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Subject: Re: [PATCH v4 4/8] rt2800: fix registers init for MT7620
To:     Stanislaw Gruszka <sgruszka@redhat.com>,
        Tom Psyborg <pozega.tomislav@gmail.com>
Cc:     linux-wireless@vger.kernel.org,
        Daniel Golle <daniel@makrotopia.org>,
        Mathias Kresin <dev@kresin.me>
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From:   Felix Fietkau <nbd@nbd.name>
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Date:   Tue, 16 Oct 2018 13:19:52 +0200
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On 2018-10-16 10:09, Stanislaw Gruszka wrote:
> On Fri, Oct 12, 2018 at 02:41:41PM +0200, Tom Psyborg wrote:
>> On 12/10/2018, Stanislaw Gruszka <sgruszka@redhat.com> wrote:
>> > On Fri, Oct 12, 2018 at 02:20:07PM +0200, Tom Psyborg wrote:
>> >> > On upstream tree where this patch is intended
>> >> > additional registers where never programmed as proper branch
>> >> > were never used, because of additional check in RT5390 branch.
>> >> >
>> >>
>> >> on my hardware additional registers were programmed in regardless of
>> >> redundant check. that why i opened whole thread on forum since i
>> >> couldn't understand how's that happening
>> >
>> > I don't understand how that possible either.
>> 
>> i'd assume because device use external lna
> 
> 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.

- Felix