Re: [PATCH 01/18] drm/amdgpu: More compact VCN IB emission

Fri, 18 Sep 2026 11:14:38 -0700 


On 18/09/2026 14:23, Lazar, Lijo wrote:

On 18-Sep-26 6:43 PM, Tvrtko Ursulin wrote:


On 18/09/2026 14:08, Lazar, Lijo wrote:



On 18-Sep-26 5:35 PM, Tvrtko Ursulin wrote:

Avoid constant register reloads while emitting IBs by using a local 
write

pointer and only updating the size at the end of each helper.

Signed-off-by: Tvrtko Ursulin <[email protected]>
Cc: Timur Kristóf <[email protected]>
Cc: Alex Deucher <[email protected]>
---
v2:
  *  Fix decode buffer overwrite


Overall for this series, below type of calculation  -

     ib->length_dw = ptr - ib->ptr;

is not anticipating a case like below.

         r = amdgpu_vcn_enc_get_create_msg(ring, 1, &ib, NULL);
         if (r)
                 goto error;

         r = amdgpu_vcn_enc_get_destroy_msg(ring, 1, &ib, &fence);
         if (r)
                 goto error;



So amdgpu_vcn_enc_ring_test_ib() - are you saying there is a bug 
there? Help me see it please.



ib->length_dw update is enclosed in both of those helpers, same as 
before the patch.




What I'm saying is -

It's the same ib pointer that is passed to back to back calls.

Legacy implementation fills like -

ib->ptr[ib->length_dw++]


So no assumption that it is starting from offset 0 always. After the 
function is filled, length gets updated based on what is filled.


I don't see the same with an assignment like

     ib->length_dw = ptr - ib->ptr;



Hm, but both of those helpers emit separate jobs with their own 
respective IB.


Regards,

Tvrtko


---

  drivers/gpu/drm/amd/amdgpu/amdgpu_vcn.c | 134 ++++++++++++ 
+-----------

  1 file changed, 75 insertions(+), 59 deletions(-)


diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vcn.c b/drivers/gpu/ 
drm/amd/amdgpu/amdgpu_vcn.c

index 17db7264269e..2c367ee6e182 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vcn.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vcn.c

@@ -622,9 +622,12 @@ static int amdgpu_vcn_dec_send_msg(struct 
amdgpu_ring *ring,

  {
      u64 addr = AMDGPU_GPU_PAGE_ALIGN(ib_msg->gpu_addr);
      struct amdgpu_device *adev = ring->adev;

+    struct amdgpu_vcn_reg *internal = &adev->vcn.inst[ring- 
>me].internal;

+    uint32_t nop = PACKET0(internal->nop, 0);
      struct dma_fence *f = NULL;
      struct amdgpu_job *job;
      struct amdgpu_ib *ib;
+    uint32_t *ptr;
      int i, r;
      r = amdgpu_job_alloc_with_ib(ring->adev, NULL, NULL,

@@ -635,15 +638,16 @@ static int amdgpu_vcn_dec_send_msg(struct 
amdgpu_ring *ring,

          goto err;
      ib = &job->ibs[0];
-    ib->ptr[0] = PACKET0(adev->vcn.inst[ring->me].internal.data0, 0);
-    ib->ptr[1] = addr;
-    ib->ptr[2] = PACKET0(adev->vcn.inst[ring->me].internal.data1, 0);
-    ib->ptr[3] = addr >> 32;
-    ib->ptr[4] = PACKET0(adev->vcn.inst[ring->me].internal.cmd, 0);
-    ib->ptr[5] = 0;
+    ptr = ib->ptr;
+    *ptr++ = PACKET0(internal->data0, 0);
+    *ptr++ = addr;
+    *ptr++ = PACKET0(internal->data1, 0);
+    *ptr++ = addr >> 32;
+    *ptr++ = PACKET0(internal->cmd, 0);
+    *ptr++ = 0;
      for (i = 6; i < 16; i += 2) {

-        ib->ptr[i] = PACKET0(adev->vcn.inst[ring->me].internal.nop, 
0);

-        ib->ptr[i+1] = 0;
+        *ptr++ = nop;
+        *ptr++ = 0;
      }
      ib->length_dw = 16;

@@ -763,17 +767,20 @@ int amdgpu_vcn_dec_ring_test_ib(struct 
amdgpu_ring *ring, long timeout)
  static uint32_t *amdgpu_vcn_unified_ring_ib_header(struct 
amdgpu_ib *ib,

