> Am 21.08.2016 um 19:09 schrieb One Thousand Gnomes > <gno...@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
