GPIO-driven RTS on TI hardware with 8250_omap driver

[Matwey V. Kornilov]

Andy,

The half of what is described here are implemented in my patches.
But I cannot understand the other half. Each of six AM335x UARTs has
RTS/CTS pins which are controlled by pinmux in device tree, no magic
required here.


2015-12-27 15:47 GMT+03:00 Andy Shevchenko <andy.shevchenko at gmail.com>:
> +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
>



-- 
With best regards,
Matwey V. Kornilov.
Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
119991, Moscow, Universitetsky pr-k 13, +7 (495) 9392382