Re: [RFC PATCH 0/3] UART slave device bus

H. Nikolaus Schaller
2016-08-21T18:23:24Z

> Am 21.08.2016 um 19:09 schrieb One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>:
>
>> Let me ask a question about your centralized and pre-cooked buffering approach.
>>
>> As far as I see, even then the kernel API must notify the driver at the right moment
>> that a new block has arrived. Right?
>
> The low level driver queues words (data byte, flag byte)
> The buffer processing workqueue picks those bytes from the queue and
> atomically empties the queue

When and how fast is the work queue scheduled?
And by which event?

> The workqueue involves the receive handler.

This should be faster than if a driver directly processes incoming bytes?

>
>> But how does the kernel API know how long such a block is?
>
> It's as long as the data that has arrived in that time.

Which means the work queue handler have to decide if it is enough for a
frame to decode and if not, wait a little until more arrives.

Or you have to assemble chunks into a frame, i.e. copy data around.

Both seems a waste of scarce cpu cycles in high-speed situations to me.

>
>> Usually there is a start byte/character, sometimes a length indicator, then payload data,
>> some checksum and finally a stop byte/character. For NMEA it is $, no length, * and \r\n.
>> For other serial protocols it might be AT, no length, and \r. Or something different.
>> HCI seems to use 2 byte op-code or 1 byte event code and 1 byte parameter length.
>
> It doesn't look for any kind of protocol block headers.

Which might become the pitfall of the design because as I have described it is an
essential part of processing UART based protocols. You seem to focus on efficiently
buffering only but not about efficiently processing the queued data.

> The routine
> invoked by the work queue does any frame recovery.

>
>> So I would even conclude that you usually can't even use DMA based UART receive
>> processing for arbitrary and not well-defined protocols. Or have to assume that the
>
> We do, today for bluetooth and other protocols just fine

I think it works (even with user-space HCI daemon) because bluetooth HCI is slow (<300kByte/s).

> - it's all about
> data flows not about framing in the protocol sense.

Yes, but you should also take framing into account for a solution that helps to implement
UART slave devices. That is my concern.

BR,
Nikolaus