linux-kernel - Re: [gpu-drm-radeon] question about potential dead code in vce_v2_0_enable

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Date:   Wed, 28 Jun 2017 18:08:43 -0500
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     "Deucher, Alexander" <Alexander.Deucher@....com>
Cc:     "Koenig, Christian" <Christian.Koenig@....com>,
        David Airlie <airlied@...ux.ie>, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [gpu-drm-radeon] question about potential dead code in
 vce_v2_0_enable_mgcg()

Hi Alex,

Quoting "Deucher, Alexander" <Alexander.Deucher@....com>:

>> -----Original Message-----
>> From: Gustavo A. R. Silva [mailto:garsilva@...eddedor.com]
>> Sent: Wednesday, June 28, 2017 10:22 AM
>> To: Deucher, Alexander; Koenig, Christian; David Airlie
>> Cc: amd-gfx@...ts.freedesktop.org; dri-devel@...ts.freedesktop.org; linux-
>> kernel@...r.kernel.org
>> Subject: [gpu-drm-radeon] question about potential dead code in
>> vce_v2_0_enable_mgcg()
>>
>>
>> Hello everybody,
>>
>> While looking into Coverity ID 1198635 I ran into the following piece
>> of code at drivers/gpu/drm/radeon/vce_v2_0.c:107:
>>
>> 107void vce_v2_0_enable_mgcg(struct radeon_device *rdev, bool enable)
>> 108{
>> 109        bool sw_cg = false;
>> 110
>> 111        if (enable && (rdev->cg_flags &
>> RADEON_CG_SUPPORT_VCE_MGCG)) {
>> 112                if (sw_cg)
>> 113                        vce_v2_0_set_sw_cg(rdev, true);
>> 114                else
>> 115                        vce_v2_0_set_dyn_cg(rdev, true);
>> 116        } else {
>> 117                vce_v2_0_disable_cg(rdev);
>> 118
>> 119                if (sw_cg)
>> 120                        vce_v2_0_set_sw_cg(rdev, false);
>> 121                else
>> 122                        vce_v2_0_set_dyn_cg(rdev, false);
>> 123        }
>> 124}
>>
>> The issue here is that local variable sw_cg is never updated again
>> after its initialization; which cause some code to be logically dead.
>>
>> My question here is if such variable is there for testing purposes or
>> if it is a sort of an old code leftover that should be removed?
>>
>> In any case I can send a patch to add a comment or remove the dead code.
>>
>> I'd really appreciate any comments on this.
>
> I wanted to leave the code in for debugging if we ran into problems  
> with dynamic clockgating.
>

Do you mind if I send a patch to add such comment and make it clear  
the purpose of that variable?

--- a/drivers/gpu/drm/radeon/vce_v2_0.c
+++ b/drivers/gpu/drm/radeon/vce_v2_0.c
@@ -104,6 +104,10 @@ static void vce_v2_0_disable_cg(struct  
radeon_device *rdev)
         WREG32(VCE_CGTT_CLK_OVERRIDE, 7);
  }

+/*
+ * Local variable sw_cg is used for debugging purposes, in case we
+ * ran into problems with dynamic clock gating. Don't remove it.
+ */
  void vce_v2_0_enable_mgcg(struct radeon_device *rdev, bool enable)
  {
         bool sw_cg = false;


Thanks for clarifying!
--
Gustavo A. R. Silva