[Libre-soc-dev] Wishbone B4 Streaming Specification, Bug 244

[Abhishek Sharma]

Thanks alot for this explanation.I ll look into the details and get back.

On Mon, 8 Nov, 2021, 6:38 pm Luke Kenneth Casson Leighton, <lkcl at lkcl.net>
wrote:

> On Mon, Nov 8, 2021 at 9:35 AM Abhishek Sharma
> <abhisheksharma at object-automation.com> wrote:
> >
> > Hello All, I am new to this project.  I have briefly gone through
> > https://bugs.libre-soc.org/show_bug.cgi?id=244, started reading Wishbone
> > and AXI. Any suggestions for headstart.
>
> hi abishek, thanks for getting in touch.
>
> as you can see, i also went briefly over AXI stream:
> https://bugs.libre-soc.org/show_bug.cgi?id=244#c2
>
> you should also read up on the wishbone B4 specification
> as to what the concept of "TAGs" means.  with that in mind,
> below will make sense:
>
> it really does look to be as simple as adding a couple of extra
> tags, one to indicate "data bus is position code not data" and
> another to indicate "data bus is NULL data, please ignore",
> each on a per-byte-level basis.
>
> however if you then look further down in the axi_stream.pdf
> you will find that there are several *pages* describing "optional
> behaviour".
>
> this will be the majority of the "wishbone streaming" specification:
> the "error handling" and "what to do when things are optional".
>
> the fact that there are two tags:
> * tag 1: data-or-position
> * tag 2: data-is-NULL / data-is-not-NULL
>
> means that there are FOUR possible combinations there of
> behaviours:
>
> * data non-NULL
> * position non-NULL
> * data NULL
> * position NULL
>
> and, furthermore, there are sixteen possible permutations of "options":
>
> * master has data and NULL tags, slave has data and NULL tags
> * master has data no NULL tags, slave has data and NULL tags
> * master has no data but NULL tags, slave has data and NULL tags
> * master has no data or NULL tags, slave has data and NULL tags
> ...
> ...
> * master has no data or NULL tags, slave has no data or NULL tags
>
> each of these combinations will need to be covered and described in the
> specification, as to what behaviour is required when that situation is
> encountered.
>
> so although it is "very simple" (just add 2 extra tags) it is actually
> quite a lot of implications and combinations.
>
> l.
>
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