Re: [PATCH 1/2] arm_mpam: Force __iomem casts

[Krzysztof Kozlowski]

On 27/02/2026 15:06, Ben Horgan wrote:
> Hi Krzysztof,
> 
> On 2/16/26 11:02, Krzysztof Kozlowski wrote:
>> Code allocates standard kernel memory to pass to the MPAM, which expects
>> __iomem.  The code is safe, because __iomem accessors should work fine
>> on kernel mapped memory, however leads to sparse warnings:
>>
>>   test_mpam_devices.c:327:42: warning: incorrect type in initializer (different address spaces)
>>   test_mpam_devices.c:327:42:    expected char [noderef] __iomem *buf
>>   test_mpam_devices.c:327:42:    got void *
>>   test_mpam_devices.c:342:24: warning: cast removes address space '__iomem' of expression
>>
>> Cast the pointer to memory via __force to silence them.
>>
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxxxxxxxx>
>> ---
>>  drivers/resctrl/test_mpam_devices.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/resctrl/test_mpam_devices.c b/drivers/resctrl/test_mpam_devices.c
>> index 3e8d564a0c64..2de41b47c138 100644
>> --- a/drivers/resctrl/test_mpam_devices.c
>> +++ b/drivers/resctrl/test_mpam_devices.c
>> @@ -324,7 +324,7 @@ static void test_mpam_enable_merge_features(struct kunit *test)
>>  
>>  static void test_mpam_reset_msc_bitmap(struct kunit *test)
>>  {
>> -	char __iomem *buf = kunit_kzalloc(test, SZ_16K, GFP_KERNEL);
>> +	char __iomem *buf = (__force char __iomem *)kunit_kzalloc(test, SZ_16K, GFP_KERNEL);
>>  	struct mpam_msc fake_msc = {};
>>  	u32 *test_result;
>>  
>> @@ -339,7 +339,7 @@ static void test_mpam_reset_msc_bitmap(struct kunit *test)
>>  	mutex_init(&fake_msc.part_sel_lock);
>>  	mutex_lock(&fake_msc.part_sel_lock);
>>  
>> -	test_result = (u32 *)(buf + MPAMCFG_CPBM);
>> +	test_result = (__force u32 *)(buf + MPAMCFG_CPBM);
>>  
>>  	mpam_reset_msc_bitmap(&fake_msc, MPAMCFG_CPBM, 0);
>>  	KUNIT_EXPECT_EQ(test, test_result[0], 0);
> 
> 
> This change looks good to me. As sparse is more broken I needed to use
> the patch from [1] to reproduce this. Copied here for convenience.

The branch from Al Viro was working fine at that time, now merged to master.

> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 2b30a0529d48..90536b2bc42e 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -14,8 +14,8 @@ struct vm_area_struct;
>  struct mempolicy;
> 
>  /* Helper macro to avoid gfp flags if they are the default one */
> -#define __default_gfp(a,...) a
> -#define default_gfp(...) __default_gfp(__VA_ARGS__ __VA_OPT__(,)
> GFP_KERNEL)
> +#define __default_gfp(a,b,...) b
> +#define default_gfp(...) __default_gfp(,##__VA_ARGS__,GFP_KERNEL)
> 
>  /* Convert GFP flags to their corresponding migrate type */
>  #define GFP_MOVABLE_MASK (__GFP_RECLAIMABLE|__GFP_MOVABLE)
> 
> There is a kernel test robot report [2] and that asks for these tags:

That I did not know. Anyone can run sparse, as I am doing every now and
then, and find issues.

> 
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes:
> https://lore.kernel.org/oe-kbuild-all/202512160133.eAzPdJv2-lkp@xxxxxxxxx/
> 
> Acked-by: Ben Horgan <ben.horgan@xxxxxxx>

Best regards,
Krzysztof