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

H. Nikolaus Schaller
2016-08-19T17:42:50Z

> Am 19.08.2016 um 13:06 schrieb One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>:
>
>> If possible, please do a callback for every character that arrives.
>> And not only if the rx buffer becomes full, to give the slave driver
>> a chance to trigger actions almost immediately after every character.
>> This probably runs in interrupt context and can happen often.
>
> We don't realistically have the clock cycles to do that on a low end
> embedded processor handling high speed I/O.

well, if we have a low end embedded processor and high-speed I/O, then
buffering the data before processing doesn't help either since processing
still will eat up clock cycles.

> The best you can do is
> trigger a workqueue to switch the buffer data around and call the helper
> while the uart may be receiving more bytes.

Ok, assuming DMA double buffering might (almost) double throughput.

The question is if this is needed at all. If we have a bluetooth stack with HCI the
fastest UART interface I am aware of is running at 3 Mbit/s. 10 bits incl. framing
means 300kByte/s equiv. 3µs per byte to process. Should be enough to decide
if the byte should go to a buffer or not, check checksums, or discard and move
the protocol engine to a different state. This is what I assume would be done in
a callback. No processing needing some ms per frame.

>
> What you are asking for you'd get out of the first parts of tidying up
> the receive paths because you'd set a different port->rx() method and get
> bursts of characters, flags and length data.
>
> Alan