On Thu, Aug 18, 2016 at 10:04 AM, One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx> wrote: >> No, the code should be fast as it is so simple. I assume there is some >> reason the tty buffering is more complex than just a circular buffer. > > I would suggest you read n_tty.c carefully and then it'll make a fair bit > of sense. It has to interlock multiple reader/writes with discipline > changes and flushes of pending data. At the same time a received > character may cause output changes including bytes to be queued for > transmit and the entire lot must not excessively recurse. > > It's fun and it took years to make work safely but basically you need to > handle a simultaneous ldisc change, config change, read of data from the > buffers, receive, transmit and the receive causing the transmit status to > change and maybe other transmits, that might have to be sent with > priority. It's fun 8) > > The good news is that nobody but n_tty and maybe n_irda cares on the rx > side. Every other ldisc consumes the bytes immediately. IRDA hasn't worked > for years anyway. > >> My best guess is because the tty layer has to buffer things for >> userspace and userspace can be slow to read? Do line disciplines make >> assumptions about the tty buffering? Is 4KB enough buffering? > > RTFS but to save you a bit of effort > > 1. 4K is not enough, 64K is not always sufficient, this is why we have > all the functionality you appear to want to re-invent already in the tty > buffer logic of the tty_port > 2. Only n_tty actually uses the tty_port layer buffering > 3. The ring buffer used for dumb uarts is entirely about latency limits > on low end processors and only used by some uarts anyway. > >> Also, the current receive implementation has no concept of blocking or >> timeout. Should the uart_dev_rx() function return when there's no more >> data or wait (with timeout) until all requested data is received? >> (Probably do all of them is my guess). > > Your rx routine needs to be able to run in IRQ context, not block and > complete in very very short time scales because on some hardware you have > exactly 9 bit times to recover the data byte and clear the IRQ done. > Serial really stretches some of the low end embedded processors running > at 56K/115200, and RS485 at 4Mbits even with 8 bytes of buffering is > pretty tight. Thus you need very fast buffers for just about any use case. > Dumb uarts you'll need to keep the existing ring buffer or similar > (moving to a kfifo would slightly improve performance I think) and queue > after. > >> >> - Convert a real driver/line discipline over to UART bus. >> > >> > That's going to be the real test, I recommend trying that as soon as >> > possible as it will show where the real pain points are :) > > The locking. It's taken ten years to debug the current line discipline > change locking. If you want to be able to switch stuff kernel side > however it's somewhat easier. > > The change should be > > Add tty_port->rx(uint8_t *data,uint8_t *flags, unsigned int len) > > The semantics of tty_port->rx are > > - You may not assume a tty is bound to this port > - You may be called in IRQ context, but are guaranteed not to get > parallel calls for the same port > - When you return the bytes you passed are history > > At that point you can set tty_port->rx to point to the > tty_flip_buffer_push() and everyone can use it. Slow ones will want to > queue to a ring buffer then do tty_port->rx (where we do the flush_buffer > now), fast ones will do the ->rx directly. Other than doing DMA, I did not find any examples of UARTs doing internal rx ring buffers. Most/all the non-DMA cases do tty_insert_flip_char directly in the ISR. The flow is insert a series of flags and characters as we process the receive status and then trigger a flush of the buffer at the end. That doesn't match up with what you are proposing for how tty_port->rx would work. That would change the receive ISR processing in all the drivers quite a bit. Either we'd have to call tty_port->rx a character at a time or implement some temporary buffer. I don't think we want to call things like BT receive code a byte at a time. This needs to be a layer higher. flush_to_ldisc either needs to be duplicated to handle tty_port->rx or generalized to call either tty_port->rx or ldisc receive_buf. I'm not sure what to do about ldisc ref counting in the latter case. Rob -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
