linux-kernel - Re: [PATCH v2 1/3] tty: serial: introduce transmit helper generators

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Date:   Fri, 2 Sep 2022 12:24:10 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] tty: serial: introduce transmit helper generators

On 02. 09. 22, 12:22, Ilpo Järvinen wrote:
> On Thu, 1 Sep 2022, 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 helper generators:
>> * DEFINE_UART_PORT_TX_HELPER_LIMITED() -- it performs the above taking
>>    the written characters limit into account, and
>> * DEFINE_UART_PORT_TX_HELPER() -- the same as above, except it only
>>    checks the HW readiness, not the characters limit.
>>
>> The HW specific operations (as stated as "differences" above) are passed
>> as arguments to the macros. They are:
>> * 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.
>>
>> Note that the above macros are generators. This means the code is
>> generated in place and the above 3 arguments are "inlined". I.e. no
>> added penalty by generating call instructions for every single
>> character. Nor any indirect calls. (As in previous versions of this
>> patchset.)
>>
>> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
>> ---
>>
>> Notes:
>>      [v2] instead of a function (uart_port_tx_limit()) in serial_core,
>>           generate these in-place using macros. Thus eliminating "call"
>>           penalty.
>>
>>   Documentation/driver-api/serial/driver.rst |  3 +
>>   include/linux/serial_core.h                | 86 ++++++++++++++++++++++
>>   2 files changed, 89 insertions(+)
>>
>> diff --git a/Documentation/driver-api/serial/driver.rst b/Documentation/driver-api/serial/driver.rst
>> index 23c6b956cd90..25775bf1fcc6 100644
>> --- a/Documentation/driver-api/serial/driver.rst
>> +++ b/Documentation/driver-api/serial/driver.rst
>> @@ -78,6 +78,9 @@ Other functions
>>              uart_get_lsr_info uart_handle_dcd_change uart_handle_cts_change
>>              uart_try_toggle_sysrq uart_get_console
>>   
>> +.. kernel-doc:: include/linux/serial_core.h
>> +   :identifiers: DEFINE_UART_PORT_TX_HELPER_LIMITED DEFINE_UART_PORT_TX_HELPER
>> +
>>   Other notes
>>   -----------
>>   
>> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
>> index 6e4f4765d209..715778160ae1 100644
>> --- a/include/linux/serial_core.h
>> +++ b/include/linux/serial_core.h
>> @@ -646,6 +646,92 @@ struct uart_driver {
>>   
>>   void uart_write_wakeup(struct uart_port *port);
>>   
>> +#define __DEFINE_UART_PORT_TX_HELPER(name, port, ch, tx_ready, put_char,  \
>> +		tx_done, for_test, for_post, ...)			  \
>> +unsigned int name(struct uart_port *port __VA_OPT__(,) __VA_ARGS__)	  \
>> +{									  \
>> +	struct circ_buf *xmit = &port->state->xmit;			  \
>> +	unsigned int pending;						  \
>> +	u8 ch;								  \
>> +									  \
>> +	for (; (for_test) && (tx_ready); (for_post), port->icount.tx++) { \
> 
>> + * The functions in parameters shall be designed as follows:
>> + *  * **tx_ready(port):** the function shall return true if the HW can accept
>> + *    more data to be sent. This function can be %NULL, which means the HW is
>> + *    always ready.
> 
> So if tx_ready can be NULL, how does that for() loop above work??

Sorry, the docs is wrong (corresponded to v1). I fixed it locally already:

https://git.kernel.org/pub/scm/linux/kernel/git/jirislaby/linux.git/commit/?h=devel&id=989c488c69faf2987648e09358c79d75b4a3b5c7

thanks,
-- 
js
suse labs