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Date: Mon, 14 Feb 2022 11:38:36 +0200
From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Zev Weiss <zev@bewilderbeest.net>
Subject: Re: [PATCH v2] serial: 8250_aspeed_vuart: add PORT_ASPEED_VUART port
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Cc: linux-aspeed@lists.ozlabs.org, Konstantin Aladyshev <aladyshev22@gmail.com>,
 Andrew Jeffery <andrew@aj.id.au>,
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On Thu, Feb 10, 2022 at 04:42:03PM -0800, Zev Weiss wrote:
> Commit 54da3e381c2b ("serial: 8250_aspeed_vuart: use UPF_IOREMAP to
> set up register mapping") fixed a bug that had, as a side-effect,
> prevented the 8250_aspeed_vuart driver from enabling the VUART's
> FIFOs.  However, fixing that (and hence enabling the FIFOs) has in
> turn revealed what appears to be a hardware bug in the ASPEED VUART in
> which the host-side THRE bit doesn't get if the BMC-side receive FIFO
> trigger level is set to anything but one byte.  This causes problems
> for polled-mode writes from the host -- for example, Linux kernel
> console writes proceed at a glacial pace (less than 100 bytes per
> second) because the write path waits for a 10ms timeout to expire
> after every character instead of being able to continue on to the next
> character upon seeing THRE asserted.  (GRUB behaves similarly.)
> 
> As a workaround, introduce a new port type for the ASPEED VUART that's
> identical to PORT_16550A as it had previously been using, but with
> UART_FCR_R_TRIG_00 instead to set the receive FIFO trigger level to
> one byte, which (experimentally) seems to avoid the problematic THRE
> behavior.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

> Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
> Tested-by: Konstantin Aladyshev <aladyshev22@gmail.com>
> Fixes: 54da3e381c2b ("serial: 8250_aspeed_vuart: use UPF_IOREMAP to set up register mapping")
> ---
> 
> Changes since v1 [0]:
>  - Added Fixes: tag
>  - Shifted PORT_* constant down into an unused gap
> 
> [0] https://lore.kernel.org/all/20220209203414.23491-1-zev@bewilderbeest.net/
> 
>  drivers/tty/serial/8250/8250_aspeed_vuart.c | 2 +-
>  drivers/tty/serial/8250/8250_port.c         | 8 ++++++++
>  include/uapi/linux/serial_core.h            | 3 +++
>  3 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_aspeed_vuart.c b/drivers/tty/serial/8250/8250_aspeed_vuart.c
> index 2350fb3bb5e4..c2cecc6f47db 100644
> --- a/drivers/tty/serial/8250/8250_aspeed_vuart.c
> +++ b/drivers/tty/serial/8250/8250_aspeed_vuart.c
> @@ -487,7 +487,7 @@ static int aspeed_vuart_probe(struct platform_device *pdev)
>  	port.port.irq = irq_of_parse_and_map(np, 0);
>  	port.port.handle_irq = aspeed_vuart_handle_irq;
>  	port.port.iotype = UPIO_MEM;
> -	port.port.type = PORT_16550A;
> +	port.port.type = PORT_ASPEED_VUART;
>  	port.port.uartclk = clk;
>  	port.port.flags = UPF_SHARE_IRQ | UPF_BOOT_AUTOCONF | UPF_IOREMAP
>  		| UPF_FIXED_PORT | UPF_FIXED_TYPE | UPF_NO_THRE_TEST;
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 3b12bfc1ed67..973870ebff69 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -307,6 +307,14 @@ static const struct serial8250_config uart_config[] = {
>  		.rxtrig_bytes	= {1, 32, 64, 112},
>  		.flags		= UART_CAP_FIFO | UART_CAP_SLEEP,
>  	},
> +	[PORT_ASPEED_VUART] = {
> +		.name		= "ASPEED VUART",
> +		.fifo_size	= 16,
> +		.tx_loadsz	= 16,
> +		.fcr		= UART_FCR_ENABLE_FIFO | UART_FCR_R_TRIG_00,
> +		.rxtrig_bytes	= {1, 4, 8, 14},
> +		.flags		= UART_CAP_FIFO,
> +	},
>  };
>  
>  /* Uart divisor latch read */
> diff --git a/include/uapi/linux/serial_core.h b/include/uapi/linux/serial_core.h
> index c4042dcfdc0c..8885e69178bd 100644
> --- a/include/uapi/linux/serial_core.h
> +++ b/include/uapi/linux/serial_core.h
> @@ -68,6 +68,9 @@
>  /* NVIDIA Tegra Combined UART */
>  #define PORT_TEGRA_TCU	41
>  
> +/* ASPEED AST2x00 virtual UART */
> +#define PORT_ASPEED_VUART	42
> +
>  /* Intel EG20 */
>  #define PORT_PCH_8LINE	44
>  #define PORT_PCH_2LINE	45
> -- 
> 2.35.1
> 

-- 
With Best Regards,
Andy Shevchenko