Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755828AbaDGS0h (ORCPT ); Mon, 7 Apr 2014 14:26:37 -0400 Received: from www.linutronix.de ([62.245.132.108]:53141 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753281AbaDGS0e (ORCPT ); Mon, 7 Apr 2014 14:26:34 -0400 Message-ID: <5342EDD6.3050706@linutronix.de> Date: Mon, 07 Apr 2014 20:26:30 +0200 From: Sebastian Andrzej Siewior User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Icedove/24.4.0 MIME-Version: 1.0 To: Gerhard Sittig CC: jamie@jamieiles.com, atull@altera.com, gnurou@gmail.com, linux-gpio@vger.kernel.org, linux-kernel@vger.kernel.org, dinguyen@altera.com, linus.walleij@linaro.org, sebastian.hesselbarth@gmail.com Subject: Re: [PATCH 5/5] gpio: dwapb: use d->mask instead od BIT(bit) References: <1396865589-9972-1-git-send-email-bigeasy@linutronix.de> <1396865589-9972-6-git-send-email-bigeasy@linutronix.de> <20140407122620.GX11339@book.gsilab.sittig.org> In-Reply-To: <20140407122620.GX11339@book.gsilab.sittig.org> X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/