Re: [PATCH] serial: 8250_aspeed_vuart: add PORT_ASPEED_VUART port type

[Zev Weiss <zev@bewilderbeest.net>]

On Wed, Feb 09, 2022 at 11:40:42PM PST, Greg Kroah-Hartman wrote:
>On Wed, Feb 09, 2022 at 12:34:14PM -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.
>>
>> Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
>> Tested-by: Konstantin Aladyshev <aladyshev22@gmail.com>
>
>Do we need a "Fixes:" tag here as well?

I was wondering the same -- I left it out because it didn't seem like it 
was strictly a bug in the earlier commit that's really being fixed per 
se, but perhaps that's an overly pedantic distinction.  I can certainly 
add it if you'd prefer.

>
>> ---
>>  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..cd11748833e6 100644
>> --- a/include/uapi/linux/serial_core.h
>> +++ b/include/uapi/linux/serial_core.h
>> @@ -274,4 +274,7 @@
>>  /* Freescale LINFlexD UART */
>>  #define PORT_LINFLEXUART	122
>>
>> +/* ASPEED AST2x00 virtual UART */
>> +#define PORT_ASPEED_VUART	123
>
>Why does this value have to be in a uapi header file?  What userspace
>tool is going to need this?
>

I only put it there because that was where all the other port type 
constants were defined, and wondered the same thing about the lot of 
them.  Is there a userspace tool that makes use of any of these?


Zev