On Thu, Jul 24, 2008 at 12:57:06PM +0100, Alan Cox wrote: > On Thu, 24 Jul 2008 13:47:29 +0200 > Laurent Pinchart <laurentp@xxxxxxxxxxxxxxxxx> wrote: > > > This patch adds support for the automatic RS485 direction control feature > > present in 16850 UARTs. > > > > A new termios c_cflag, CARTS, is introduced to configure automatic direction > > control from userspace. > > > > This is a first proposal. I'm open to suggestions regarding the CARTS name. > > I assume the CARTS flag will have to be added to all asm/termbits.h headers. > > Why are the termios bits definitions platform specific ? > > Because many of them are made to match up with the existing previous > OS on those systems (eg to match with Alpha OSF). > > I've no fundamental objection to this but I do wonder whether this > feature belongs in setserial rather than termios as it is so chip > specific ? It may be specific to certain chips, but of those drivers which support hardware RS485 modes to date, everyone has invented their own ioctl to switch to RS485 mode. There's no unification between any of the serial drivers for this. On devices which don't support hardware RS485, what should be done is the termios bit remains clear, so that programs can tell if the port doesn't support it (as per POSIX.) I would also stress that this feature should be limited to enabling _hardware_ RS485 support, and not software emulation of that. The reason being is that with plain 16550 UARTs, the best you can do with interrupts is to know when the last character is transferred out of the transmit holding register into the transmit shift register - in other words, before the last character has finished transmission. Knowing when that character has been transmitted involves polling the LSR - and having the kernel sit in a loop waiting for that event is extremely wasteful. Scheduling to other processes... well, you don't know if there's some timing constraint - eg, must deassert RTS 1us after the stop bit of the last character. Basically, software RS485 is very yucky, and we've always resisted having that support in the kernel. -- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: -- 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
