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

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Date:   Thu, 1 Sep 2022 14:25:14 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] tty: serial: introduce transmit helper generators

On Thu, Sep 01, 2022 at 01:06:55PM +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 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.

Much nicer, but:

> +#define __DEFINE_UART_PORT_TX_HELPER(name, port, ch, tx_ready, put_char,  \
> +		tx_done, for_test, for_post, ...)			  \

Do you really need "port" and "ch" as part of this macro?  You always
set that to the same thing in your patches, so is it really needed?

thanks,

greg k-h