                          uint32_t ib_pack_in_dw, bool enc)
  {
+    u32 *ptr = &ib->ptr[ib->length_dw];
      uint32_t *ib_checksum;
-    ib->ptr[ib->length_dw++] = 0x00000010; /* single queue checksum */
-    ib->ptr[ib->length_dw++] = 0x30000002;
-    ib_checksum = &ib->ptr[ib->length_dw++];
-    ib->ptr[ib->length_dw++] = ib_pack_in_dw;
+    *ptr++ = 0x00000010; /* single queue checksum */
+    *ptr++ = 0x30000002;
+    ib_checksum = ptr++;
+    *ptr++ = ib_pack_in_dw;
-    ib->ptr[ib->length_dw++] = 0x00000010; /* engine info */
-    ib->ptr[ib->length_dw++] = 0x30000001;
-    ib->ptr[ib->length_dw++] = enc ? 0x2 : 0x3;
-    ib->ptr[ib->length_dw++] = ib_pack_in_dw * sizeof(uint32_t);
+    *ptr++ = 0x00000010; /* engine info */
+    *ptr++ = 0x30000001;
+    *ptr++ = enc ? 0x2 : 0x3;
+    *ptr++ = ib_pack_in_dw * sizeof(uint32_t);
+
+    ib->length_dw = ptr - ib->ptr;
      return ib_checksum;
  }

@@ -803,7 +810,8 @@ static int amdgpu_vcn_dec_sw_send_msg(struct 
amdgpu_ring *ring,

      uint64_t addr = AMDGPU_GPU_PAGE_ALIGN(ib_msg->gpu_addr);
      uint32_t *ib_checksum;
      uint32_t ib_pack_in_dw;
-    int i, r;
+    u32 *ptr;
+    int r;
      if (adev->vcn.inst[ring->me].using_unified_queue)
          ib_size_dw += 8;

@@ -825,18 +833,20 @@ static int amdgpu_vcn_dec_sw_send_msg(struct 
amdgpu_ring *ring,
          ib_checksum = amdgpu_vcn_unified_ring_ib_header(ib, 
ib_pack_in_dw, false);

      }

-    ib->ptr[ib->length_dw++] = sizeof(struct 
amdgpu_vcn_decode_buffer) + 8;
-    ib->ptr[ib->length_dw++] = 
cpu_to_le32(AMDGPU_VCN_IB_FLAG_DECODE_BUFFER);
-    decode_buffer = (struct amdgpu_vcn_decode_buffer *)&(ib- 
>ptr[ib- >length_dw]);

-    ib->length_dw += sizeof(struct amdgpu_vcn_decode_buffer) / 4;
+    ptr = &ib->ptr[ib->length_dw];
+
+    *ptr++ = sizeof(struct amdgpu_vcn_decode_buffer) + 8;
+    *ptr++ = cpu_to_le32(AMDGPU_VCN_IB_FLAG_DECODE_BUFFER);
+    decode_buffer = (struct amdgpu_vcn_decode_buffer *)ptr;
+    ptr += sizeof(struct amdgpu_vcn_decode_buffer) / 4;
+    ib->length_dw = ptr - ib->ptr;

      memset(decode_buffer, 0, sizeof(struct 
amdgpu_vcn_decode_buffer));
      decode_buffer->valid_buf_flag |= 
cpu_to_le32(AMDGPU_VCN_CMD_FLAG_MSG_BUFFER);

      decode_buffer->msg_buffer_address_hi = cpu_to_le32(addr >> 32);
      decode_buffer->msg_buffer_address_lo = cpu_to_le32(addr);
-    for (i = ib->length_dw; i < ib_size_dw; ++i)
-        ib->ptr[i] = 0x0;
+    memset32(ptr, 0, ib_size_dw - ib->length_dw);
      if (adev->vcn.inst[ring->me].using_unified_queue)

          amdgpu_vcn_unified_ring_ib_checksum(&ib_checksum, 
ib_pack_in_dw);
@@ -934,7 +944,8 @@ static int amdgpu_vcn_enc_get_create_msg(struct 
amdgpu_ring *ring, uint32_t hand

      struct dma_fence *f = NULL;
      uint32_t *ib_checksum = NULL;
      uint64_t addr;
-    int i, r;
+    u32 *ptr;
+    int r;
      if (adev->vcn.inst[ring->me].using_unified_queue)
          ib_size_dw += 8;

@@ -947,31 +958,33 @@ static int 
amdgpu_vcn_enc_get_create_msg(struct amdgpu_ring *ring, uint32_t hand

          return r;
      ib = &job->ibs[0];
-    addr = AMDGPU_GPU_PAGE_ALIGN(ib_msg->gpu_addr);
-
      ib->length_dw = 0;
+    addr = AMDGPU_GPU_PAGE_ALIGN(ib_msg->gpu_addr);
      if (adev->vcn.inst[ring->me].using_unified_queue)

          ib_checksum = amdgpu_vcn_unified_ring_ib_header(ib, 0x11, 
true);

