[PATCH 2/2] drm/amdgpu: fix uninitialized variable warning

[Zhou, Bob]

[Public]

Thanks for your comments.

I should clarify the issue. As you see the amdgpu_i2c_get_byte code:
                if (i2c_transfer(&i2c_bus->adapter, msgs, 2) == 2) {
                        *val = in_buf[0];
                        DRM_DEBUG("val = 0x%02x\n", *val);
                } else {
                        DRM_DEBUG("i2c 0x%02x 0x%02x read failed\n",  addr, *val);
                }
If the read failure by amdgpu_i2c_get_byte(), the value will not be modified.
Then the amdgpu_i2c_put_byte() successfully written the random value and it will cause unexpected issue.

Regards,
Bob

-----Original Message-----
From: Koenig, Christian <Christian.Koenig at amd.com>
Sent: 2024年4月23日 14:05
To: Zhou, Bob <Bob.Zhou at amd.com>; amd-gfx at lists.freedesktop.org; Deucher, Alexander <Alexander.Deucher at amd.com>; Koenig, Christian <Christian.Koenig at amd.com>
Subject: Re: [PATCH 2/2] drm/amdgpu: fix uninitialized variable warning

Am 23.04.24 um 07:33 schrieb Bob Zhou:
> Because the val isn't initialized, a random variable is set by amdgpu_i2c_put_byte.
> So fix the uninitialized issue.

Well that isn't correct. See the code here:

         amdgpu_i2c_get_byte(amdgpu_connector->router_bus,
                             amdgpu_connector->router.i2c_addr,
                             0x3, &val);
         val &= ~amdgpu_connector->router.cd_mux_control_pin;
         amdgpu_i2c_put_byte(amdgpu_connector->router_bus,
                             amdgpu_connector->router.i2c_addr,
                             0x3, val);

The value is first read by amdgpu_i2c_get_byte(), then modified and then written again by amdgpu_i2c_put_byte().

Was this an automated warning?

Either way the patch is clearly rejected.

Regards,
Christian.

>
> Signed-off-by: Bob Zhou <bob.zhou at amd.com>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_i2c.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_i2c.c
> b/drivers/gpu/drm/amd/amdgpu/amdgpu_i2c.c
> index 82608df43396..d4d2dc792b60 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_i2c.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_i2c.c
> @@ -368,7 +368,7 @@ amdgpu_i2c_router_select_ddc_port(const struct amdgpu_connector *amdgpu_connecto
>   void
>   amdgpu_i2c_router_select_cd_port(const struct amdgpu_connector *amdgpu_connector)
>   {
> -     u8 val;
> +     u8 val = 0;
>
>       if (!amdgpu_connector->router.cd_valid)
>               return;