On Mon, Mar 7, 2022 at 12:01 AM Lukas Wunner <lukas@xxxxxxxxx> wrote: > On Wed, Mar 02, 2022 at 11:56:02AM +0200, Ilpo Järvinen wrote: > > This patch enables support for SW half-duplex mode. Synopsys > > DesignWare UART has a build-in support for the RS485 protocol > > from IP version 4.0 onward with dedicated RE/DE_EN registers. > > This patch enables RS485 either using dedicated registers or > > em485 as fallback. > > > > In order to select preference for SW half-duplex mode (em485 + > > RE/DE_EN) over HW managed one, as both are supported under > > some configurations, SER_RS485_SW_RX_OR_TX flag is added to > > serial_rs485. > > > > This patch depends on UART_CAP_NOTEMT which is not provided > > by this series but another one: > > https://lore.kernel.org/all/20210204161158.643-1-etremblay@xxxxxxxxxxxxxxxxxxxx/ > > I don't see any benefit in using software emulated RTS assertion > if hardware support is present. It just consumes more CPU time > and is slower to deassert RTS, thereby increasing bus turn-around time. > > So if hardware support is present, I think you always want to > use that and you need to fallback to software emulation only > if hardware support is missing (i.e. on IP versions < 4). No, this is not a good idea because presence of the HW support does not guarantee we will have pins on PCB level and/or properly muxed by firmware, or be able to be properly muxed by software (OS). Or am I missing something here and you have an idea how to enable RS485 in that case? > The registers you're using here, DW_UART_RE_EN and DW_UART_DE_EN > don't seem to be present on older IP versions. I'm looking at > the databook for version 3.04a and those registers aren't mentioned: > > https://linux-sunxi.org/images/d/d2/Dw_apb_uart_db.pdf > > So the software emulation you've implemented here won't help with > older IP and the newer IP doesn't need it because it supports > RTS assertion in hardware. Is that correct? If so, I'd suggest > not supporting DW_UART_TCR_XFER_MODE_SW_DE_OR_RE at all. > > A number of people have attempted to add rs485 software emulation > to 8250_dw.c but noone ever pursued it into mainline. The last > attempt was by Heiko Stübner, who used patches by Giulio Benetti (+cc): > > https://lore.kernel.org/linux-serial/20200517215610.2131618-1-heiko@xxxxxxxxx/ > > FWIW, newer TI Sitara SoCs have also added hardware support for > rs485 and in this (not yet upstreamed) patch, I likewise chose > to use software emulation only if hardware support is not available: > > https://github.com/l1k/linux/commit/82c989617a05 -- With Best Regards, Andy Shevchenko
