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Date: Tue, 29 Mar 2016 08:59:29 -0700
From: Bjorn Andersson <bjorn.andersson@...aro.org>
To: Stephen Boyd <stephen.boyd@...aro.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-arm@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-serial@...r.kernel.org, linux-arm-msm@...r.kernel.org,
"Ivan T. Ivanov" <iivanov.xz@...il.com>,
Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
Andy Gross <andy.gross@...aro.org>,
Matthew McClintock <mmcclint@...eaurora.org>
Subject: Re: [PATCH] tty: serial: msm: Support more bauds
On Fri 25 Mar 14:35 PDT 2016, Stephen Boyd wrote:
> The msm_find_best_baud() function is written with the assumption
> that the port->uartclk rate is fixed to a particular rate at boot
> time, but now this driver changes that clk rate at runtime when
> the baud is changed.
>
> The way the hardware works is that an input clk rate comes from
> the clk controller into the uart hw block. That rate is typically
> 1843200 or 3686400 Hz. That rate can then be divided by an
> internal divider in the hw block to achieve a particular baud on
> the serial wire. msm_find_best_baud() is looking for that divider
> value.
>
> A few things are wrong with the way the code is written. First,
> it assumes that the maximum baud that the uart can support if the
> clk rate is fixed at boot is 460800, which would correspond to an
> input clk rate of 230400 * 16 == 3686400 Hz. Except some devices
> have a boot rate of 1843200 Hz or max baud of 115200, so
> achieving 230400 on those devices doesn't work at all because we
> don't increase the clk rate unless max baud is 460800.
>
> Second, we can't achieve bauds higher than 460800 that require
> anything besides a divisor of 1, because we always call
> msm_find_best_baud() with a fixed port->uartclk rate that will
> eventually be changed after we calculate the divisor. So if we
> need to get a baud of 500000, we'll just multiply that by 16 and
> hope that the clk can give us 500000 * 16 == 8000000 Hz, which it
> typically can't do. To really achieve 500000 baud, we need to get
> an input clk rate of 24000000 Hz and then divide that by 3 inside
> the uart hardware.
>
> Finally, we return success for bauds even when we can't actually
> achieve them. This means that when the user asks for 500000 baud,
> we actually get 921600 right now, but the user doesn't know that.
>
> Fix all of this by searching through the divisor and clk rate
> space with a combination of clk_round_rate() and baud
> calculations, keeping track of the best clk rate and divisor we
> find if we can't get an exact match. Typically we can get an
> exact match with a divisor of 1, but sometimes we need to keep
> track and try more frequencies. On my msm8916 device, this
> results in all standard bauds in baud_table being supported
> except for 1800, 576000, 1152000, and 4000000.
>
> Fixes: 850b37a71bde ("tty: serial: msm: Remove 115.2 Kbps maximum baud rate limitation")
> Cc: "Ivan T. Ivanov" <iivanov.xz@...il.com>
> Cc: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
> Cc: Andy Gross <andy.gross@...aro.org>
> Cc: Matthew McClintock <mmcclint@...eaurora.org>
> Signed-off-by: Stephen Boyd <stephen.boyd@...aro.org>
Acked-by: Bjorn Andersson <bjorn.andersson@...aro.org>
Regards,
Bjorn
> ---
> drivers/tty/serial/msm_serial.c | 99 +++++++++++++++++++++++++++--------------
> 1 file changed, 66 insertions(+), 33 deletions(-)
>
> diff --git a/drivers/tty/serial/msm_serial.c b/drivers/tty/serial/msm_serial.c
> index 96d3ce8dc2dc..3d28be85bd46 100644
