Date:   Fri, 15 Apr 2022 09:47:26 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] tty: serial: introduce uart_port_tx{,_limit}()
 helpers

On 14. 04. 22, 18:32, Greg KH wrote:
> On Mon, Apr 11, 2022 at 12:54:03PM +0200, Jiri Slaby wrote:
>> Many serial drivers do the same thing:
>> * send x_char if set
>> * keep sending from the xmit circular buffer until either
>>    - the loop reaches the end of the xmit buffer
>>    - TX is stopped
>>    - HW fifo is full
>> * check for pending characters and:
>>    - wake up tty writers to fill for more data into xmit buffer
>>    - stop TX if there is nothing in the xmit buffer
>>
>> The only differences are:
>> * how to write the character to the HW fifo
>> * the check of the end condition:
>>    - is the HW fifo full?
>>    - is limit of the written characters reached?
>>
>> So unify the above into two helpers:
>> * uart_port_tx_limit() -- the generic one, it performs the above taking
>>    into account the written characters limit
>> * uart_port_tx() -- calls the above with ~0 as the limit. So it only
>>    checks the HW fullness.
>>
>> We need three more hooks in struct uart_ops for all this to work:
>> * tx_ready() -- returns true if HW can accept more data.
>> * put_char() -- write a character to the device.
>> * tx_done() -- when the write loop is done, perform arbitrary action
>>    before potential invocation of ops->stop_tx() happens.
>>
>> NOTE1: Maybe the three hooks in uart_ops above are overkill. We can
>> instead pass pointers to the three functions directly to the new helpers
>> as they are not used elsewhere. Similar to uart_console_write() and its
>> putchar().
>>
>> NOTE2: These two new helper functions call the hooks per every character
>> processed. I was unable to measure any difference, provided most time is
>> spent by readb (or alike) in the hooks themselves.  First, LTO might
>> help to eliminate these explicit calls (we might need NOTE1 to be
>> implemented for this to be true). Second, if this turns out to be a
>> problem, we can introduce a macro to build the helper in the driver's
>> code instead of serial_core. That is, similar to wait_event().
>>
>> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
>> ---
>>   Documentation/driver-api/serial/driver.rst | 28 ++++++++++++
>>   drivers/tty/serial/serial_core.c           | 53 ++++++++++++++++++++++
>>   include/linux/serial_core.h                |  9 ++++
>>   3 files changed, 90 insertions(+)
>>
>> diff --git a/Documentation/driver-api/serial/driver.rst b/Documentation/driver-api/serial/driver.rst
>> index 06ec04ba086f..7dc3791addeb 100644
>> --- a/Documentation/driver-api/serial/driver.rst
>> +++ b/Documentation/driver-api/serial/driver.rst
>> @@ -80,6 +80,34 @@ hardware.
>>   
>>   	This call must not sleep
>>   
>> +  tx_ready(port)
>> +	The driver returns true if the HW can accept more data to be sent.
>> +
>> +	Locking: port->lock taken.
>> +
>> +	Interrupts: locally disabled.
>> +
>> +	This call must not sleep.
>> +
>> +  put_char(port, ch)
>> +	The driver is asked to write ch to the device.
>> +
>> +	Locking: port->lock taken.
>> +
>> +	Interrupts: locally disabled.
>> +
>> +	This call must not sleep.
>> +
>> +  tx_done(port)
>> +	When the write loop is done, the driver can perform arbitrary action
>> +	here before potential invocation of ops->stop_tx() happens.
>> +
>> +	Locking: port->lock taken.
>> +
>> +	Interrupts: locally disabled.
>> +
>> +	This call must not sleep.
>> +
>>     set_mctrl(port, mctrl)
>>   	This function sets the modem control lines for port described
>>   	by 'port' to the state described by mctrl.  The relevant bits
>> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
>> index 6a8963caf954..1be14e90066c 100644
>> --- a/drivers/tty/serial/serial_core.c
>> +++ b/drivers/tty/serial/serial_core.c
>> @@ -107,6 +107,59 @@ void uart_write_wakeup(struct uart_port *port)
>>   }
>>   EXPORT_SYMBOL(uart_write_wakeup);
>>   
>> +static bool uart_port_tx_always_ready(struct uart_port *port)
>> +{
>> +	return true;
>> +}
>> +
>> +/**
>> + * uart_port_tx_limit -- transmit helper for uart_port
>> + * @port: from which port to transmit
>> + * @count: limit count
>> + *
>> + * uart_port_tx_limit() transmits characters from the xmit buffer to the
>> + * hardware using @uart_port::ops::put_char(). It does so until @count
>> + * characters are sent and while @uart_port::ops::tx_ready() still returns
>> + * non-zero (if non-NULL).
>> + *
>> + * Return: number of characters in the xmit buffer when done.
>> + */
>> +unsigned int uart_port_tx_limit(struct uart_port *port, unsigned int count)
>> +{
>> +	struct circ_buf *xmit = &port->state->xmit;
>> +	bool (*tx_ready)(struct uart_port *) = port->ops->tx_ready ? :
>> +		uart_port_tx_always_ready;
>> +	unsigned int pending;
>> +
>> +	for (; count && tx_ready(port); count--, port->icount.tx++) {
>> +		if (port->x_char) {
>> +			port->ops->put_char(port, port->x_char);
>> +			port->x_char = 0;
>> +			continue;
>> +		}
>> +
>> +		if (uart_circ_empty(xmit) || uart_tx_stopped(port))
>> +			break;
>> +
>> +		port->ops->put_char(port, xmit->buf[xmit->tail]);
> 
> That's a lot of redirection and function pointer mess per each character
> sent now.  With the spectre overhead here (and only getting worse), this
> feels like a step backwards.
> 
> I doubt throughput matters here given cpu speeds now, _but_ the cpu load
> should go up.
> 
> Although on smaller cpus with slower Mhz and faster line rates, this
> feels like a lot of extra work happening for no real good reason.

I know… Did you miss NOTE2 in the commit log? Any idea on that?

thanks,
-- 
js
suse labs

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