>
Date:   Mon, 6 Nov 2023 15:19:22 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc:     linux-renesas-soc@...r.kernel.org,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] gnss: ubx: add support for the reset gpio

On Mon, Oct 23, 2023 at 09:09:03AM +0200, Wolfram Sang wrote:
> > So this means that the reset line will be asserted indefinitely in case
> > the GNSS receiver is not used. Are you sure that's a good idea?
> 
> Yup. Saves power. We do this for our ethernet PHYs as well. Any reasons
> not to do this?

AFAIK you should generally not try to use reset this way as you may end
up leaking current (and possibly other complications).
 
> > I don't know yet which module this is for, or how this signal is
> > supposed to be used, but the above makes this look more like an
> > active-high (regulator) enable signal. Perhaps that's what it is or
> > should be modelled as (i.e. as a fixed regulator).
> 
> Nope, it is a RESET_N pin.

And the neo-m8 hardware integration manual explicitly says that it shall
not be used as a disable signal:

	1.5 I/O pins
	RESET_N: Reset input
	Driving RESET_N low activates a hardware reset of the system.
	Use this pin only to reset the module. Do not use RESET_N to
	turn the module on and off, since the reset state increases
	power consumption.

Some of the other modules in this family says so explicitly also in the
datasheet.

Johan

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