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

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Message-Id: <af980df4-a308-4aab-8628-ca02b55de43f@www.fastmail.com>
Date:   Wed, 07 Sep 2022 15:47:37 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
Cc:     "Jiri Slaby" <jirislaby@...nel.org>,
        "Johan Hovold" <johan@...nel.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Tobias Klauser" <tklauser@...tanz.ch>,
        "Richard Genoud" <richard.genoud@...il.com>,
        "Nicolas Ferre" <nicolas.ferre@...rochip.com>,
        "Alexandre Belloni" <alexandre.belloni@...tlin.com>,
        "Claudiu Beznea" <claudiu.beznea@...rochip.com>,
        "Vladimir Zapolskiy" <vz@...ia.com>,
        "Liviu Dudau" <liviu.dudau@....com>,
        "Sudeep Holla" <sudeep.holla@....com>,
        "Lorenzo Pieralisi" <lorenzo.pieralisi@....com>,
        "Shawn Guo" <shawnguo@...nel.org>,
        "Sascha Hauer" <s.hauer@...gutronix.de>,
        "Pengutronix Kernel Team" <kernel@...gutronix.de>,
        "Fabio Estevam" <festevam@...il.com>,
        "NXP Linux Team" <linux-imx@....com>,
        Andreas Färber <afaerber@...e.de>,
        "Manivannan Sadhasivam" <mani@...nel.org>,
        "Russell King" <linux@...linux.org.uk>,
        "Florian Fainelli" <f.fainelli@...il.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Pali Rohár <pali@...nel.org>,
        "Kevin Cernekee" <cernekee@...il.com>,
        "Palmer Dabbelt" <palmer@...belt.com>,
        "Paul Walmsley" <paul.walmsley@...ive.com>,
        "Orson Zhai" <orsonzhai@...il.com>,
        "Baolin Wang" <baolin.wang7@...il.com>,
        "Chunyan Zhang" <zhang.lyra@...il.com>,
        "Patrice Chotard" <patrice.chotard@...s.st.com>,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v3 0/4] tty: TX helpers

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