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

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Date:	Wed, 4 Nov 2015 10:34:50 +0100
From:	Aleksander Morgado <aleksander@...ksander.es>
To:	Marek Vasut <marex@...x.de>
Cc:	Oliver Hartkopp <socketcan@...tkopp.net>,
	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 Tue, Nov 3, 2015 at 10:43 PM, Marek Vasut <marex@...x.de> wrote:
> 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.

Agree on this, the three non-label bytes in an ARINC word should be
taken as opaque payload. The only exception would be the parity most
significant bit, but I don't think it'd be an issue to have that in
the opaque payload.

-- 
Aleksander
https://aleksander.es
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