On Wed, May 18, 2022 at 07:11:38PM +0100, christopher tyerman wrote: > On 18/05/2022 09:48, Greg KH wrote:> On Tue, May 17, 2022 at 07:42:46PM > +0100, linuxkernel wrote: > >> hello I, looking at writing a kernel driver for the Texas instruments > >> TPS92662A/TPS92663A which uses a "UART bus" e.g > >> |-----------| |-----------| |-----------| |-----------| > >> | | | TPS92662A | | TPS92662A | | TPS92663A | > >> | UART | | -Q1 | | -Q1 | | -Q1 | > >> | | | | | | | | > >> |-----------| |-----------| |-----------| |-----------| > >> | | | | > >> | | | | > >> | | | | > >> ----------------------------------------------------- > >> and control communications by specific sequencing. configuring this > >> appears to require being able to pull RX Low > >>> The MCU unit can reset the device UART and protocol state machine at > >>> any time by holding the RX input low for a period of at least 12 bit > >>> times (16 × 12 CLK periods). > >> practically i can always pull RX low via other means, but if the UART > >> can do this it would be better and be more generic. > > How can your UART do that in a generic way? thanks, greg k-h sorry > > im not sure i explained that clearly > > i also appear to have, not got the previous diagram right, (corrected in > this reply for clarity) Im still getting used to plain text emails. > > im not that familiar with UARTs in practice, and after re-reading i think i > could have phrased that better, as had issue with MCU/IC UART TX/RX > perspectives > > e.g > > |-----------| |-----------| > | TX|-----------|RX | > | MCU UART | | IC | > | RX|-----------|TX | > |-----------| |-----------| > > from the mcu perspective its the TX pin, while from the RX pin from the ICs > perspective. > > from that perspective i need to pull the ICs RX pin low which means pull the > MCU UART TX pin Low. > > now this seems conceptually similar to a "break condition" > > which some tty functions can produce > e.g > static int send_break(struct tty_struct *tty, unsigned int duration) > [https://elixir.bootlin.com/linux/latest/source/drivers tty/tty_io.c#L2461] > > with which i might me half way to answering my own question > > i can't see call to that via serdev, or equivalent > That is correct, serdev can not handle break conditions on the tty port right now. You need to write a "real" serial driver if you want to have that type of control over the connection. You could propose a patch for serdev to handle break, try that and submit it and see if the serdev maintainer agrees that it is a worth addition. thanks, greg k-h
