# [PATCH] drm/amdgpu: simplify padding calculations (v3)

Luben Tuikov luben.tuikov at amd.com
Wed Dec 18 23:16:34 UTC 2019

```This patch seems to have been dropped after posting
and reposting. I'm not sure why it keeps being dropped.

In v3, I added an explanation in the commit text
below the original commit text of the single sentence
of "Simplify padding calculations." Identical diffstat
as v2.

Regards,
Luben

On 2019-12-18 6:12 p.m., Luben Tuikov wrote:
> Simplify padding calculations.
>
> 1. Taking the remainder of a binary value when
> the divisor is a power of two, such as,
> a % 2^n is identical to, a & (2^n - 1) in base-2
> arithmetic, and so expressions like this:
>
> (12 - (lower_32_bits(ring->wptr) & 7)) % 8
>
> are superflous. And can be reduced to a single
> remainder-taking operation.
>
> 2. Subtracting the remainder modulo 8 out of 12
> and then again taking the remainder modulo 8,
> yields the same result as simply subtracting
> the value out of 4 and then taking the remainder.
> This is true because 4 belongs to the congruence
> (residue) class {4 + 8*k}, k in Z. (So using,
>  {..., -12, -4, 4, 12, 20, ...}, would yield
>  the same final result.
>
> So the above expression, becomes,
>
> (4 - lower_32_bits(ring->wptr)) & 7
>
> 3. Now since 8 is part of the conguence class
> {0 + 8*k}, k in Z, and from 1) and 2) above, then,
>
> (8 - (ib->length_dw & 0x7)) % 8
>
> becomes, simply,
>
> (-ib->length_dw) & 7.
>
> This patch implements all of the above in this code.
>
> v2: Comment update and spacing.
> v3: Add 1, 2, 3, above, in this commit message.
>
> Signed-off-by: Luben Tuikov <luben.tuikov at amd.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/cik_sdma.c  |  4 ++--
>  drivers/gpu/drm/amd/amdgpu/sdma_v2_4.c |  4 ++--
>  drivers/gpu/drm/amd/amdgpu/sdma_v3_0.c |  4 ++--
>  drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c |  4 ++--
>  drivers/gpu/drm/amd/amdgpu/sdma_v5_0.c | 17 ++++++++++++-----
>  5 files changed, 20 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/cik_sdma.c b/drivers/gpu/drm/amd/amdgpu/cik_sdma.c
> index 1f22a8d0f7f3..4274ccf765de 100644
> --- a/drivers/gpu/drm/amd/amdgpu/cik_sdma.c
> +++ b/drivers/gpu/drm/amd/amdgpu/cik_sdma.c
> @@ -228,7 +228,7 @@ static void cik_sdma_ring_emit_ib(struct amdgpu_ring *ring,
>  	u32 extra_bits = vmid & 0xf;
>
>  	/* IB packet must end on a 8 DW boundary */
> -	cik_sdma_ring_insert_nop(ring, (12 - (lower_32_bits(ring->wptr) & 7)) % 8);
> +	cik_sdma_ring_insert_nop(ring, (4 - lower_32_bits(ring->wptr)) & 7);
>
>  	amdgpu_ring_write(ring, SDMA_PACKET(SDMA_OPCODE_INDIRECT_BUFFER, 0, extra_bits));
>  	amdgpu_ring_write(ring, ib->gpu_addr & 0xffffffe0); /* base must be 32 byte aligned */
> @@ -811,7 +811,7 @@ static void cik_sdma_ring_pad_ib(struct amdgpu_ring *ring, struct amdgpu_ib *ib)
>  	int i;
>
> -	pad_count = (8 - (ib->length_dw & 0x7)) % 8;
> +	pad_count = (-ib->length_dw) & 7;
>  	for (i = 0; i < pad_count; i++)
>  		if (sdma && sdma->burst_nop && (i == 0))
>  			ib->ptr[ib->length_dw++] =
> diff --git a/drivers/gpu/drm/amd/amdgpu/sdma_v2_4.c b/drivers/gpu/drm/amd/amdgpu/sdma_v2_4.c
> index 606b621145a1..fd7fa6082563 100644
> --- a/drivers/gpu/drm/amd/amdgpu/sdma_v2_4.c
> +++ b/drivers/gpu/drm/amd/amdgpu/sdma_v2_4.c
> @@ -255,7 +255,7 @@ static void sdma_v2_4_ring_emit_ib(struct amdgpu_ring *ring,
>  	unsigned vmid = AMDGPU_JOB_GET_VMID(job);
>
>  	/* IB packet must end on a 8 DW boundary */
> -	sdma_v2_4_ring_insert_nop(ring, (10 - (lower_32_bits(ring->wptr) & 7)) % 8);
> +	sdma_v2_4_ring_insert_nop(ring, (2 - lower_32_bits(ring->wptr)) & 7);
>
>  	amdgpu_ring_write(ring, SDMA_PKT_HEADER_OP(SDMA_OP_INDIRECT) |
>  			  SDMA_PKT_INDIRECT_HEADER_VMID(vmid & 0xf));
> @@ -750,7 +750,7 @@ static void sdma_v2_4_ring_pad_ib(struct amdgpu_ring *ring, struct amdgpu_ib *ib
>  	int i;
>
