Currently, there are 7 checks for whether to enable
the internal fifo device of a 8250/16550 type uart.

Instead of checking all 7 values again whenever
we need to know whether we have the fifo device enabled,
store the result as a struct member of uart_8250_port.

This can, for example, lessen the amount
of calculations done during a write operation.

Signed-off-by: Michael Pratt <mcpratt@pm.me>
---
V1 -> V2: new commit

 drivers/tty/serial/8250/8250_port.c | 2 ++
 include/linux/serial_8250.h         | 1 +
 2 files changed, 3 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index fc9dd5d45295..5b0cfe6bc98c 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -3392,6 +3392,8 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
 		 */
 		!(up->port.flags & UPF_CONS_FLOW);
 
+	up->fifo_enable = use_fifo;
+
 	if (likely(use_fifo))
 		serial8250_console_fifo_write(up, s, count);
 	else
diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index fd59ed2cca53..017429f0e743 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -127,6 +127,7 @@ struct uart_8250_port {
 	struct list_head	list;		/* ports on this IRQ */
 	u32			capabilities;	/* port capabilities */
 	u16			bugs;		/* port bugs */
+	unsigned int		fifo_enable;	/* fifo enabled if capable */
 	unsigned int		tx_loadsz;	/* transmit fifo load size */
 	unsigned char		acr;
 	unsigned char		fcr;
-- 
2.30.2

On Tue, Apr 16, 2024 at 06:29:56PM +0000, Michael Pratt wrote:
> Currently, there are 7 checks for whether to enable
> the internal fifo device of a 8250/16550 type uart.
> 
> Instead of checking all 7 values again whenever
> we need to know whether we have the fifo device enabled,
> store the result as a struct member of uart_8250_port.
> 
> This can, for example, lessen the amount
> of calculations done during a write operation.

...

> @@ -3392,6 +3392,8 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,

> +	up->fifo_enable = use_fifo;

This seems incorrect / not the only one place to assign this. What if the
console not enabled at compile time? What if it's not enabled at boot time?

-- 
With Best Regards,
Andy Shevchenko

Hi Andy,

On Tuesday, April 16th, 2024 at 14:55, Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:

> 
> > @@ -3392,6 +3392,8 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
> 
> > + up->fifo_enable = use_fifo;
> 
> 
> This seems incorrect / not the only one place to assign this. What if the
> console not enabled at compile time? What if it's not enabled at boot time?
>

This is 8250 specific, and currently, it's the only place there
where it's decided whether or not to use the fifo device
by checking a bunch of flags and values.

If you're suggesting that these checks are moved out of this function somewhere else,
I would probably agree with that, but let's save that idea for the future...

If you're suggesting that there could be a null pointer, I don't think that's possible
in this function... (the name of the pointer being "up" might be confusing?)

Sorry if I'm misunderstanding what you mean.

> --
> With Best Regards,
> Andy Shevchenko

--
MCP

On Tue, Apr 16, 2024 at 07:09:52PM +0000, Michael Pratt wrote:
> On Tuesday, April 16th, 2024 at 14:55, Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:

> > > @@ -3392,6 +3392,8 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
> > 
> > > + up->fifo_enable = use_fifo;
> > 
> > This seems incorrect / not the only one place to assign this. What if the
> > console not enabled at compile time? What if it's not enabled at boot time?
> 
> This is 8250 specific, and currently, it's the only place there
> where it's decided whether or not to use the fifo device
> by checking a bunch of flags and values.

Exactly, as initial commit is related to the kernel console _only_.
While your code, IIUC (correct me, if I'm wrong) is for any use of the port.

> If you're suggesting that these checks are moved out of this function somewhere else,
> I would probably agree with that, but let's save that idea for the future...

Not really (again, IIUC above), as console can be not enabled, and hence
serial8250_console_write() never been called and you will have false impression
that there is no FIFO in use.

> If you're suggesting that there could be a null pointer, I don't think that's possible
> in this function... (the name of the pointer being "up" might be confusing?)
> 
> Sorry if I'm misunderstanding what you mean.

-- 
With Best Regards,
Andy Shevchenko

Hi Andy,

On Tuesday, April 16th, 2024 at 15:18, Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:

> 
> 
> On Tue, Apr 16, 2024 at 07:09:52PM +0000, Michael Pratt wrote:
> 
> > On Tuesday, April 16th, 2024 at 14:55, Andy Shevchenko andriy.shevchenko@linux.intel.com wrote:
> 
> > > > @@ -3392,6 +3392,8 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
> > > 
> > > > + up->fifo_enable = use_fifo;
> > > 
> > > This seems incorrect / not the only one place to assign this. What if the
> > > console not enabled at compile time? What if it's not enabled at boot time?
> > 
> > This is 8250 specific, and currently, it's the only place there
> > where it's decided whether or not to use the fifo device
> > by checking a bunch of flags and values.
> 
> 
> Exactly, as initial commit is related to the kernel console only.
> While your code, IIUC (correct me, if I'm wrong) is for any use of the port.
> 
> > If you're suggesting that these checks are moved out of this function somewhere else,
> > I would probably agree with that, but let's save that idea for the future...
> 
> 
> Not really (again, IIUC above), as console can be not enabled, and hence
> serial8250_console_write() never been called and you will have false impression
> that there is no FIFO in use.

Ah ok, I understand now...

So there are cases where the function with the checks will never be called,
yet the device itself will be configured the same way and the struct member I am adding
will still be instantiated with value of 0 and never be set elsewhere... and because
it is declared in a major struct "uart_8250_port", it appears to apply to a larger scope
compared to the way it is actually being used...
(or at the very least, the name "fifo_enable" would be misleading).

Thanks for pointing that out, I'll take a deeper dive into the file...


> 
> --
> With Best Regards,
> Andy Shevchenko

--
MCP