> --- a/drivers/tty/serial/msm_serial.c
> +++ b/drivers/tty/serial/msm_serial.c
> @@ -861,37 +861,72 @@ struct msm_baud_map {
> };
>
> static const struct msm_baud_map *
> -msm_find_best_baud(struct uart_port *port, unsigned int baud)
> +msm_find_best_baud(struct uart_port *port, unsigned int baud,
> + unsigned long *rate)
> {
> - unsigned int i, divisor;
> - const struct msm_baud_map *entry;
> + struct msm_port *msm_port = UART_TO_MSM(port);
> + unsigned int divisor, result;
> + unsigned long target, old, best_rate = 0, diff, best_diff = ULONG_MAX;
> + const struct msm_baud_map *entry, *end, *best;
> static const struct msm_baud_map table[] = {
> - { 1536, 0x00, 1 },
> - { 768, 0x11, 1 },
> - { 384, 0x22, 1 },
> - { 192, 0x33, 1 },
> - { 96, 0x44, 1 },
> - { 48, 0x55, 1 },
> - { 32, 0x66, 1 },
> - { 24, 0x77, 1 },
> - { 16, 0x88, 1 },
> - { 12, 0x99, 6 },
> - { 8, 0xaa, 6 },
> - { 6, 0xbb, 6 },
> - { 4, 0xcc, 6 },
> - { 3, 0xdd, 8 },
> - { 2, 0xee, 16 },
> { 1, 0xff, 31 },
> - { 0, 0xff, 31 },
> + { 2, 0xee, 16 },
> + { 3, 0xdd, 8 },
> + { 4, 0xcc, 6 },
> + { 6, 0xbb, 6 },
> + { 8, 0xaa, 6 },
> + { 12, 0x99, 6 },
> + { 16, 0x88, 1 },
> + { 24, 0x77, 1 },
> + { 32, 0x66, 1 },
> + { 48, 0x55, 1 },
> + { 96, 0x44, 1 },
> + { 192, 0x33, 1 },
> + { 384, 0x22, 1 },
> + { 768, 0x11, 1 },
> + { 1536, 0x00, 1 },
> };
>
> - divisor = uart_get_divisor(port, baud);
> + best = table; /* Default to smallest divider */
> + target = clk_round_rate(msm_port->clk, 16 * baud);
> + divisor = DIV_ROUND_CLOSEST(target, 16 * baud);
> +
> + end = table + ARRAY_SIZE(table);
> + entry = table;
> + while (entry < end) {
> + if (entry->divisor <= divisor) {
> + result = target / entry->divisor / 16;
> + diff = abs(result - baud);
> +
> + /* Keep track of best entry */
> + if (diff < best_diff) {
> + best_diff = diff;
> + best = entry;
> + best_rate = target;
> + }
>
> - for (i = 0, entry = table; i < ARRAY_SIZE(table); i++, entry++)
> - if (entry->divisor <= divisor)
> - break;
> + if (result == baud)
> + break;
> + } else if (entry->divisor > divisor) {
> + old = target;
> + target = clk_round_rate(msm_port->clk, old + 1);
> + /*
> + * The rate didn't get any faster so we can't do
> + * better at dividing it down
> + */
> + if (target == old)
> + break;
> +
> + /* Start the divisor search over at this new rate */
> + entry = table;
> + divisor = DIV_ROUND_CLOSEST(target, 16 * baud);
> + continue;
> + }
> + entry++;
> + }
>
> - return entry; /* Default to smallest divider */
> + *rate = best_rate;
> + return best;
> }
>
> static int msm_set_baud_rate(struct uart_port *port, unsigned int baud,
> @@ -900,22 +935,20 @@ static int msm_set_baud_rate(struct uart_port *port, unsigned int baud,
> unsigned int rxstale, watermark, mask;
> struct msm_port *msm_port = UART_TO_MSM(port);
> const struct msm_baud_map *entry;
> - unsigned long flags;
> -
> - entry = msm_find_best_baud(port, baud);
> -
> - msm_write(port, entry->code, UART_CSR);
> -
> - if (baud > 460800)
> - port->uartclk = baud * 16;
> + unsigned long flags, rate;
>
> flags = *saved_flags;
> spin_unlock_irqrestore(&port->lock, flags);
>
> - clk_set_rate(msm_port->clk, port->uartclk);
> + entry = msm_find_best_baud(port, baud, &rate);
> + clk_set_rate(msm_port->clk, rate);
> + baud = rate / 16 / entry->divisor;
>
> spin_lock_irqsave(&port->lock, flags);
> *saved_flags = flags;
> + port->uartclk = rate;
> +
> + msm_write(port, entry->code, UART_CSR);
>
> /* RX stale watermark */
> rxstale = entry->rxstale;
> --
> 2.8.0.rc4
>
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