linux-kernel - Re: [PATCH v8 4/4] serial: pl011: add console matching function

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Message-ID: <20160622204548.GB28766@yury-N73SV>
Date:	Wed, 22 Jun 2016 23:45:48 +0300
From:	Yury Norov <ynorov@...iumnetworks.com>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	Aleksey Makarov <aleksey.makarov@...aro.org>,
	"Rafael J . Wysocki" <rjw@...ysocki.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-serial@...r.kernel.org>, <linux-acpi@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King <linux@....linux.org.uk>,
	Len Brown <lenb@...nel.org>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Graeme Gregory <graeme.gregory@...aro.org>,
	Al Stone <ahs3@...hat.com>,
	Christopher Covington <cov@...eaurora.org>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	"Zheng, Lv" <lv.zheng@...el.com>, Mark Salter <msalter@...hat.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Russell King <linux@...linux.org.uk>,
	Jiri Slaby <jslaby@...e.com>
Subject: Re: [PATCH v8 4/4] serial: pl011: add console matching function

Hi Peter, 

Nice to meet you.

On Wed, Jun 22, 2016 at 07:08:33AM -0700, Peter Hurley wrote:
> 
> 
> > On Jun 22, 2016, at 5:18 AM, Yury Norov <ynorov@...iumnetworks.com> wrote:
> > 
> >> On Fri, May 20, 2016 at 04:03:23PM +0300, Aleksey Makarov wrote:
> >> This patch adds function pl011_console_match() that implements
> >> method match of struct console.  It allows to match consoles against
> >> data specified in a string, for example taken from command line or
> >> compiled by ACPI SPCR table handler.
> >> 
> >> Signed-off-by: Aleksey Makarov <aleksey.makarov@...aro.org>
> >> Reviewed-by: Peter Hurley <peter@...leysoftware.com>
> >> ---
> >> drivers/tty/serial/amba-pl011.c | 56 +++++++++++++++++++++++++++++++++++++++++
> >> 1 file changed, 56 insertions(+)
> >> 
> >> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> >> index a2aa655..388edc8 100644
> >> --- a/drivers/tty/serial/amba-pl011.c
> >> +++ b/drivers/tty/serial/amba-pl011.c
> >> @@ -2288,12 +2288,68 @@ static int __init pl011_console_setup(struct console *co, char *options)
> >>    return uart_set_options(&uap->port, co, baud, parity, bits, flow);
> >> }
> >> 
> >> +/**
> >> + *    pl011_console_match - non-standard console matching
> >> + *    @co:      registering console
> >> + *    @name:      name from console command line
> >> + *    @idx:      index from console command line
> >> + *    @options: ptr to option string from console command line
> >> + *
> >> + *    Only attempts to match console command lines of the form:
> >> + *        console=pl011,mmio|mmio32,<addr>[,<options>]
> >> + *        console=pl011,0x<addr>[,<options>]
> >> + *    This form is used to register an initial earlycon boot console and
> >> + *    replace it with the amba_console at pl011 driver init.
> >> + *
> >> + *    Performs console setup for a match (as required by interface)
> >> + *    If no <options> are specified, then assume the h/w is already setup.
> >> + *
> >> + *    Returns 0 if console matches; otherwise non-zero to use default matching
> >> + */
> >> +static int __init pl011_console_match(struct console *co, char *name, int idx,
> >> +                      char *options)
> >> +{
> >> +    char match[] = "pl011";    /* pl011-specific earlycon name */
> >> +    unsigned char iotype;
> >> +    unsigned long addr;
> >> +    int i;
> >> +
> >> +    if (strncmp(name, match, 5) != 0)
> >> +        return -ENODEV;
> >> +
> >> +    if (uart_parse_earlycon(options, &iotype, &addr, &options))
> >> +        return -ENODEV;
> >> +
> >> +    /* try to match the port specified on the command line */
> >> +    for (i = 0; i < ARRAY_SIZE(amba_ports); i++) {
> >> +        struct uart_port *port;
> >> +
> >> +        if (!amba_ports[i])
> >> +            continue;
> >> +
> >> +        port = &amba_ports[i]->port;
> >> +
> >> +        if (iotype != UPIO_MEM && iotype != UPIO_MEM32)
> >> +            continue;
> > 
> > So it looks like iotype is constant inside the loop, and UPIO_MEM
> > and UPIO_MEM32 too, of course. It means you can move this check out of
> > cycle and avoid ports traversing at all in specific case. Am I wrong?
> > 
> 
> No you're not wrong but I'd prefer if we don't use assumptions like that in port enumeration.

I don't think this is an assumption. I think this is solid fact.
There's no a single chance for stack-allocated variable to be
shared with other thread of execution, or be unexpectedly changed
somehow else. So this code not only decreases performance (I don't
think it's really hot path), but also confuses reader, and makes him
spend more time on reading this than it deserves.

If you still insist on current version, I'd ask you or Alexey to add
clear description why we check the same condition again and again
inside the loop.

Yury.

> >> +
> >> +        if (port->mapbase != addr)
> >> +            continue;
> >> +
> >> +        co->index = i;
> >> +        port->cons = co;
> >> +        return pl011_console_setup(co, options);
> >> +    }
> >> +
> >> +    return -ENODEV;
> >> +}
> >> +
> >> static struct uart_driver amba_reg;
> >> static struct console amba_console = {
> >>    .name        = "ttyAMA",
> >>    .write        = pl011_console_write,
> >>    .device        = uart_console_device,
> >>    .setup        = pl011_console_setup,
> >> +    .match        = pl011_console_match,
> >>    .flags        = CON_PRINTBUFFER,
> >>    .index        = -1,
> >>    .data        = &amba_reg,
> >> -- 
> >> 2.8.2