linux-kernel - Re: [Intel-gfx] [RESEND PATCH] drm/i915: constify pointers to hwmon_channel

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Date:   Fri, 18 Aug 2023 14:03:19 +0300
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Subject: Re: [Intel-gfx] [RESEND PATCH] drm/i915: constify pointers to
 hwmon_channel_info

On Thu, 25 May 2023, Jani Nikula <jani.nikula@...ux.intel.com> wrote:
> On Thu, 11 May 2023, Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> wrote:
>> Statically allocated array of pointers to hwmon_channel_info can be made
>> const for safety.
>
> Btw if you want to further make things const, the compound literals
> defined by HWMON_CHANNEL_INFO() still end up mutable, even if they're
> only referenced inline using a const pointer. If possible, would be nice
> to add const there too.

Krzysztof, can I persuade you to follow up on this one? ;)

With HWMON_CHANNEL_INFO defined like this:

#define HWMON_CHANNEL_INFO(stype, ...)	\
	(&(struct hwmon_channel_info) {	\
		.type = hwmon_##stype,	\
		.config = (u32 []) {	\
			__VA_ARGS__, 0	\
		}			\
	})

and initializers like this all over the kernel:

static const struct hwmon_channel_info * const hwm_info[] = {
	HWMON_CHANNEL_INFO(in, HWMON_I_INPUT),
	HWMON_CHANNEL_INFO(power, HWMON_P_MAX | HWMON_P_RATED_MAX | HWMON_P_CRIT),
	HWMON_CHANNEL_INFO(energy, HWMON_E_INPUT),
	HWMON_CHANNEL_INFO(curr, HWMON_C_CRIT),
	NULL
};

You'll actually end up with *mutable non-const* struct
hwmon_channel_info's being allocated in .data sections, and having the
const pointers in the arrays point at the mutable stuff. Check with
readelf or objdump.

To put all of it in .rodata, you'd need to make the compound literals
const too:

 #define HWMON_CHANNEL_INFO(stype, ...)	\
-	(&(struct hwmon_channel_info) {	\
+	(&(const struct hwmon_channel_info) {	\
 		.type = hwmon_##stype,	\

But I'm not up for going throw all of the use sites to see if they can
all be const.


BR,
Jani.



>
> BR,
> Jani.
>
>>
>> Acked-by: Jani Nikula <jani.nikula@...el.com>
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
>> ---
>>  drivers/gpu/drm/i915/i915_hwmon.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c b/drivers/gpu/drm/i915/i915_hwmon.c
>> index 8e7dccc8d3a0..e99e8c97ef01 100644
>> --- a/drivers/gpu/drm/i915/i915_hwmon.c
>> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
>> @@ -267,7 +267,7 @@ static const struct attribute_group *hwm_groups[] = {
>>  	NULL
>>  };
>>  
>> -static const struct hwmon_channel_info *hwm_info[] = {
>> +static const struct hwmon_channel_info * const hwm_info[] = {
>>  	HWMON_CHANNEL_INFO(in, HWMON_I_INPUT),
>>  	HWMON_CHANNEL_INFO(power, HWMON_P_MAX | HWMON_P_RATED_MAX | HWMON_P_CRIT),
>>  	HWMON_CHANNEL_INFO(energy, HWMON_E_INPUT),
>> @@ -275,7 +275,7 @@ static const struct hwmon_channel_info *hwm_info[] = {
>>  	NULL
>>  };
>>  
>> -static const struct hwmon_channel_info *hwm_gt_info[] = {
>> +static const struct hwmon_channel_info * const hwm_gt_info[] = {
>>  	HWMON_CHANNEL_INFO(energy, HWMON_E_INPUT),
>>  	NULL
>>  };

-- 
Jani Nikula, Intel Open Source Graphics Center