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

[Ben Horgan <ben.horgan@arm.com>]

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@oss.qualcomm.com>
> ---
>  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.

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:

Reported-by: kernel test robot <lkp@intel.com>
Closes:
https://lore.kernel.org/oe-kbuild-all/202512160133.eAzPdJv2-lkp@intel.com/

Acked-by: Ben Horgan <ben.horgan@arm.com>

[1]
https://lore.kernel.org/all/CAHk-=wijD-giccF6sJ+BdJpGDX9kPEUT6kryaQG0GRyJ3QQwng@mail.gmail.com/
[2] https://lore.kernel.org/all/202512160133.eAzPdJv2-lkp@intel.com/

Thanks,

Ben