> -	pad_count = (8 - (ib->length_dw & 0x7)) % 8;
> +	pad_count = (-ib->length_dw) & 7;
>  	for (i = 0; i < pad_count; i++)
>  		if (sdma && sdma->burst_nop && (i == 0))
>  			ib->ptr[ib->length_dw++] =
> diff --git a/drivers/gpu/drm/amd/amdgpu/sdma_v3_0.c b/drivers/gpu/drm/amd/amdgpu/sdma_v3_0.c
> index a559573ec8fd..4a8a7f0f3a9c 100644
> --- a/drivers/gpu/drm/amd/amdgpu/sdma_v3_0.c
> +++ b/drivers/gpu/drm/amd/amdgpu/sdma_v3_0.c
> @@ -429,7 +429,7 @@ static void sdma_v3_0_ring_emit_ib(struct amdgpu_ring *ring,
>  	unsigned vmid = AMDGPU_JOB_GET_VMID(job);
>
>  	/* IB packet must end on a 8 DW boundary */
> -	sdma_v3_0_ring_insert_nop(ring, (10 - (lower_32_bits(ring->wptr) & 7)) % 8);
> +	sdma_v3_0_ring_insert_nop(ring, (2 - lower_32_bits(ring->wptr)) & 7);
>
>  	amdgpu_ring_write(ring, SDMA_PKT_HEADER_OP(SDMA_OP_INDIRECT) |
>  			  SDMA_PKT_INDIRECT_HEADER_VMID(vmid & 0xf));
> @@ -1021,7 +1021,7 @@ static void sdma_v3_0_ring_pad_ib(struct amdgpu_ring *ring, struct amdgpu_ib *ib
>  	int i;
>
> -	pad_count = (8 - (ib->length_dw & 0x7)) % 8;
> +	pad_count = (-ib->length_dw) & 7;
>  	for (i = 0; i < pad_count; i++)
>  		if (sdma && sdma->burst_nop && (i == 0))
>  			ib->ptr[ib->length_dw++] =
> diff --git a/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c b/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c
> index bd9ed33bab43..c69df0cb21ec 100644
> --- a/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c
> +++ b/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c
> @@ -698,7 +698,7 @@ static void sdma_v4_0_ring_emit_ib(struct amdgpu_ring *ring,
>  	unsigned vmid = AMDGPU_JOB_GET_VMID(job);
>
>  	/* IB packet must end on a 8 DW boundary */
> -	sdma_v4_0_ring_insert_nop(ring, (10 - (lower_32_bits(ring->wptr) & 7)) % 8);
> +	sdma_v4_0_ring_insert_nop(ring, (2 - lower_32_bits(ring->wptr)) & 7);
>
>  	amdgpu_ring_write(ring, SDMA_PKT_HEADER_OP(SDMA_OP_INDIRECT) |
>  			  SDMA_PKT_INDIRECT_HEADER_VMID(vmid & 0xf));
> @@ -1579,7 +1579,7 @@ static void sdma_v4_0_ring_pad_ib(struct amdgpu_ring *ring, struct amdgpu_ib *ib
>  	int i;
>
> -	pad_count = (8 - (ib->length_dw & 0x7)) % 8;
> +	pad_count = (-ib->length_dw) & 7;
>  	for (i = 0; i < pad_count; i++)
>  		if (sdma && sdma->burst_nop && (i == 0))
>  			ib->ptr[ib->length_dw++] =
> diff --git a/drivers/gpu/drm/amd/amdgpu/sdma_v5_0.c b/drivers/gpu/drm/amd/amdgpu/sdma_v5_0.c
> index 119364293cec..3912937f878f 100644
> --- a/drivers/gpu/drm/amd/amdgpu/sdma_v5_0.c
> +++ b/drivers/gpu/drm/amd/amdgpu/sdma_v5_0.c
> @@ -382,8 +382,15 @@ static void sdma_v5_0_ring_emit_ib(struct amdgpu_ring *ring,
>  	unsigned vmid = AMDGPU_JOB_GET_VMID(job);
>
> -	/* IB packet must end on a 8 DW boundary */
> -	sdma_v5_0_ring_insert_nop(ring, (10 - (lower_32_bits(ring->wptr) & 7)) % 8);
> +	/* An IB packet must end on a 8 DW boundary--the next dword
> +	 * must be on a 8-dword boundary. Our IB packet below is 6
> +	 * dwords long, thus add x number of NOPs, such that, in
> +	 * modular arithmetic,
> +	 * wptr + 6 + x = 8k, k >= 0, which in C is,
> +	 * (wptr + 6 + x) % 8 = 0.
> +	 * The expression below, is a solution of x.
> +	 */
> +	sdma_v5_0_ring_insert_nop(ring, (2 - lower_32_bits(ring->wptr)) & 7);
>
>  	amdgpu_ring_write(ring, SDMA_PKT_HEADER_OP(SDMA_OP_INDIRECT) |
>  			  SDMA_PKT_INDIRECT_HEADER_VMID(vmid & 0xf));
> @@ -1076,10 +1083,10 @@ static void sdma_v5_0_vm_set_pte_pde(struct amdgpu_ib *ib,
>  }
>
>  /**
> - * sdma_v5_0_ring_pad_ib - pad the IB to the required number of dw
> - *
> + * sdma_v5_0_ring_pad_ib - pad the IB
>   * @ib: indirect buffer to fill with padding
>   *
> + * Pad the IB with NOPs to a boundary multiple of 8.
>   */
>  static void sdma_v5_0_ring_pad_ib(struct amdgpu_ring *ring, struct amdgpu_ib *ib)
>  {
> @@ -1087,7 +1094,7 @@ static void sdma_v5_0_ring_pad_ib(struct amdgpu_ring *ring, struct amdgpu_ib *ib