What can I do so that this small but important change becomes part of
trunk & Linux kernel trunk?
2014-02-28 18:56 GMT+01:00 Alex Walker <alexaaronwal...@gmail.com>:
> Dear folks,
>
> I noticed that the bit timing of the UART in the WR703N / AR9331
> device is inaccurate and provide a patch to improve timing accuracy.
>
> I suppose to change
> #define AR933X_UART_MAX_STEP 0xffff
> to
> #define AR933X_UART_MAX_STEP 0x3333
> in
> linux-3.10.32/drivers/tty/serial/ar933x_uart.c
>
> The maximum value of UART_STEP should be 13107 as suggested by the
> AR9331 datasheet.
> See section 6.2.3:
> This value should range between 1310-13107 to
> maintain a better than +5% accuracy.
> 13107 corresponds to 0x3333.
>
> I did a detailed timing analysis using an oscilloscope. I'm using a
> different version of picocom which in turn uses termios2 to support
> arbitrary baud rates [2]. These are my findings. See [3] for the
> screenshots.
>
>
> Sending ASCII 'U' = 0x55 = 0b01010101
> This results in a nice square wave with the start and stop bit. The
> bit timing is generated from the 25 MHz internal frequency by a
> fractional baud rate generator. This is state of the art to generate
> 'odd' baud rates as 115200 from 'even' clock rates as 25 MHz.
>
> The setting of UART_CLOCK_SCALE and UART_CLOCK_STEP determine the bit
> rate of the UART. These settings are dynamically calculated when
> changing the baud rate.
>
> On: BARRIER BREAKER (Bleeding Edge, r39757)
> Linux myhost 3.10.32 #1 Fri Feb 28 11:03:52 CET 2014 mips GNU/Linux
>
> Baud 115200
> Screenshot 10 - 22
>
> You can see a significant jitter of each bit time. At this baud rate
> this is not yet a problem.
>
> Baud 921600
> Screenshot 23 - 38
>
> At this baud rate the jitter already accumulated to a significant
> portion of the bit time. This can make the connection unreliable.
>
> Baud 500000
> Screenshot 39 - 41
>
> The setting of UART_BAUD_STEP is much to high resulting in a jitter
> that prevented me from having a reliable loopback connection.
>
> I assume that for
>
> Uart Clock: 25000000
> Uart Baud: 500000
>
> the kernel calculates the following value
>
> Uart Scale: 15
> Uart Step: 41943
>
> This gist [1] for a user-landed code from the driver.
>
> The granulation of the bit timing is 640 nsec and the UART_STEP is
> 41943. This value violates the recommendation from the data sheet.
>
> I then applied my proposed change.
>
> New Kernel with patch 104-tty-ar933x_uart-fix-clock-step-max.patch applied
> Linux myhost 3.10.32 #1 Fri Feb 28 17:00:25 CET 2014 mips GNU/Linux
>
> 115200 Baud
> [ 182.150000] ar933x_uart_get_scale_step: clk 25000000, baud 115200,
> scale 20, step 12683
> Screenshot 42
> compare to screenshot 10 and see that the jitter is a little smaller.
>
> 500000 Baud
> [ 426.870000] ar933x_uart_get_scale_step: clk 25000000, baud 500000,
> scale 4, step 13107
> Screenshot 43
> As remainder(25E6, 5E5) is 0 there is no jitter with this
> configuration. The bit timing is near perfect.
>
> 1 MBaud
> [ 690.900000] ar933x_uart_get_scale_step: clk 25000000, baud 1000000,
> scale 1, step 10485
> Screenshot 44, 45
>
> 2 MBaud
> [ 804.230000] ar933x_uart_get_scale_step: clk 25000000, baud 2000000,
> scale 0, step 10485
> Screenshot 46, 47
>
> 3 MBaud
> The code fails so I propose to work on that ...
> [ 919.830000] ar933x_uart_get_scale_step: clk 25000000, baud 115200,
> scale 20, step 12683
>
> 921600 Baud
> [ 993.680000] ar933x_uart_get_scale_step: clk 25000000, baud 921600,
> scale 1, step 9663
> Screenshot 48, 49
> Finally, for the highest baudrate that is supported by an unpatched
> picoterm the timing is nearly perfect, too.
>
> Here is the patch:
>
> Signed-off-by: Alex Aaron Walker <alexaaronwal...@gmail.com>
> ---
> Index: linux-3.10.32/drivers/tty/serial/ar933x_uart.c
> ===================================================================
> --- linux-3.10.32.orig/drivers/tty/serial/ar933x_uart.c 2014-02-28
> 12:12:03.356386552 +0100
> +++ linux-3.10.32/drivers/tty/serial/ar933x_uart.c 2014-02-28
> 12:15:21.716502356 +0100
> @@ -33,7 +33,7 @@
> #define DRIVER_NAME "ar933x-uart"
>
> #define AR933X_UART_MAX_SCALE 0xff
> -#define AR933X_UART_MAX_STEP 0xffff
> +#define AR933X_UART_MAX_STEP 0x3333
>
> #define AR933X_UART_MIN_BAUD 300
> #define AR933X_UART_MAX_BAUD 3000000
> @@ -221,6 +221,7 @@
> *step = tstep;
> }
> }
> + printk("ar933x_uart_get_scale_step: clk %u, baud %u, scale %u,
> step %u\n", clk, baud, *scale, *step);
> }
>
> static void ar933x_uart_set_termios(struct uart_port *port,
>
> --
>
> All feedback welcome
>
> Alex
>
> [1] https://gist.github.com/anonymous/9275937
> [2] https://github.com/jmesmon/picocom.git
> [3] https://plus.google.com/113941111747937474864/posts/ZnmUHycHXbJ
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