linux-kernel - Re: [PATCH] of: overlay: user space synchronization

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Message-ID: <CANk1AXQU=3Ap=wbO5b8c2pMq61hzuRkeWYNv2Esq+c9d+0e_BA@mail.gmail.com>
Date:   Mon, 15 Oct 2018 15:38:55 -0500
From:   Alan Tull <atull@...nel.org>
To:     Frank Rowand <frowand.list@...il.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Rob Herring <robh+dt@...nel.org>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Pantelis Antoniou <panto@...oniou-consulting.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] of: overlay: user space synchronization

On Mon, Oct 15, 2018 at 1:09 PM Frank Rowand <frowand.list@...il.com> wrote:
>
> On 10/15/18 01:24, Geert Uytterhoeven wrote:
> >
> > Please say explicitly that tree_version contains a 32-bit unsigned
> > decimal number, which is incremented before and after every change.
> > I had to deduce that from the code.
>
> Good point.  I'll add that.

Looking at the code, tree_version being odd or even means something.
tree_version is incremented every time the of_mutex is locked or
unlocked, such that:
 * tree_version is odd  == of_mutex is locked and the tree is
currently be in the process of being changed
 * tree_version is even == of_version is unlocked again and the
changes are finished.

How about making this explicit in the interface by breaking it up into
two attributes:

/sys/firmware/devicetree/tree_lock == "locked" or "unlocked".  If the
tree is locked, you know that the tree may still be changing and the
sysfs can't be trusted to be stable yet.  Or maybe even more
specifically tree_overlay_lock?

/sys/firmware/devicetree/tree_version = a u32 that is incremented once
for every overlay added or removed.

Alan

>
>
> >
> > IMHO that is more important than having the sample script here.
> >
> > Gr{oetje,eeting}s,
> >
> >                         Geert
> >
>