[PATCH][next] drm/amd/pm: Replace one-element array with flexible-array in struct SISLANDS_SMC_SWSTATE

[Gustavo A. R. Silva]

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code according to the use of a flexible-array member in
struct SISLANDS_SMC_SWSTATE, instead of a one-element array, and use
the struct_size() helper to calculate the size for the allocation.

Also, this helps with the ongoing efforts to enable -Warray-bounds and
fix the following warnings:

drivers/gpu/drm/amd/amdgpu/../pm/powerplay/si_dpm.c:2448:20: warning: array
subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’
{aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]
drivers/gpu/drm/amd/amdgpu/../pm/powerplay/si_dpm.c:2449:20: warning: array
subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’
{aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]
drivers/gpu/drm/amd/amdgpu/../pm/powerplay/si_dpm.c:2450:20: warning: array
subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’
{aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]
drivers/gpu/drm/amd/amdgpu/../pm/powerplay/si_dpm.c:2451:20: warning: array
subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’
{aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]
drivers/gpu/drm/amd/amdgpu/../pm/powerplay/si_dpm.c:2452:20: warning: array
subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’
{aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]
drivers/gpu/drm/amd/amdgpu/../pm/powerplay/si_dpm.c:5570:20: warning: array
subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’
{aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Build-tested-by: kernel test robot <lkp at intel.com>
Link: https://lore.kernel.org/lkml/6023be58.sk66L%2FV4vuSJI5mI%25lkp@intel.com/ 
Signed-off-by: Gustavo A. R. Silva <gustavoars at kernel.org>
---
 drivers/gpu/drm/amd/pm/powerplay/si_dpm.c       |  6 ++----
 drivers/gpu/drm/amd/pm/powerplay/sislands_smc.h | 10 +++++-----
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/amd/pm/powerplay/si_dpm.c b/drivers/gpu/drm/amd/pm/powerplay/si_dpm.c
index afa1711c9620..62291358fb1c 100644
--- a/drivers/gpu/drm/amd/pm/powerplay/si_dpm.c
+++ b/drivers/gpu/drm/amd/pm/powerplay/si_dpm.c
@@ -5715,11 +5715,9 @@ static int si_upload_sw_state(struct amdgpu_device *adev,
 	int ret;
 	u32 address = si_pi->state_table_start +
 		offsetof(SISLANDS_SMC_STATETABLE, driverState);
-	u32 state_size = sizeof(SISLANDS_SMC_SWSTATE) +
-		((new_state->performance_level_count - 1) *
-		 sizeof(SISLANDS_SMC_HW_PERFORMANCE_LEVEL));
 	SISLANDS_SMC_SWSTATE *smc_state = &si_pi->smc_statetable.driverState;
-
+	size_t state_size = struct_size(smc_state, levels,
+					new_state->performance_level_count);
 	memset(smc_state, 0, state_size);
 
 	ret = si_convert_power_state_to_smc(adev, amdgpu_new_state, smc_state);
diff --git a/drivers/gpu/drm/amd/pm/powerplay/sislands_smc.h b/drivers/gpu/drm/amd/pm/powerplay/sislands_smc.h
index d2930eceaf3c..0f7554052c90 100644
--- a/drivers/gpu/drm/amd/pm/powerplay/sislands_smc.h
+++ b/drivers/gpu/drm/amd/pm/powerplay/sislands_smc.h
@@ -182,11 +182,11 @@ typedef struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL SISLANDS_SMC_HW_PERFORMANCE_LEV
 
 struct SISLANDS_SMC_SWSTATE
 {
-    uint8_t                             flags;
-    uint8_t                             levelCount;
-    uint8_t                             padding2;
-    uint8_t                             padding3;
-    SISLANDS_SMC_HW_PERFORMANCE_LEVEL   levels[1];
+	uint8_t                             flags;
+	uint8_t                             levelCount;
+	uint8_t                             padding2;
+	uint8_t                             padding3;
+	SISLANDS_SMC_HW_PERFORMANCE_LEVEL   levels[];
 };
 
 typedef struct SISLANDS_SMC_SWSTATE SISLANDS_SMC_SWSTATE;
-- 
2.27.0