2016-04-18 14:34 GMT+02:00 Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>:
> Hi Arnd,
>
> CC Richard (serial-mctrl-gpio)
> CC Grant (ePAPR successor) and Frank
>
> On Sat, Apr 16, 2016 at 6:30 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> On Thursday 14 April 2016 14:13:19 Geert Uytterhoeven wrote:
>>> Document a set of generic properties for describing UARTs in a
>>> device tree:
>>> 1. The GPIO modem control properties are currently duplicated across
>>> hardware-specific binding documentation,
>>> 2. The property for dedicated RTS/CTS hardware flow control lines is
>>> already supported by several drivers, albeit with a vendor-specific
>>> prefix, hence make it generic.
>>>
>>> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
>>
>> Originally the ISA 8250 uart binding (from ieee) was used as the
>> template for other uart bindings. How about documenting the parts that
>> are used in 8250-of today (current-speed, clock-frequency,
>> reg-offset, reg-shift, fifo-size, reg-io-width, auto-flow-control)
>> in the same file?
>
> I don't think we have the habit of documenting (duplicating) bindings for ePAPR
> under Documentation/devicetree/bindings/. Perhaps we should?
>
> Apart from that, most of the properties you mention look legacy or overly
> broad too me.
>
> - current-speed: This is configuration, not a property of the hardware.
> For the console, this has been deprecated by appending the serial config
> to chosen/stdout-path (e.g. "serial0:115200n8").
> For non-consoles, its use is debatable, IMHO.
> It's users are mostly legacy powerpc and early adaptors of DT on ARM.
> - clock-frequency: Legacy predating the Common Clock Framework.
> Any modern SoC uses clock specifiers with clock handles pointing to clock
> providers.
> - reg-offset, reg-shift, reg-io-width: These are much broader than serial,
> and IMHO thus don't belong in
> Documentation/devicetree/bindings/serial/serial.txt.
> - auto-flow-control: Looks a bit like a vague version of "uart-has-rtscts".
> Documentation/devicetree/bindings/serial/8250.txt doesn't make it clear
> whether this is about hardware capabilities or software configuration.
> Even the driver doesn't make it clear:
> #define UART_CAP_AFE (1 << 11) /* MCR-based hw flow control */
> "MCR" could mean RTS/CTS, DSR/DTR, ...
> - fifo-size: This one could be generic. atmel-usart uses a vendor-specific
> version "atmel,fifo-size".
>
> I suggest we move forward with my initial set, as I have patches that depend on
> them? We can always add more properties later.
>
>>> - out1-gpios: Must contain a GPIO specifier, referring to the GPIO pin to be
>>> used as the UART's OUT1 line.
>>> - out2-gpios: Must contain a GPIO specifier, referring to the GPIO pin to be
>>> used as the UART's OUT2 line.
>>
>> I had to look up what OUT1 and OUT2 are, but I still don't see how you'd
>> implement them using a GPIO line: From all I can tell, these are usually
>> internal registers in a hardware uart but they are not assigned to an
>> external line on the standard db9 or even the old db25 connectors. Should
>> we drop these instead?
>
> They're indeed fairly exotic, and they're burried deeply in the ns16550
> datasheet. We do have TIOCM_OUT1 and TIOCM_LOOP in asm-generic/termios.h,
> probably for obscure historical reasons.
>
> If we drop them, I guess they should be removed from the helper code in
> drivers/tty/serial/serial_mctrl_gpio.c, too? There don't seem to be any
> current users.
I must confess that I don't really know what TIOCM_OUT1&2 are for.
(I implemented them for completeness)
But it seems that OUT2 is used in some drivers:
drivers/tty/serial/omap-serial.c:
/*
* Most PC uarts need OUT2 raised to enable interrupts.
*/
up->port.mctrl |= TIOCM_OUT2;
>
>> On a related note, do you think it would be possible to do a bit-banged
>> uart if we defined gpio lines for rxd and txd?
>
> Sure we can. Whether it would work well is another question ;-)
> Regardless of flow control, byte transmission and reception has hard real-time
> requirements due to the implicit clocking.
> Bit-banging i2c and spi (master) is much easier, as clocking is explicit.
> Even i2c slave is easier, as the slave can stretch cycles.
+1
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