linux-kernel - Re: [PATCH v6 2/2] hwspinlock: add sun6i hardware spinlock support

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Date:   Tue, 2 Mar 2021 18:20:02 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Wilken Gottwalt <wilken.gottwalt@...teo.net>
Cc:     linux-kernel@...r.kernel.org, Ohad Ben-Cohen <ohad@...ery.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Baolin Wang <baolin.wang7@...il.com>,
        Rob Herring <robh+dt@...nel.org>, Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>
Subject: Re: [PATCH v6 2/2] hwspinlock: add sun6i hardware spinlock support

Hi,

On Mon, Mar 01, 2021 at 03:06:08PM +0100, Wilken Gottwalt wrote:
> On Mon, 1 Mar 2021 14:13:05 +0100
> Maxime Ripard <mripard@...nel.org> wrote:
> 
> > On Sat, Feb 27, 2021 at 02:03:54PM +0100, Wilken Gottwalt wrote:
> > > Adds the sun6i_hwspinlock driver for the hardware spinlock unit found in
> > > most of the sun6i compatible SoCs.
> > >
> > > This unit provides at least 32 spinlocks in hardware. The implementation
> > > supports 32, 64, 128 or 256 32bit registers. A lock can be taken by
> > > reading a register and released by writing a 0 to it. This driver
> > > supports all 4 spinlock setups, but for now only the first setup (32
> > > locks) seem to exist in available devices. This spinlock unit is shared
> > > between all ARM cores and the embedded companion core. All of them can
> > > take/release a lock with a single cycle operation. It can be used to
> > > sync access to devices shared by the ARM cores and the companion core.
> > >
> > > There are two ways to check if a lock is taken. The first way is to read
> > > a lock. If a 0 is returned, the lock was free and is taken now. If an 1
> > > is returned, the caller has to try again. Which means the lock is taken.
> > > The second way is to read a 32bit wide status register where every bit
> > > represents one of the 32 first locks. According to the datasheets this
> > > status register supports only the 32 first locks. This is the reason the
> > > first way (lock read/write) approach is used to be able to cover all 256
> > > locks in future devices. The driver also reports the amount of supported
> > > locks via debugfs.
> > >
> > > Signed-off-by: Wilken Gottwalt <wilken.gottwalt@...teo.net>
> 
> Nope, I had to replace the devm_hwspin_lock_register function by the
> hwspin_lock_register function because like Bjorn pointed out that it can
> fail and needs to handled correctly. And having a devm_* function does not
> play well with the non-devm clock/reset functions and winding back if an
> error occurs. It also messes with the call order in the remove function. So
> I went back to the classic way where I have full control over the call order.

If you're talking about the clock and reset line reassertion, I don't
really see what the trouble is. Sure, it's not going to be in the exact
same order in remove, but it's still going to execute in the proper
order (ie, clock disable, then reset disable, then clock put and reset
put). And you can use devm_add_action if you want to handle things
automatically.

Maxime

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