Patch "tty/serial: at91: fix hardware handshake on Atmel platforms" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

tty/serial: at91: fix hardware handshake on Atmel platforms

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tty-serial-at91-fix-hardware-handshake-on-atmel-platforms.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Richard Genoud <[email protected]>
Date: Thu, 27 Oct 2016 18:04:06 +0200
Subject: tty/serial: at91: fix hardware handshake on Atmel platforms
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From: Richard Genoud <[email protected]>

commit 9bcffe7575b721d7b6d9b3090fe18809d9806e78 upstream.

After commit 1cf6e8fc8341 ("tty/serial: at91: fix RTS line management when hardware handshake is enabled"), the hardware handshake wasn't functional anymore on Atmel platforms (beside 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. (Nevertheless, it's very useful if low latencies are needed.)

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 must 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 (DMAC flavour), AT91SAM9G20-EK (PDC flavour), SAMA5D2xplained (FIFO flavour).

* the list may not be exhaustive

---
 drivers/tty/serial/atmel_serial.c |   26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

@@ -2178,11 +2178,29 @@ static void atmel_set_termios(struct uar
 		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 (there must be an internal pull-up
+			 * responsible for this behaviour).
+			 * 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 */

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/tty-serial-at91-fix-hardware-handshake-on-atmel-platforms.patch