On 04/07/2014 02:26 PM, Gerhard Sittig wrote:
> On Mon, 2014-04-07 at 12:13 +0200, Sebastian Andrzej Siewior wrote:
>>
>> d->mask contains exact the same information as BIT(bit) so we could save
>> a few cycles here.
>
> ISTR that the benefit of saving cycles was questioned in previous
> review comments. On ARM, the shift "comes for free".
I can't recall that some pointed this out. However:
- you load one variable in both cases. Not performing the shift means
there is at least one instruction less to be performed.
- that gpio controller is generic IP core from Synopsys. Every can buy
it and but into their IP core so it is not limited to ARM.
>> --- a/drivers/gpio/gpio-dwapb.c
>> +++ b/drivers/gpio/gpio-dwapb.c
>> @@ -113,7 +113,7 @@ static void dwapb_irq_enable(struct irq_data *d)
>>
>> irq_gc_lock(igc);
>> val = readl(gpio->regs + GPIO_INTEN);
>> - val |= BIT(d->hwirq);
>> + val |= d->mask;
>
> these are equally costly or cheap, nothing saved here
I still thing not performing an instruction is more efficient than
performing one.
>> struct dwapb_gpio *gpio = igc->private;
>> - int bit = d->hwirq;
>> + u32 mask = d->mask;
>> unsigned long level, polarity;
>>
>> if (type & ~(IRQ_TYPE_EDGE_RISING | IRQ_TYPE_EDGE_FALLING |
>> @@ -171,24 +171,24 @@ static int dwapb_irq_set_type(struct irq_data *d, u32
>> type)
>>
>> switch (type) {
>> case IRQ_TYPE_EDGE_BOTH:
>> - level |= BIT(bit);
>> - dwapb_toggle_trigger(gpio, bit);
>> + level |= mask;
>> + dwapb_toggle_trigger(gpio, d->hwirq);
>
> these introduce another pointer dereference, unless 'bit' was
> assigned from a pointer dereference (as is shown above), so
> nothing was gained
dwapb_toggle_trigger() is a bit special and it needs both. However,
size on ARM says
text data bss dec hex filename
3264 96 0 3360 d20 drivers/gpio/gpio-dwapb.o.before
3224 96 0 3320 cf8 drivers/gpio/gpio-dwapb.o.after
that with the patch the code is smaller by 40 bytes. Does 40 bytes
smaller code quality for "safe a few cycles" statement?
> virtually yours
> Gerhard Sittig
Sebastian
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
