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Date: Thu, 10 Feb 2022 01:22:09 -0800
From: Zev Weiss <zev@bewilderbeest.net>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH] serial: 8250_aspeed_vuart: add PORT_ASPEED_VUART port type
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Cc: linux-aspeed@lists.ozlabs.org, Konstantin Aladyshev <aladyshev22@gmail.com>,
 Andrew Jeffery <andrew@aj.id.au>, openbmc@lists.ozlabs.org,
 linux-kernel@vger.kernel.org, Oskar Senft <osk@google.com>,
 linux-serial@vger.kernel.org,
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 Jiri Slaby <jirislaby@kernel.org>, linux-arm-kernel@lists.infradead.org
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On Thu, Feb 10, 2022 at 12:20:12AM PST, Greg Kroah-Hartman wrote:
>On Wed, Feb 09, 2022 at 11:49:47PM -0800, Zev Weiss wrote:
>> 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.
>
>This obviously fixes an issue, if you don't have a specific commit that
>caused it, a cc: stable@vger.kernel.org should be added so we know to
>backport this to all stable kernels.
>

Okay -- well, I suppose it's a fix in the sense that if you have the 
earlier commit, you'll also want this one, so I'll add the tag.

>>
>> >
>> > > ---
>> > >  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?
>
>Not really, please don't add it if you do not require it.
>

It seems like an odd inconsistency to put this one particular definition 
somewhere else when the other 100+ of its siblings are in the uapi 
header; would you want a preceding patch to move them all somewhere 
under include/linux?  (Which in turn doesn't really seem like a change 
for -stable I'd think.)

Though actually, on further investigation I see those constants are in 
fact exposed to userspace in struct serial_struct->type (via 
TIOCGSERIAL/TIOCSSERIAL), and via the 'type' sysfs attribute, so I'd 
think we'd probably want to keep them as they are?


Zev