linux-kernel - Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions

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Date:   Tue, 17 Apr 2018 17:07:28 +0800
From:   yuankuiz@...eaurora.org
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andy Whitcroft <apw@...onical.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>,
        Len Brown <len.brown@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pm-owner@...r.kernel.org
Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool
 member definitions

Hi julia,

On 2018-04-15 05:19 AM, Julia Lawall wrote:
> On Wed, 11 Apr 2018, Joe Perches wrote:
> 
>> On Thu, 2018-04-12 at 08:22 +0200, Julia Lawall wrote:
>> > On Wed, 11 Apr 2018, Joe Perches wrote:
>> > > On Wed, 2018-04-11 at 09:29 -0700, Andrew Morton wrote:
>> > > > We already have some 500 bools-in-structs
>> > >
>> > > I got at least triple that only in include/
>> > > so I expect there are at probably an order
>> > > of magnitude more than 500 in the kernel.
>> > >
>> > > I suppose some cocci script could count the
>> > > actual number of instances.  A regex can not.
>> >
>> > I got 12667.
>> 
>> Could you please post the cocci script?
>> 
>> > I'm not sure to understand the issue.  Will using a bitfield help if there
>> > are no other bitfields in the structure?
>> 
>> IMO, not really.
>> 
>> The primary issue is described by Linus here:
>> https://lkml.org/lkml/2017/11/21/384
>> 
>> I personally do not find a significant issue with
>> uncontrolled sizes of bool in kernel structs as
>> all of the kernel structs are transitory and not
>> written out to storage.
>> 
>> I suppose bool bitfields are also OK, but for the
>> RMW required.
>> 
>> Using unsigned int :1 bitfield instead of bool :1
>> has the negative of truncation so that the uint
>> has to be set with !! instead of a simple assign.
> 
> At least with gcc 5.4.0, a number of structures become larger with
> unsigned int :1. bool:1 seems to mostly solve this problem.  The 
> structure
> ichx_desc, defined in drivers/gpio/gpio-ich.c seems to become larger 
> with
> both approaches.
[ZJ] Hopefully, this could make it better in your environment.
      IMHO, this is just for double check.

diff --git a/drivers/gpio/gpio-ich.c b/drivers/gpio/gpio-ich.c
index 4f6d643..b46e170 100644
--- a/drivers/gpio/gpio-ich.c
+++ b/drivers/gpio/gpio-ich.c
@@ -70,6 +70,18 @@ static const u8 avoton_reglen[3] = {
  #define ICHX_READ(reg, base_res)       inl((reg) + (base_res)->start)

  struct ichx_desc {
+       /* GPO_BLINK is available on this chipset */
+       bool uses_gpe0:1;
+
+       /* Whether the chipset has GPIO in GPE0_STS in the PM IO region 
*/
+        bool uses_gpe0:1;
+
+        /*
+         * Some chipsets don't let reading output values on GPIO_LVL 
register
+         * this option allows driver caching written output values
+         */
+        bool use_outlvl_cache:1;
+
         /* Max GPIO pins the chipset can have */
         uint ngpio;

@@ -77,24 +89,12 @@ struct ichx_desc {
         const u8 (*regs)[3];
         const u8 *reglen;

-       /* GPO_BLINK is available on this chipset */
-       bool have_blink;
-
-       /* Whether the chipset has GPIO in GPE0_STS in the PM IO region 
*/
-       bool uses_gpe0;
-
         /* USE_SEL is bogus on some chipsets, eg 3100 */
         u32 use_sel_ignore[3];

         /* Some chipsets have quirks, let these use their own 
request/get */
         int (*request)(struct gpio_chip *chip, unsigned offset);
         int (*get)(struct gpio_chip *chip, unsigned offset);
-
-       /*
-        * Some chipsets don't let reading output values on GPIO_LVL 
register
-        * this option allows driver caching written output values
-        */
-       bool use_outlvl_cache;
  };


ZJ