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

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Message-Id: <715b40ba-1bcc-4582-bed1-ef41126c7b94@www.fastmail.com>
Date:   Wed, 07 Sep 2022 13:59:05 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        "Jiri Slaby" <jirislaby@...nel.org>,
        "Johan Hovold" <johan@...nel.org>
Cc:     "Greg Kroah-Hartman" <gregkh@...uxfoundation.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 12:16 PM, Ilpo Järvinen wrote:
> On Wed, 7 Sep 2022, Jiri Slaby wrote:
>> On 06. 09. 22, 13:30, Johan Hovold wrote:
>> > On Tue, Sep 06, 2022 at 12:48:01PM +0200, Jiri Slaby wrote:
>> > NAK
>> 
>> I'd love to come up with something nicer. That would be a function in
>> serial-core calling hooks like I had [1] for example. But provided all those
>> CPU workarounds/thunks, it'd be quite expensive to call two functions per
>> character.
>> 
>> Or creating a static inline (having ± the macro content) and the hooks as
>> parameters and hope for optimizations to eliminate thunks (also suggested in
>> the past [1]).
>> 
>> [1] https://lore.kernel.org/all/20220411105405.9519-1-jslaby@suse.cz/
>
> I second Jiri here.
>
> Saving lines in drivers is not that important compared with all removing 
> all the variants of the same thing that have crept there over the years.
>
> I suspect the main reason for the variants is that everybody just used 
> other drivers as examples and therefore we've a few "main" variant 
> branches depending on which of the drivers was used as an example for the 
> other. That is hardly a good enough reason to keep them different and as 
> long as each driver keeps its own function for this, it will eventually 
> lead to similar differentiation so e.g. a one-time band-aid similarization 
> would not help in the long run.
>
> Also, I don't understand why you see it unreadable when the actual code is 
> out in the open in that macro. It's formatted much better than e.g. 
> read_poll_timeout() if you want an example of something that is hardly 
> readable ;-). I agree though there's a learning-curve, albeit small, that 
> it actually creates a function but that doesn't seem to me as big of an 
> obstacle you seem to think.

I think it would help to replace the macro that defines
the function with a set of macros that can be used in
function bodies. This would avoid the __VA_ARGS__ stuff
and allow readers that are unfamiliar with tty drivers to
treat it as a function call.

So e.g. instead of 

static DEFINE_UART_PORT_TX_HELPER_LIMITED(altera_jtaguart_do_tx_chars,
		true,
		writel(ch, port->membase + ALTERA_JTAGUART_DATA_REG),
		({}));

the altera_jtaguart driver would contain a function like

static int altera_jtaguart_do_tx_chars(struct uart_port *port,
                                       unsigned int count)
{
       char ch;

       return uart_port_tx_helper_limited(port, ch, count, true,
                writel(ch, port->membase + ALTERA_JTAGUART_DATA_REG),
                ({}));
}

or some variation of that. It's a few more lines, but those
extra lines would help me understand what is actually going on
while still avoiding the usual bugs and duplication.

If the caller of that function is itself trivial (like
serial21285_tx_chars), then the intermediate function can
be omitted in order to save some of the extra complexity.

       Arnd