linux-kernel - Re: [PATCH] Optimized division operation to shift operation

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Message-ID: <b5ffc6b7-bfa5-0827-a267-4e8c20027982@amd.com>
Date:   Wed, 15 Apr 2020 09:57:16 +0200
From:   Christian König <christian.koenig@....com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Alex Deucher <alexdeucher@...il.com>,
        Bernard Zhao <bernard@...o.com>
Cc:     Alex Sierra <alex.sierra@....com>, Oak Zeng <Oak.Zeng@....com>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>, David Airlie <airlied@...ux.ie>,
        Felix Kuehling <Felix.Kuehling@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>, kernel@...o.com,
        Huang Rui <ray.huang@....com>,
        Kent Russell <kent.russell@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Sam Ravnborg <sam@...nborg.org>,
        Xiaojie Yuan <xiaojie.yuan@....com>
Subject: Re: [PATCH] Optimized division operation to shift operation

Am 15.04.20 um 09:41 schrieb Jani Nikula:
> On Tue, 14 Apr 2020, Alex Deucher <alexdeucher@...il.com> wrote:
>> On Tue, Apr 14, 2020 at 9:05 AM Bernard Zhao <bernard@...o.com> 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@...o.com>
>> 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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