linux-kernel - Re: [PATCH 1/2] amba: Take device out of reset before reading pid and cid values

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Date:   Thu, 28 Mar 2019 17:43:04 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Xu Wei <xuwei5@...ilicon.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Griffin <peter.griffin@...aro.org>,
        Guodong Xu <guodong.xu@...aro.org>,
        Haojian Zhuang <haojian.zhuang@...aro.org>
Subject: Re: [PATCH 1/2] amba: Take device out of reset before reading pid and
 cid values

On Thu, Mar 21, 2019 at 12:17 PM Daniel Thompson
<daniel.thompson@...aro.org> wrote:
> On Wed, Mar 20, 2019 at 05:29:56PM +0000, Russell King - ARM Linux admin wrote:
> > On Wed, Mar 20, 2019 at 12:26:58PM +0530, Manivannan Sadhasivam wrote:

> > That said, if a reset line is shared between multiple devices, and a
> > driver wants to assert the reset line, it would disrupt the operation
> > of all those devices, so there would need to be some kind of
> > synchronisation between the drivers.
>
> That is what shared ownership of the reset line provides. When a line is
> shared a single driver does not have the authority to unilaterally
> assert reset because deasserts and asserts are counted and the line only
> goes high again when they balance.

This is what we want for this I'm pretty sure. Regulators have the same
thing with internal reference counting so that if you turn a shared regulator
on from several clients it just increases the refcount and it will not really
be turned off until the last consumer is disabling it.

Yours,
Linus Walleij