On Thu, Aug 18, 2016 at 01:01:24PM +0200, Marcel Holtmann wrote: > Hi Greg, > > >>>> Currently, devices attached via a UART are not well supported in the > >>>> kernel. The problem is the device support is done in tty line disciplines, > >>>> various platform drivers to handle some sideband, and in userspace with > >>>> utilities such as hciattach. > >>>> > >>>> There have been several attempts to improve support, but they suffer from > >>>> still being tied into the tty layer and/or abusing the platform bus. This > >>>> is a prototype to show creating a proper UART bus for UART devices. It is > >>>> tied into the serial core (really struct uart_port) below the tty layer > >>>> in order to use existing serial drivers. > >>>> > >>>> This is functional with minimal testing using the loopback driver and > >>>> pl011 (w/o DMA) UART under QEMU (modified to add a DT node for the slave > >>>> device). It still needs lots of work and polish. > >>>> > >>>> TODOs: > >>>> - Figure out the port locking. mutex plus spinlock plus refcounting? I'm > >>>> hoping all that complexity is from the tty layer and not needed here. > >>>> - Split out the controller for uart_ports into separate driver. Do we see > >>>> a need for controller drivers that are not standard serial drivers? > >>>> - Implement/test the removal paths > >>>> - Fix the receive callbacks for more than character at a time (i.e. DMA) > >>>> - Need better receive buffering than just a simple circular buffer or > >>>> perhaps a different receive interface (e.g. direct to client buffer)? > >>>> - Test with other UART drivers > >>>> - Convert a real driver/line discipline over to UART bus. > >>>> > >>>> Before I spend more time on this, I'm looking mainly for feedback on the > >>>> general direction and structure (the interface with the existing serial > >>>> drivers in particular). > >>> > >>> Some quick comments (can't do any real life tests in the next weeks) from my (biased) view: > >>> > >>> * tieing the solution into uart_port is the same as we had done. The difference seems to > >>> me that you completely bypass serial_core (and tty) while we want to integrate it with standard tty operation. > >>> > >>> We have tapped the tty layer only because it can not be 100% avoided if we use serial_core. > >>> > >>> * one feedback I had received was that there may be uart device drivers not using serial_core. I am not sure if your approach addresses that. > >>> > >>> * what I don't see is how we can implement our GPS device power control driver: > >>> - the device should still present itself as a tty device (so that cat /dev/ttyO1 reports NMEA records) and should > >>> not be completely hidden from user space or represented by a new interface type invented just for this device > >>> (while the majority of other GPS receivers are still simple tty devices). > >>> - how we can detect that the device is sending data to the UART while no user space process has the uart port open > >>> i.e. when does the driver know when to start/stop the UART. > >> > >> I am actually not convinced that GPS should be represented as > >> /dev/ttyS0 or similar TTY. It think they deserve their own driver > >> exposing them as simple character devices. That way we can have a > >> proper DEVTYPE and userspace can find them correctly. We can also > >> annotate them if needed for special settings. > > > > I would _love_ to see that happen, but what about the GPS line > > discipline that we have today? How would that match up with a char > > device driver? > > we have a GPS line discipline? What is that one doing? As far as I > know all GPS implementations are fully userspace. Hm, for some reason I thought that was what n_gsm.c was being used for, but I could be wrong, I've never seen the hardware that uses that code... greg k-h -- 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
