[PATCH] tty/serial: at91: fix hardware handshake on Atmel platforms

[Richard Genoud <richard.genoud@gmail.com>]

commit 1cf6e8fc8341 ("tty/serial: at91: fix RTS line management when
hardware handshake is enabled"), despite its title, broke hardware
handshake on *every* Atmel platforms.

The only one partially working is the SAMA5D2.

To understand why, one has to understand the flag ATMEL_US_USMODE_HWHS
first:
Before commit 1cf6e8fc8341 ("tty/serial: at91: fix RTS line management
when hardware handshake is enabled"), this flag was never set.
Thus, the CTS/RTS where only handled by serial_core (and everything
worked just fine).

This commit introduced the use of the ATMEL_US_USMODE_HWHS flag,
enabling it for all boards when the user space enables flow control.

When the ATMEL_US_USMODE_HWHS is set, the Atmel USART controller
handles a part of the flow control job:
- disable the transmitter when the CTS pin gets high.
- drive the RTS pin high when the DMA buffer transfer is completed or
  PDC RX buffer full or RX FIFO is beyond threshold. (depending on the
  controller version).

NB: This feature is *not* mandatory for the flow control to work.

Now, the specifics of the ATMEL_US_USMODE_HWHS flag:

- For platforms with DMAC and no FIFOs (sam9x25, sam9x35, sama5D3,
sama5D4, sam9g15, sam9g25, sam9g35)* this feature simply doesn't work.
( source: https://lkml.org/lkml/2016/9/7/598 )
Tested it on sam9g35, the RTS pins always stays up, even when RXEN=1
or a new DMA transfer descriptor is set.
=> ATMEL_US_USMODE_HWHS should not be used for those platforms

- For platforms with a PDC (sam926{0,1,3}, sam9g10, sam9g20, sam9g45,
sam9g46)*, there's another kind of problem. Once the flag
ATMEL_US_USMODE_HWHS is set, the RTS pin can't be driven anymore via
RTSEN/RTSDIS in USART Control Register. The RTS pin can only be driven
by enabling/disabling the receiver or setting RCR=RNCR=0 in the PDC
(Receive (Next) Counter Register).
=> Doing this is beyond the scope of this patch and could add other
bugs, so the original (and working) behaviour should be set for those
platforms (meaning ATMEL_US_USMODE_HWHS flag should be unset).

- For platforms with a FIFO (sama5d2)*, the RTS pin is driven according
to the RX FIFO thresholds, and can be also driven by RTSEN/RTSDIS in
USART Control Register. No problem here.
(This was the use case of commit 1cf6e8fc8341 ("tty/serial: at91: fix
RTS line management when hardware handshake is enabled"))
NB: If the CTS pin declared as a GPIO in the DTS, (for instance
cts-gpios = <&pioA PIN_PB31 GPIO_ACTIVE_LOW>), the transmitter will be
disabled.
=> ATMEL_US_USMODE_HWHS flag can be set for this platform ONLY IF the
CTS pin is not a GPIO.

So, the only case when ATMEL_US_USMODE_HWHS can be enabled is when
(atmel_use_fifo(port) &&
 !mctrl_gpio_to_gpiod(atmel_port->gpios, UART_GPIO_CTS))

Tested on all Atmel USART controller flavours:
 AT91SAM9G35-CM, AT91SAM9G20-EK and SAMA5D2xplained
      ^^^^           ^^^^               ^^^^
  (DMAC flavour), (PDC flavour) and (FIFO flavour)

Changes since v4:
 - the mctrl_gpio_use_rtscts() is gone since it was atmel_serial
 specific. (so patch 1 is gone)
 - patches 2 and 3 have been merged together since it didn't make
 a lot of sense to correct the GPIO case in one separate patch.
 - ATMEL_US_USMODE_HWHS is now unset for platform with PDC

Changes since v3:
 - remove superfluous #include <linux/err.h> (thanks to Uwe)
 - rebase on next-20160930

Changes since v2:
 - remove IS_ERR_OR_NULL() test in patch 1/3 as Uwe suggested.
 - fix typos in patch 2/3
 - rebase on next-20160927
 - simplify the logic in patch 3/3.

Changes since v1:
 - Correct patch 1 with the error found by kbuild.
 - Add Alexandre's Acked-by on patch 2
 - Rewrite patch 3 logic in the light of the on-going discussion
   with Cyrille and Alexandre.

* the list may not be exhaustive

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
---
 drivers/tty/serial/atmel_serial.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

 I think this should go in the stable tree since it fixes the flow
 control broken since v4.0.
 But It won't compile on versions before 4.9rc1 because:
 function atmel_use_fifo was introduced in 4.4.12 / 4.7
 variable atmel_port was introduced in 4.9rc1

 That's why I didn't add the Cc stable in the email body.


diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index fd8aa1f4ba78..2c7c45904ba7 100644
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -2132,11 +2132,28 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
 		mode |= ATMEL_US_USMODE_RS485;
 	} else if (termios->c_cflag & CRTSCTS) {
 		/* RS232 with hardware handshake (RTS/CTS) */
-		if (atmel_use_dma_rx(port) && !atmel_use_fifo(port)) {
-			dev_info(port->dev, "not enabling hardware flow control because DMA is used");
-			termios->c_cflag &= ~CRTSCTS;
-		} else {
+		if (atmel_use_fifo(port) &&
+		    !mctrl_gpio_to_gpiod(atmel_port->gpios, UART_GPIO_CTS)) {
+			/*
+			 * with ATMEL_US_USMODE_HWHS set, the controller will
+			 * be able to drive the RTS pin high/low when the RX
+			 * FIFO is above RXFTHRES/below RXFTHRES2.
+			 * It will also disable the transmitter when the CTS
+			 * pin is high.
+			 * This mode is not activated if CTS pin is a GPIO
+			 * because in this case, the transmitter is always
+			 * disabled.
+			 * If the RTS pin is a GPIO, the controller won't be
+			 * able to drive it according to the FIFO thresholds,
+			 * but it will be handled by the driver.
+			 */
 			mode |= ATMEL_US_USMODE_HWHS;
+		} else {
+			/*
+			 * For platforms without FIFO, the flow control is
+			 * handled by the driver.
+			 */
+			mode |= ATMEL_US_USMODE_NORMAL;
 		}
 	} else {
 		/* RS232 without hadware handshake */