Re: [PATCH RFT v2 4/5] i3c: mipi-i3c-hci: use get_parity8 helper instead of open coding it

From: David Laight <david.laight.linux@gmail.com>
To: Wolfram Sang <wsa+renesas@sang-engineering.com>
Cc: linux-i3c@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-renesas-soc@vger.kernel.org,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Alexandre Belloni <alexandre.belloni@bootlin.com>
Subject: Re: [PATCH RFT v2 4/5] i3c: mipi-i3c-hci: use get_parity8 helper instead of open coding it
Date: Fri, 3 Jan 2025 13:49:35 +0000	[thread overview]
Message-ID: <20250103134935.2a341ce0@pumpkin> (raw)
In-Reply-To: <Z3e1tuAR5GsEhYLz@shikoro>

On Fri, 3 Jan 2025 11:02:30 +0100
Wolfram Sang <wsa+renesas@sang-engineering.com> wrote:

> > > > > -		  (dynaddr_parity(address) ? DAT_0_DYNADDR_PARITY : 0);
> > > > > +		  (parity8(address) ? 0 : DAT_0_DYNADDR_PARITY);    
> 
> ...
> 
> > The old code is:  
> > > -static inline bool dynaddr_parity(unsigned int addr)
> > > -{
> > > -	addr |= 1 << 7;
> > > -	addr += addr >> 4;
> > > -	addr += addr >> 2;
> > > -	addr += addr >> 1;
> > > -	return (addr & 1);
> > > -}  
> > 
> > So:
> > 1) it always sets 0x80.  
> 
> Right, this is why the arguments of the ternary operator above are
> exchanged. The old function was basically 'is_odd'.

Provided the high bit isn't already set - which it may not be.

> > 2) it uses addition not exclusive or.  
> 
> True, but it will work nonetheless because we are only interested in bit
> 0 of the result. For one bit, XOR and addition are interchangable. The
> overflow to other bits is not important.

add: 00010001 => xxxx0010 => xx10 => x1
xor: 00010001 => xxxx0000 => 00xx => x0

> 
> > So just not the same definition of 'parity'.  
> 
> I think it is. I mean, I3C wants odd parity, otherwise it will not work.
> And Jarkko kindly confirmed it still works.

I bet the target isn't checking...

So you might be fixing a bug.

	David

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