Re: [PATCH v3 0/4] tty: TX helpers

["Arnd Bergmann" <arnd@arndb.de>]

On Wed, Sep 7, 2022, at 2:56 PM, Ilpo Järvinen wrote:
> On Wed, 7 Sep 2022, Greg Kroah-Hartman wrote:
>> Ok, no, I don't understand what Arnd meant here then :(
>
> I think he did just 2 things (compared with 2/4 of this series):
>
> - __DEFINE_UART_PORT_TX_HELPER() doesn't create a function but just wraps 
> the macro body with ({ }). Therefore, a driver must use it inside 
> a function rather than the macro creating a new function with name.
> - Use lowercase instead of uppercase (this is a trivial change)
>
> When a driver is using these "function" that are just made look functions 
> but are macros for real, it will not add function-pointer-indirection-call 
> per character but pulls the whole loop from the macro into the function 
> in the driver and injects those putchar, etc. directly into that loop. 
> ...It is just like read_poll_timeout() constructs the loop.

Right, the example I was thinking of was wait_event(), which is
another common macro that takes expressions as arguments but gets
used like a function call. Same thing really.

Note that even with an always_inline function that takes a
function pointer argument, anything from gcc-4.7 up has no 
problem inlining through that, so it should not be any less
efficient though it might not help readability either.
See https://godbolt.org/z/o8GGG8TzM for an example of that.

     Arnd