Re: [RFC PATCH 3/7] serial: 8250_dwlib: Implement SW half duplex support

[Andy Shevchenko]

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