Date:   Wed, 11 Mar 2020 12:22:04 +0000
From:   Yash Shah <yash.shah@...ive.com>
To:     Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>
CC:     Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "jslaby@...e.com" <jslaby@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "kernel-team@...roid.com" <kernel-team@...roid.com>,
        Sachin Ghadi <sachin.ghadi@...ive.com>
Subject: RE: [PATCH] tty: sifive: Finish transmission before changing the
 clock

> -----Original Message-----
> From: linux-riscv <linux-riscv-bounces@...ts.infradead.org> On Behalf Of
> Palmer Dabbelt
> Sent: 07 March 2020 09:57
> To: Paul Walmsley <paul.walmsley@...ive.com>
> Cc: Palmer Dabbelt <palmerdabbelt@...gle.com>; Greg KH
> <gregkh@...uxfoundation.org>; jslaby@...e.com; linux-
> kernel@...r.kernel.org; Palmer Dabbelt <palmer@...belt.com>; linux-
> serial@...r.kernel.org; Paul Walmsley <paul.walmsley@...ive.com>; linux-
> riscv@...ts.infradead.org; kernel-team@...roid.com
> Subject: [PATCH] tty: sifive: Finish transmission before changing the clock
> 
> From: Palmer Dabbelt <palmerdabbelt@...gle.com>
> 
> SiFive's UART has a software controller clock divider that produces the final
> baud rate clock.  Whenever the clock that drives the UART is changed this
> divider must be updated accordingly, and given that these two events are
> controlled by software they cannot be done atomically.
> During the period between updating the UART's driving clock and internal
> divider the UART will transmit a different baud rate than what the user has
> configured, which will probably result in a corrupted transmission stream.
> 
> The SiFive UART has a FIFO, but due to an issue with the programming
> interface there is no way to directly determine when the UART has finished
> transmitting.  We're essentially restricted to dead reckoning in order to figure
> that out: we can use the FIFO's TX busy register to figure out when the last
> frame has begun transmission and just delay for a long enough that the last
> frame is guaranteed to get out.
> 
> As far as the actual implementation goes: I've modified the existing existing
> clock notifier function to drain both the FIFO and the shift register in on
> PRE_RATE_CHANGE.  As far as I know there is no hardware flow control in
> this UART, so there's no good way to ask the other end to stop transmission
> while we can't receive (inserting software flow control messages seems like a
> bad idea here).
> 
> Signed-off-by: Palmer Dabbelt <palmerdabbelt@...gle.com>
> ---
> I have not tested this, as I don't have any hardware.  I'm also not even
> remotely familiar with the serial subsystem, so I don't know if there's a
> better way of going about this.  I'm specifically worried about a udelay() that
> could be quite long.  Maybe some sort of "delay for short times, sleep for
> long times" approach would be better?
> 
> I don't know if this manifests in practice on existing hardware when running
> real workloads, but I'd be willing to bet that it would be possible to trigger
> the bug on purpose as by my calculations there's about a 10k cycle window in
> which the clock can't change.  IIRC there's a lot of instability when changing
> the clock frequency on the HiFive Unleashed so I doubt people are going to
> stumble across the issue regularly in practice.
> 
>  drivers/tty/serial/sifive.c | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/serial/sifive.c b/drivers/tty/serial/sifive.c index
> d5f81b98e4d7..d34031e842d0 100644
> --- a/drivers/tty/serial/sifive.c
> +++ b/drivers/tty/serial/sifive.c
> @@ -618,10 +618,10 @@ static void sifive_serial_shutdown(struct uart_port
> *port)
>   *
>   * On the V0 SoC, the UART IP block is derived from the CPU clock source
>   * after a synchronous divide-by-two divider, so any CPU clock rate change
> - * requires the UART baud rate to be updated.  This presumably could
> corrupt any
> - * serial word currently being transmitted or received.  It would probably
> - * be better to stop receives and transmits, then complete the baud rate
> - * change, then re-enable them.
> + * requires the UART baud rate to be updated.  This presumably corrupts
> + any
> + * serial word currently being transmitted or received.  In order to
> + avoid
> + * corrupting the output data stream, we drain the transmit queue
> + before
> + * allowing the clock's rate to be changed.
>   */
>  static int sifive_serial_clk_notifier(struct notifier_block *nb,
>  				      unsigned long event, void *data) @@ -
> 629,6 +629,26 @@ static int sifive_serial_clk_notifier(struct notifier_block
> *nb,
>  	struct clk_notifier_data *cnd = data;
>  	struct sifive_serial_port *ssp = notifier_to_sifive_serial_port(nb);
> 
> +	if (event == PRE_RATE_CHANGE) {
> +		/*
> +		 * The TX watermark is always set to 1 by this driver, which
> +		 * means that the TX busy bit will lower when there are 0
> bytes
> +		 * left in the TX queue -- in other words, when the TX FIFO is
> +		 * empty.
> +		 */
> +		__ssp_wait_for_xmitr(ssp);
> +		/*
> +		 * On the cycle the TX FIFO goes empty there is still a full
> +		 * UART frame left to be transmitted in the shift register.
> +		 * The UART provides no way for software to directly
> determine
> +		 * when that last frame has been transmitted, so we just
> sleep
> +		 * here instead.  As we're not tracking the number of stop
> bits
> +		 * they're just worst cased here.  The rest of the serial
> +		 * framing parameters aren't configurable by software.
> +		 */
> +		udelay(DIV_ROUND_UP(12 * 1000 * 1000, ssp->baud_rate));
> +	}
> +
>  	if (event == POST_RATE_CHANGE && ssp->clkin_rate != cnd-
> >new_rate) {
>  		ssp->clkin_rate = cnd->new_rate;
>  		__ssp_update_div(ssp);
> --
> 2.25.1.481.gfbce0eb801-goog
> 

A quick test on HiFive Unleashed board showed some improvements.
Prior to this patch, I have been observing some random corrupted characters on serial console when continuously changing the CPU clock rate.
After applying this patch I don't see those corrupted characters anymore while changing the clock rate.

Tested-by: Yash Shah <yash.shah@...ive.com>

This observation is based on a quick initial test on HiFive Unleashed. I am planning to further test it by inducing the error on purpose. Will try to update the result soon.

- Yash

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