Re: [PATCH 4/4] drm/amdgpu: Set SDMA v3 copy_max_bytes to 0x3fff00

["Timur Kristóf" <timur.kristof@gmail.com>]

On Tue, 2025-09-02 at 08:45 +0200, Christian König wrote:
> On 01.09.25 12:00, Timur Kristóf wrote:
> > SDMA v3-v5 can copy almost 4 MiB in a single copy operation.
> > Use the	same value as PAL and Mesa for copy_max_bytes.
> > 
> > For reference, see oss2DmaCmdBuffer.cpp	in PAL:
> > "Due to HW limitation, the maximum count may not be 2^n-1,
> > can only be 2^n - 1 - start_addr[4:2]"
> 
> Is that public available? If yes better reference AMDVLK here.

I only have access to public code, so that's all I can reference.

AMDVLK uses PAL so I assume the same would apply to it as well.
The above comment is in oss2DmaCmdBuffer.cpp here:
https://github.com/GPUOpen-Drivers/pal/blob/bcec463efe5260776d486a5e3da0c549bc0a75d2/src/core/hw/ossip/oss2/oss2DmaCmdBuffer.cpp#L308



> 
> Apart from that looks good to me.
> 
> Regards,
> Christian.
> 
> > 
> > See also sid.h in Mesa:
> > "There is apparently an undocumented HW limitation that
> > prevents the HW from copying the last 255 bytes of (1 << 22) - 1"
> > 
> > Fixes: dfe5c2b76b2a ("drm/amdgpu: Correct bytes limit for SDMA 3.0
> > copy and fill")
> > Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
> > ---
> >  drivers/gpu/drm/amd/amdgpu/sdma_v3_0.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/amd/amdgpu/sdma_v3_0.c
> > b/drivers/gpu/drm/amd/amdgpu/sdma_v3_0.c
> > index 1c076bd1cf73..9302cf0b5e4b 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/sdma_v3_0.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/sdma_v3_0.c
> > @@ -1659,11 +1659,11 @@ static void
> > sdma_v3_0_emit_fill_buffer(struct amdgpu_ib *ib,
> >  }
> >  
> >  static const struct amdgpu_buffer_funcs sdma_v3_0_buffer_funcs = {
> > -	.copy_max_bytes = 0x3fffe0, /* not 0x3fffff due to HW
> > limitation */
> > +	.copy_max_bytes = 0x3fff00, /* not 0x3fffff due to HW
> > limitation */
> >  	.copy_num_dw = 7,
> >  	.emit_copy_buffer = sdma_v3_0_emit_copy_buffer,
> >  
> > -	.fill_max_bytes = 0x3fffe0, /* not 0x3fffff due to HW
> > limitation */
> > +	.fill_max_bytes = 0x3fff00, /* not 0x3fffff due to HW
> > limitation */
> >  	.fill_num_dw = 5,
> >  	.emit_fill_buffer = sdma_v3_0_emit_fill_buffer,
> >  };