Re: [PATCH v4 4/8] rt2800: fix registers init for MT7620

From: Stanislaw Gruszka <sgruszka@redhat.com>
To: Felix Fietkau <nbd@nbd.name>
Cc: Tom Psyborg <pozega.tomislav@gmail.com>,
	linux-wireless@vger.kernel.org,
	Daniel Golle <daniel@makrotopia.org>,
	Mathias Kresin <dev@kresin.me>
Subject: Re: [PATCH v4 4/8] rt2800: fix registers init for MT7620
Date: Tue, 16 Oct 2018 13:21:03 +0200	[thread overview]
Message-ID: <20181016112102.GC2621@redhat.com> (raw)
In-Reply-To: <e18b2f17-9403-f17a-babb-c4f56dedd2ca@nbd.name>

On Tue, Oct 16, 2018 at 01:19:52PM +0200, Felix Fietkau wrote:
> > I have no idea how this could be related. But I think I found
> > somewhat reasonable explenation where the problem is.
> > I think below code :
> > 
> > 	if (a || b || c) {
> > 		CODE1();
> > 	} else if (c) {
> > 		CODE2();
> > 	}
> > 
> > can not be deterministic and can be compiled differently depending
> > on compiler version and used options. Sometimes it could result
> > in this 
> > 
> > 	if (a || b || c) {
> > 		CODE1();
> > 	}
> > 
> > and sometimes in this:
> > 
> > 	if (a || b) {
> > 		CODE1();
> > 	} else if (c) {
> > 		CODE2();
> > 	}
> > 
> > So that would explain the problems you see. And indeed patch
> > could cause regression on systems where second variant of
> > initalizing RT6352 registers was used.
> I don't see how that can be non-deterministic at all. The 'else if' part
> can only be hit if the first if did not match.

I meant non-deterministic during compilation process, when compiler
do or do not some optimizations or if compiler version differs.

Regards
Stanislaw