-    ib->ptr[ib->length_dw++] = 0x00000018;
-    ib->ptr[ib->length_dw++] = 0x00000001; /* session info */
-    ib->ptr[ib->length_dw++] = handle;
-    ib->ptr[ib->length_dw++] = upper_32_bits(addr);
-    ib->ptr[ib->length_dw++] = addr;
-    ib->ptr[ib->length_dw++] = 0x00000000;
+    ptr = &ib->ptr[ib->length_dw];
-    ib->ptr[ib->length_dw++] = 0x00000014;
-    ib->ptr[ib->length_dw++] = 0x00000002; /* task info */
-    ib->ptr[ib->length_dw++] = 0x0000001c;
-    ib->ptr[ib->length_dw++] = 0x00000000;
-    ib->ptr[ib->length_dw++] = 0x00000000;
+    *ptr++ = 0x00000018;
+    *ptr++ = 0x00000001; /* session info */
+    *ptr++ = handle;
+    *ptr++ = upper_32_bits(addr);
+    *ptr++ = addr;
+    *ptr++ = 0x00000000;
-    ib->ptr[ib->length_dw++] = 0x00000008;
-    ib->ptr[ib->length_dw++] = 0x08000001; /* op initialize */
+    *ptr++ = 0x00000014;
+    *ptr++ = 0x00000002; /* task info */
+    *ptr++ = 0x0000001c;
+    *ptr++ = 0x00000000;
+    *ptr++ = 0x00000000;
-    for (i = ib->length_dw; i < ib_size_dw; ++i)
-        ib->ptr[i] = 0x0;
+    *ptr++ = 0x00000008;
+    *ptr++ = 0x08000001; /* op initialize */
+
+    ib->length_dw = ptr - ib->ptr;
+
+    memset32(ptr, 0, ib_size_dw - ib->length_dw);
      if (adev->vcn.inst[ring->me].using_unified_queue)
          amdgpu_vcn_unified_ring_ib_checksum(&ib_checksum, 0x11);

@@ -1002,7 +1015,8 @@ static int 
amdgpu_vcn_enc_get_destroy_msg(struct amdgpu_ring *ring, uint32_t han

      struct dma_fence *f = NULL;
      uint32_t *ib_checksum = NULL;
      uint64_t addr;
-    int i, r;
+    u32 *ptr;
+    int r;
      if (adev->vcn.inst[ring->me].using_unified_queue)
          ib_size_dw += 8;

@@ -1015,31 +1029,33 @@ static int 
amdgpu_vcn_enc_get_destroy_msg(struct amdgpu_ring *ring, uint32_t han

          return r;
      ib = &job->ibs[0];
-    addr = AMDGPU_GPU_PAGE_ALIGN(ib_msg->gpu_addr);
-
      ib->length_dw = 0;
+    addr = AMDGPU_GPU_PAGE_ALIGN(ib_msg->gpu_addr);
      if (adev->vcn.inst[ring->me].using_unified_queue)

          ib_checksum = amdgpu_vcn_unified_ring_ib_header(ib, 0x11, 
true);

-    ib->ptr[ib->length_dw++] = 0x00000018;
-    ib->ptr[ib->length_dw++] = 0x00000001;
-    ib->ptr[ib->length_dw++] = handle;
-    ib->ptr[ib->length_dw++] = upper_32_bits(addr);
-    ib->ptr[ib->length_dw++] = addr;
-    ib->ptr[ib->length_dw++] = 0x00000000;
+    ptr = &ib->ptr[ib->length_dw];
-    ib->ptr[ib->length_dw++] = 0x00000014;
-    ib->ptr[ib->length_dw++] = 0x00000002;
-    ib->ptr[ib->length_dw++] = 0x0000001c;
-    ib->ptr[ib->length_dw++] = 0x00000000;
-    ib->ptr[ib->length_dw++] = 0x00000000;
+    *ptr++ = 0x00000018;
+    *ptr++ = 0x00000001;
+    *ptr++ = handle;
+    *ptr++ = upper_32_bits(addr);
+    *ptr++ = addr;
+    *ptr++ = 0x00000000;
-    ib->ptr[ib->length_dw++] = 0x00000008;
-    ib->ptr[ib->length_dw++] = 0x08000002; /* op close session */
+    *ptr++ = 0x00000014;
+    *ptr++ = 0x00000002;
+    *ptr++ = 0x0000001c;
+    *ptr++ = 0x00000000;
+    *ptr++ = 0x00000000;
-    for (i = ib->length_dw; i < ib_size_dw; ++i)
-        ib->ptr[i] = 0x0;
+    *ptr++ = 0x00000008;
+    *ptr++ = 0x08000002; /* op close session */
+
+    ib->length_dw = ptr - ib->ptr;
+
+    memset32(ptr, 0, ib_size_dw - ib->length_dw);
      if (adev->vcn.inst[ring->me].using_unified_queue)
          amdgpu_vcn_unified_ring_ib_checksum(&ib_checksum, 0x11);































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