[PATCH] drm/amdkfd: Avoid integer underflow with ffs in EOP ring size calc

The low 6 bits of cp_hqd_eop_control store the base-2 logarithm
of the EOP ring size. This was calculated as

ffs(q->eop_ring_buffer_size / sizeof(unsigned int)) - 1 - 1

But ffs can in theory return 1 or 0, so this could underflow
(although in practice the ring buffer size cannot be less than 4096).

Change this to

ffs(q->eop_ring_buffer_size / sizeof(unsigned int) / 4)

using properties of logarithms.

Signed-off-by: David Francis <[email protected]>
---
 drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v10.c   | 2 +-
 drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v11.c   | 2 +-
 drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v12.c   | 2 +-
 drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v12_1.c | 2 +-
 4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v10.c b/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v10.c
index e034da638c07..4f8a8a1a6186 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v10.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v10.c
@@ -204,7 +204,7 @@ static void update_mqd(struct mqd_manager *mm, void *mqd,
 	 * is safe, giving a maximum field value of 0xA.
 	 */
 	m->cp_hqd_eop_control = q->eop_ring_buffer_size ? min(0xA,
-		ffs(q->eop_ring_buffer_size / sizeof(unsigned int)) - 1 - 1) : 0;
+		ffs(q->eop_ring_buffer_size / sizeof(unsigned int) / 4)) : 0;
 	m->cp_hqd_eop_base_addr_lo =
 			lower_32_bits(q->eop_ring_buffer_address >> 8);
 	m->cp_hqd_eop_base_addr_hi =
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v11.c b/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v11.c
index 350fcbbba4b2..bf015dc5b868 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v11.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v11.c
@@ -242,7 +242,7 @@ static void update_mqd(struct mqd_manager *mm, void *mqd,
 	 * is safe, giving a maximum field value of 0xA.
 	 */
 	m->cp_hqd_eop_control = q->eop_ring_buffer_size ? min(0xA,
-		ffs(q->eop_ring_buffer_size / sizeof(unsigned int)) - 1 - 1) : 0;
+		ffs(q->eop_ring_buffer_size / sizeof(unsigned int) / 4)) : 0;
 	m->cp_hqd_eop_base_addr_lo =
 			lower_32_bits(q->eop_ring_buffer_address >> 8);
 	m->cp_hqd_eop_base_addr_hi =
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v12.c b/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v12.c
index 7c387fa90076..3942d6675268 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v12.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v12.c
@@ -217,7 +217,7 @@ static void update_mqd(struct mqd_manager *mm, void *mqd,
 	 * is safe, giving a maximum field value of 0xA.
 	 */
 	m->cp_hqd_eop_control = q->eop_ring_buffer_size ? min(0xA,
-		ffs(q->eop_ring_buffer_size / sizeof(unsigned int)) - 1 - 1) : 0;
+		ffs(q->eop_ring_buffer_size / sizeof(unsigned int) / 4)) : 0;
 	m->cp_hqd_eop_base_addr_lo =
 			lower_32_bits(q->eop_ring_buffer_address >> 8);
 	m->cp_hqd_eop_base_addr_hi =
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v12_1.c b/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v12_1.c
index 431a940f91f3..08da097f780a 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v12_1.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_mqd_manager_v12_1.c
@@ -295,7 +295,7 @@ static void update_mqd(struct mqd_manager *mm, void *mqd,
 	 * is safe, giving a maximum field value of 0xA.
 	 */
 	m->cp_hqd_eop_control = q->eop_ring_buffer_size ? min(0xA,
-		ffs(q->eop_ring_buffer_size / sizeof(unsigned int)) - 1 - 1) : 0;
+		ffs(q->eop_ring_buffer_size / sizeof(unsigned int) / 4)) : 0;
 	m->cp_hqd_eop_base_addr_lo =
 			lower_32_bits(q->eop_ring_buffer_address >> 8);
 	m->cp_hqd_eop_base_addr_hi =
-- 
2.34.1