On Fri, 19 Aug 2016 19:42:37 +0200
"H. Nikolaus Schaller" [off-list ref] wrote:

quoted

Am 19.08.2016 um 13:06 schrieb One Thousand Gnomes <gnomes@lxorguk.ukuu=

.org.uk>:

quoted

 =20

quoted

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. =20

=20
We don't realistically have the clock cycles to do that on a low end
embedded processor handling high speed I/O. =20

=20
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.

Of course it helps. You are out of the IRQ handler within the 9 serial
clocks, so you can take another interrupt and grab the next byte. You
will also get benefits from processing the bytes further in blocks, and
if you get too far behind you'll make the flow control limit.

You've also usually got multiple cores these days - although not on the
very low end quite often.

The question is if this is needed at all. If we have a bluetooth stack wi=

th HCI the

fastest UART interface I am aware of is running at 3 Mbit/s. 10 bits incl=

. framing

means 300kByte/s equiv. 3=C2=B5s 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.

That depends on the processor - remember people run Linux on low end CPUs
including those embedded in an FPGA not just high end PC and ARM class
devices.

The more important question is - purely for the receive side of things -
is a callback which guarantees to be called "soon" after the bytes arrive
sufficient.

If it is then almost no work is needed on the receive side to allow pure
kernel code to manage recevied data directly because the current
buffering support throughout the receive side is completely capable of
providing those services without a tty structure, and to anything which
can have a tty attached.

Doesn't solve transmit or configuration but it's one step that needs no
additional real work and re-invention.

Alan