linux-kernel - Re: [RFC PATCH v3] reset: Add a defer reset object to send board specific reset

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Date:	Fri, 20 Jun 2014 11:35:20 +0200
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Houcheng Lin <houcheng@...il.com>
Cc:	linus.walleij@...aro.org, gnurou@...il.com,
	grant.likely@...aro.org, robh+dt@...nel.org, dbaryshkov@...il.com,
	dwmw2@...radead.org, p.zabel@...gutronix.de,
	stephen.gallimore@...com, srinivas.kandagatla@...com,
	ulf.hansson@...aro.org, sre@...nel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: [RFC PATCH v3] reset: Add a defer reset object to send board
 specific reset

On Wed, Jun 18, 2014 at 09:37:53PM +0800, Houcheng Lin wrote:
> The Problem
> -----------
> The reset signal on a hardware board is send either:
>     - during machine initialization
>     - during bus master's initialization
> 
> In some hardware design, devices on bus need a non-standard and extra reset
> signal after bus is initialied. Most reason is to wake up device from hanging
> state.
> 
> The board spefic reset code can not be put into machine init code, as it is
> too early. This code can not also be put onto chip's driver, as it is board
> specific and not suitable for a common chip driver.

I don't see why not. I don't get the difference with an optional
regulator, or an optional GPIO, or whatever. And there's already some
support for such a case, with reset_control_get_optional.

> 
> Defer Reset Object
> ------------------
> The defer reset object is to resolve this issue, developer can put a defer-
> reset device on the board's dts file and enable DEFER RESET OBJECT CONFIG.
> During driver init-calls, a defer-reset object is created and issue reset signal
> after the enclosing device is initialized.
> 
> This eliminate the need to rewrite a driver module with only one purpose: sending
> a board specific reset. This also allow mainstream kernel to support many boards
> that modify the common drivers to send board specific reset. Configuring defer-reset
> device in dts leave the board specific reset rules on board level and simple to
> maintain.
> 
> Example dts File
> ----------------
> Example 1:
>     defer_reset_vbus {
>         compatible = "defer-reset";
>         reset-gpios = <&gpx3 5 GPIO_ACTIVE_LOW>;

This doesn't follow the generic reset bindings.

>         duration = <5>;
>     };
> 
> Example 2:
>     defer_reset_vbus {
>         compatible = "defer-reset";
>         reset-gpios = <&gpx3 5 GPIO_ACTIVE_HIGH>;
>         duration = <0>;
>     };
> 
> Block Diagram of dts File
> -------------------------
>     +-------------------------------------+
>     | usb-ehci-chip@...1000               |
>     |   +-------------------------+       |
>     |   | defer-reset(gpx3)       |       |
>     |   +-------------------------+       |
>     +-------------------------------------+

And this goes against the current way of doing things, with the resets
property holding a phandle.

Why not just having something like:

usb {
    compatible = "whatever";
    resets = <&rst_gpio <duration>>;
}

Which uses the generic bindings, and doesn't require anything fancy
either in the driver or the DT.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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