linux-kernel - Re: [PATCH 3/7] soc: apple: Add driver for Apple PMGR power state controls

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Date:   Thu, 7 Oct 2021 01:11:04 +0900
From:   Hector Martin <marcan@...can.st>
To:     Philipp Zabel <p.zabel@...gutronix.de>,
        linux-arm-kernel@...ts.infradead.org
Cc:     Marc Zyngier <maz@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mark Kettenis <mark.kettenis@...all.nl>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        devicetree@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        linux-serial@...r.kernel.org
Subject: Re: [PATCH 3/7] soc: apple: Add driver for Apple PMGR power state
 controls

On 06/10/2021 18.24, Philipp Zabel wrote:
>> +static int apple_pmgr_reset_reset(struct reset_controller_dev *rcdev, unsigned long id)
>> +{
>> +	int ret;
>> +
>> +	ret = apple_pmgr_reset_assert(rcdev, id);
>> +	if (ret)
>> +		return ret;
>> +
>> +	usleep_range(APPLE_PMGR_RESET_TIME, 2 * APPLE_PMGR_RESET_TIME);
> 
> Is this delay known to be long enough for all consumers using the
> reset_control_reset() functionality? Are there any users at all?

I tested this for UART and ANS outside of Linux, and found that even 
back to back register writes worked, so the 1us thing is already 
conservative. I have no idea if we'll run into some weirdo block that 
needs more time, though. If we do then this will have to be bumped or 
turned into a property.

> Is it ok for a genpd transition to happen during this sleep?

I expect consumers to call reset while the device is active; it won't 
even work without that, as the reset is synchronous and just doesn't 
take effect while clock gated (at least for UART). See the dev_err()s 
that fire when that happens.

-- 
Hector Martin (marcan@...can.st)
Public Key: https://mrcn.st/pub