[PATCH v2] drm/radeon/dpm: Replace one-element array and use struct_size() helper

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

The current codebase makes use of one-element arrays in the following
form:

struct something {
    int length;
    u8 data[1];
};

struct something *instance;

instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
instance->length = size;
memcpy(instance->data, source, size);

but the preferred mechanism to declare variable-length types such as
these ones is a flexible array member[1][2], introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on. So, replace
the one-element array with a flexible-array member.

Also, make use of the new struct_size() helper to properly calculate the
size of struct NISLANDS_SMC_SWSTATE.

This issue was found with the help of Coccinelle and, audited and fixed
_manually_.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
Changes in v2:
 - Use type size_t instead of u16 for state_size variable.

 drivers/gpu/drm/amd/amdgpu/si_dpm.h | 2 +-
 drivers/gpu/drm/radeon/ni_dpm.c     | 7 ++++---
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/si_dpm.h b/drivers/gpu/drm/amd/amdgpu/si_dpm.h
index 6b7d292b919f3..bc0be6818e218 100644
--- a/drivers/gpu/drm/amd/amdgpu/si_dpm.h
+++ b/drivers/gpu/drm/amd/amdgpu/si_dpm.h
@@ -781,7 +781,7 @@ struct NISLANDS_SMC_SWSTATE
     uint8_t                             levelCount;
     uint8_t                             padding2;
     uint8_t                             padding3;
-    NISLANDS_SMC_HW_PERFORMANCE_LEVEL   levels[1];
+    NISLANDS_SMC_HW_PERFORMANCE_LEVEL   levels[];
 };
 
 typedef struct NISLANDS_SMC_SWSTATE NISLANDS_SMC_SWSTATE;
diff --git a/drivers/gpu/drm/radeon/ni_dpm.c b/drivers/gpu/drm/radeon/ni_dpm.c
index b57c37ddd164c..abb6345bfae32 100644
--- a/drivers/gpu/drm/radeon/ni_dpm.c
+++ b/drivers/gpu/drm/radeon/ni_dpm.c
@@ -2685,11 +2685,12 @@ static int ni_upload_sw_state(struct radeon_device *rdev,
 	struct rv7xx_power_info *pi = rv770_get_pi(rdev);
 	u16 address = pi->state_table_start +
 		offsetof(NISLANDS_SMC_STATETABLE, driverState);
-	u16 state_size = sizeof(NISLANDS_SMC_SWSTATE) +
-		((NISLANDS_MAX_SMC_PERFORMANCE_LEVELS_PER_SWSTATE - 1) * sizeof(NISLANDS_SMC_HW_PERFORMANCE_LEVEL));
+	NISLANDS_SMC_SWSTATE *smc_state;
+	size_t state_size = struct_size(smc_state, levels,
+			NISLANDS_MAX_SMC_PERFORMANCE_LEVELS_PER_SWSTATE);
 	int ret;
-	NISLANDS_SMC_SWSTATE *smc_state = kzalloc(state_size, GFP_KERNEL);
 
+	smc_state = kzalloc(state_size, GFP_KERNEL);
 	if (smc_state == NULL)
 		return -ENOMEM;
 
-- 
2.26.2