GPIO-driven RTS on TI hardware with 8250_omap driver

[Andy Shevchenko]

+Peter, Russell, and Matwey.

I suggest you to ask people I added to the Cc list.

On Sat, Dec 26, 2015 at 6:17 PM, Ильяс Гасанов <torso.nafi at gmail.com> wrote:
> Hello.
>
> We are upgrading to the 4.1.x kernel for our smart metering appliance
> project, which is based on TI's Sitara hardware (AM335x SoC), and I
> decided to switch from omap-serial legacy driver to the newer
> 8250-based one. It marginally increases throughput efficiency, CPU
> cycle wise, among other goodies, but I'm looking to implement a rather
> important feature that is present in the legacy driver, but the newer
> one is lacking.
>
> Namely, our project makes use of RS232<->RS485 converters, which in
> turn need to consume RTS signals to switch between Rx and Tx modes at
> the RS485 side, due to the bus variant we use being half-duplex.
> However, the already manufactured hardware is already designed to make
> the use of certain pins to take the RTS signal from, which can only be
> configured as GPIO for that purpose (in other words, no "native" UART
> RTS) - and basically redesigning the h/w configuration now is
> definitely out of question. The omap-serial driver already provides
> FDT options for that, named "rts-gpio", "rs485-rts-active-high" etc.
>
> As far as I could ascertain, the 8250_omap driver (as well as the 8250
> framework itself) at the moment lacks the means to make use of GPIO
> pins for that purpose. While trying to implement it myself, I noticed
> that the legacy driver has it made in a comparably straightforward
> approach, via dispatching the code to switch the pin in its .start_tx
> and .stop_tx handlers, and some timing adjustments. Unfortunately, the
> situation with 8250-based drivers is different - the aforementioned
> handlers are provided by the 8250_core module and are common for all
> drivers within the framework.
>
> At first, I thought that implementing such feature for the 8250
> framework itself sounds like a good idea, but after reading this
> particular post:
> http://lists.infradead.org/pipermail/linux-arm-kernel/2014-July/271377.html
> I decided to comply with the point of view specified there. However,
> I'm not that familiar with the 8250 framework internals (or serial
> internals at all, for that matter), and my time is quite short, so I
> would appreciate much any useful directions on how to do it
> hardware-specific style, which functions/structs/handlers to use, etc.
> Of particular interest is the following part:
>
>> I don't care whether the drive does it via serial_out magic or a more explicit hook but it doesn't belong here in core code.
>
> Any ideas/clarifications on what might be meant on that part?
>
> Regards,
> Ilyas G.
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-- 
With Best Regards,
Andy Shevchenko