linux-kernel - Re: [PATCH] powerpc: sysdev: cpm1: Optimise gpio bit calculation

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Date:   Fri, 10 Mar 2017 15:41:23 +0100
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: sysdev: cpm1: Optimise gpio bit calculation



Le 10/03/2017 à 15:32, Segher Boessenkool a écrit :
> On Fri, Mar 10, 2017 at 03:04:48PM +0100, Christophe LEROY wrote:
>> Le 10/03/2017 à 14:06, Segher Boessenkool a écrit :
>>> On Fri, Mar 10, 2017 at 11:54:19AM +0100, Christophe LEROY wrote:
>>>> gpio_get() and gpio_set() are used extensively by some GPIO based
>>>> drivers like SPI, NAND, so it may be worth it as it doesn't impair
>>>> readability (if anyone prefers, we could write  (1 << 31) >> i  instead
>>>> of  0x80000000 >> i )
>>>
>>> 1 << 31 is undefined behaviour, of course.
>>>
>>
>> Shall it be 1U << 31 ?
>
> Sure, that works.  "1 << (31 - i)" is most readable (but it doesn't yet
> generate the code you want).
>
>

Euh .... I'm a bit lost. Do you mean the form we have today is the 
driver is wrong ?


@@ -684,9 +682,7 @@ static int cpm1_gpio32_get(struct gpio_chip *gc, 
unsigned int gpio)
  {
  	struct of_mm_gpio_chip *mm_gc = to_of_mm_gpio_chip(gc);
  	struct cpm_ioport32b __iomem *iop = mm_gc->regs;
-	u32 pin_mask;
-
-	pin_mask = 1 << (31 - gpio);
+	u32 pin_mask = 0x80000000 >> gpio;

  	return !!(in_be32(&iop->dat) & pin_mask);
  }


Which I thought could also become


@@ -684,9 +682,7 @@ static int cpm1_gpio32_get(struct gpio_chip *gc, 
unsigned int gpio)
  {
  	struct of_mm_gpio_chip *mm_gc = to_of_mm_gpio_chip(gc);
  	struct cpm_ioport32b __iomem *iop = mm_gc->regs;
-	u32 pin_mask;
-
-	pin_mask = 1 << (31 - gpio);
+	u32 pin_mask = (1 << 31) >> gpio;

  	return !!(in_be32(&iop->dat) & pin_mask);
  }


Christophe