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

[Matthias Brugger]

On 22/06/16 22:45, Yury Norov wrote:
> 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@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > > > Reviewed-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
> > > > ---
> > > > 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.

I agree with Yury on this. From my point of view, it makes no sense to 
check the iotype in every loop iteration.

I tried to apply these patches against linux-next. They needed some 
changes to apply, which is quite normal after such a long time. Apart 
from that the DBG2 subtype patch didn't end up in mainline, so this 
patches do not compile.

I followed up on that on the corresponding thread. Please don't forget 
to pin-point in the introduction mail to any other series which is 
needed for your patches to compile/work.

Thanks a lot,
Matthias


> 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
>
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