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

[Zev Weiss]

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@xxxxxxxxxxxxxxxxx>
> > > > Tested-by: Konstantin Aladyshev <aladyshev22@xxxxxxxxx>
> > >
> > > 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@xxxxxxxxxxxxxxx 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