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

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Date:	Tue, 3 Nov 2015 22:43:02 +0100
From:	Marek Vasut <marex@...x.de>
To:	Oliver Hartkopp <socketcan@...tkopp.net>
Cc:	Aleksander Morgado <aleksander@...ksander.es>,
	"Marc Kleine-Budde" <mkl@...gutronix.de>,
	Vostrikov Andrey <andrey.vostrikov@...entembedded.com>,
	"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 Tuesday, November 03, 2015 at 08:28:43 PM, Oliver Hartkopp wrote:
> On 11/03/2015 08:19 PM, Marek Vasut wrote:
> > On Tuesday, November 03, 2015 at 07:03:26 PM, Oliver Hartkopp wrote:
> >> On 11/03/2015 06:41 PM, Marek Vasut wrote:
> >>> On Tuesday, November 03, 2015 at 06:32:12 PM, Oliver Hartkopp wrote:
> >>> 
> >>> [...]
> >>> 
> >>>> It looks like you need to shift the stuff in user space every time.
> >>>> 
> >>>> So you might better think of something like this:
> >>>>     struct a429_frame {
> >>>>     
> >>>>             __u32   label;   /* ARINC 429 label */
> >>>>             __u8    length;  /* always set to 8 */
> >>>>             __u8    __pad;   /* padding */
> >>>>             __u8    __res0;  /* reserved / padding */
> >>>>             __u8    __res1;  /* reserved / padding */
> >>>>             __u32   data __attribute__((aligned(8)));
> >>>>             __u8    p;       /* p */
> >>>>             __u8    ssm;     /* ssm */
> >>>>             __u8    sdi;     /* sdi */
> >>>>             __u8    __end;   /* padding */
> >>>>     
> >>>>     };
> >>> 
> >>> You don't want to interpret those P(arity)/SSM/SDI bits, since they
> >>> differ depending on whatever the remote party sends. That's why I
> >>> decided to just make those into 3-bytes of data and let the userland
> >>> application deal with it as seen fit. Besides, the ARINC "FTP" really
> >>> uses those 3 bytes as plain data.
> >> 
> >> Ok. I did not know what P was for :-)
> > 
> > Oh yeah. P is parity and it's optional as well and can be odd/even
> > depending on the remote endpoint (sigh).
> > 
> >> Btw. it can make sense to introduce an union struct where different
> >> options to access the content are possible.
> > 
> > This would be pretty nasty I think. By reading the ARINC specification,
> > the SSM can be either 2 or 3 bits, the SDI is who-knows-what depending
> > on the remote endpoint and the P is also not always present. I'm not
> > convinced that the kernel should interpret the 3 byte ARINC payload in
> > any way. (but I wonder if my argument presented above is convincing at
> > all either ...).
> 
> Right.
> 
> When we define a user visible data structure, this is written into stone.
> 
> When ARINC isn't even sure about the detailed interpretation we should
> definitely keep our fingers away from doing it ourselves.

Right. Besides, such extension to the ABI can be done later if the need
arises (which I seriously doubt), can't it ? Handling the payload as a CAN
payload makes sense.

Best regards,
Marek Vasut
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