Re: [PATCH] Optimized division operation to shift operation

[Christian König <christian.koenig@xxxxxxx>]

Am 15.04.20 um 09:41 schrieb Jani Nikula:
> On Tue, 14 Apr 2020, Alex Deucher <alexdeucher@xxxxxxxxx> wrote:
> > On Tue, Apr 14, 2020 at 9:05 AM Bernard Zhao <bernard@xxxxxxxx> wrote:
> > > On some processors, the / operate will call the compiler`s div lib,
> > > which is low efficient, We can replace the / operation with shift,
> > > so that we can replace the call of the division library with one
> > > shift assembly instruction.
> This was applied already, and it's not in a driver I look after... but
> to me this feels like something that really should be
> justified. Using >> instead of / for multiples of 2 division mattered 20
> years ago, I'd be surprised if it still did on modern compilers.

I have similar worries, especially since we replace the "/ (4 * 2)" with 
">> 3" it's making the code just a bit less readable.

And that the code runs exactly once while loading the driver and pushing 
the firmware into the hardware. So performance is completely irrelevant 
here.

Regards,
Christian.

> BR,
> Jani.
>
>
> > > Signed-off-by: Bernard Zhao <bernard@xxxxxxxx>
> > Applied.  thanks.
> >
> > Alex
> >
> > > ---
> > >   drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c | 4 ++--
> > >   drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c | 4 ++--
> > >   drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c | 4 ++--
> > >   3 files changed, 6 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c b/drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c
> > > index b205039..66cd078 100644
> > > --- a/drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c
> > > +++ b/drivers/gpu/drm/amd/amdgpu/gmc_v6_0.c
> > > @@ -175,10 +175,10 @@ static int gmc_v6_0_mc_load_microcode(struct amdgpu_device *adev)
> > >          amdgpu_ucode_print_mc_hdr(&hdr->header);
> > >
> > >          adev->gmc.fw_version = le32_to_cpu(hdr->header.ucode_version);
> > > -       regs_size = le32_to_cpu(hdr->io_debug_size_bytes) / (4 * 2);
> > > +       regs_size = le32_to_cpu(hdr->io_debug_size_bytes) >> 3;
> > >          new_io_mc_regs = (const __le32 *)
> > >                  (adev->gmc.fw->data + le32_to_cpu(hdr->io_debug_array_offset_bytes));
> > > -       ucode_size = le32_to_cpu(hdr->header.ucode_size_bytes) / 4;
> > > +       ucode_size = le32_to_cpu(hdr->header.ucode_size_bytes) >> 2;
> > >          new_fw_data = (const __le32 *)
> > >                  (adev->gmc.fw->data + le32_to_cpu(hdr->header.ucode_array_offset_bytes));
> > >
> > > diff --git a/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c b/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c
> > > index 9da9596..ca26d63 100644
> > > --- a/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c
> > > +++ b/drivers/gpu/drm/amd/amdgpu/gmc_v7_0.c
> > > @@ -193,10 +193,10 @@ static int gmc_v7_0_mc_load_microcode(struct amdgpu_device *adev)
> > >          amdgpu_ucode_print_mc_hdr(&hdr->header);
> > >
> > >          adev->gmc.fw_version = le32_to_cpu(hdr->header.ucode_version);
> > > -       regs_size = le32_to_cpu(hdr->io_debug_size_bytes) / (4 * 2);
> > > +       regs_size = le32_to_cpu(hdr->io_debug_size_bytes) >> 3;
> > >          io_mc_regs = (const __le32 *)
> > >                  (adev->gmc.fw->data + le32_to_cpu(hdr->io_debug_array_offset_bytes));
> > > -       ucode_size = le32_to_cpu(hdr->header.ucode_size_bytes) / 4;
> > > +       ucode_size = le32_to_cpu(hdr->header.ucode_size_bytes) >> 2;
> > >          fw_data = (const __le32 *)
> > >                  (adev->gmc.fw->data + le32_to_cpu(hdr->header.ucode_array_offset_bytes));
> > >
> > > diff --git a/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c b/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c
> > > index 27d83204..295039c 100644
> > > --- a/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c
> > > +++ b/drivers/gpu/drm/amd/amdgpu/gmc_v8_0.c
> > > @@ -318,10 +318,10 @@ static int gmc_v8_0_tonga_mc_load_microcode(struct amdgpu_device *adev)
> > >          amdgpu_ucode_print_mc_hdr(&hdr->header);
> > >
> > >          adev->gmc.fw_version = le32_to_cpu(hdr->header.ucode_version);
> > > -       regs_size = le32_to_cpu(hdr->io_debug_size_bytes) / (4 * 2);
> > > +       regs_size = le32_to_cpu(hdr->io_debug_size_bytes) >> 3;
> > >          io_mc_regs = (const __le32 *)
> > >                  (adev->gmc.fw->data + le32_to_cpu(hdr->io_debug_array_offset_bytes));
> > > -       ucode_size = le32_to_cpu(hdr->header.ucode_size_bytes) / 4;
> > > +       ucode_size = le32_to_cpu(hdr->header.ucode_size_bytes) >> 2;
> > >          fw_data = (const __le32 *)
> > >                  (adev->gmc.fw->data + le32_to_cpu(hdr->header.ucode_array_offset_bytes));
> > >
> > > --
> > > 2.7.4
> > >
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