netdev - Re: [RFC][PATCH] net: arinc429: Add ARINC-429 stack

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Date:	Tue, 10 Nov 2015 17:15:39 +0100
From:	Marek Vasut <marex@...x.de>
To:	Aleksander Morgado <aleksander@...ksander.es>
Cc:	Vostrikov Andrey <andrey.vostrikov@...entembedded.com>,
	Oliver Hartkopp <socketcan@...tkopp.net>,
	"Marc Kleine-Budde" <mkl@...gutronix.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Wolfgang Grandegger <wg@...ndegger.com>,
	Andrew Lunn <andrew@...n.ch>
Subject: Re: [RFC][PATCH] net: arinc429: Add ARINC-429 stack

On Wednesday, November 04, 2015 at 04:45:20 PM, Aleksander Morgado wrote:
> On Wed, Nov 4, 2015 at 4:33 PM, Marek Vasut <marex@...x.de> wrote:
> > On Wednesday, November 04, 2015 at 04:19:45 PM, Aleksander Morgado wrote:
> >> On Wed, Nov 4, 2015 at 4:18 PM, Vostrikov Andrey
> >> 
> >> <andrey.vostrikov@...entembedded.com> wrote:
> >> >>> > About the parity -- can we add some flag into the datagram to
> >> >>> > indicate we want hardware to calculate the parity for that
> >> >>> > particular datagram for us? And we'd also need to indicate what
> >> >>> > type of parity. I dunno if this is worth the hassle.
> >> >>> 
> >> >>> This  is HW configuration property, it does not belong to  datagram.
> >> >>> Also for TX channels,  parity could  be  two  kinds:  odd and even,
> >> >>> for RX it is only on/off.
> >> >> 
> >> >> There are datagrams which do contain parity and ones which do not
> >> >> contain it, correct ? Thus, it's a property of that particular
> >> >> datagram.
> >> 
> >> All ARINC words have bit #31 as parity bit; whether it's used or not
> >> depends on the setup as Andrey says below.
> > 
> > Can bit 31 be ever used for DATA instead of parity ? Or is this just me
> > not understanding the parlance of the specification, where "DATA"
> > actually means "DATA with parity" ?
> 
> Well, as far as I know bit 31 is always parity bit, never used for
> actual data contents. Which is the spec section that got you confused?
> Maybe I'm the one which didn't read it well?

Sorry for being so late into the discussion.

I got confused by hi-3585_v-rev-l.pdf page 7 right, CR4 lets you treat bit
32 as either data or parity. But I guess this is not the general case.

So I wonder, does it make sense to treat the P bit as data always and do
parity in software or not ?

Best regards,
Marek Vasut
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