clang warning: implicit conversion in intel_ddi.c:1481

[Ozan Alpay]

Dear Rodrigo Vivi, Ville Syrjälä,

My name is Ozan Alpay, and I am a student mentored by Lukas Bulwahn. We 
intend to use static analysis tools on the kernel source to identify, 
analyze and report issues. As a very first step, we are looking into 
clang compiler warnings and will then move to more sophisticated tools. 

When compiling Linux 4.15 with clang, we have discovered that your commit 
2952cd6fb4cc ("drm/i915: Let's use more enum intel_dpll_id pll_id.") 
introduced the following warning:

drivers/gpu/drm/i915/intel_ddi.c:1481:30: warning: implicit conversion from enumeration type 'enum port' to different enumeration type 'enum intel_dpll_id' [-Wenum-conversion] 
	enum intel_dpll_id pll_id = port;

To reproduce it, you can compile Linux 4.15 with clang with this command: 

make HOSTCC=clang-5.0 defconfig && make -j32 HOSTCC=clang-5.0 CC=clang-5.0

If you don't have clang installed in your system, you can use this simple 
docker setup to compile the kernel with clang:

wget https://raw.githubusercontent.com/bulwahn/linux-kernel-analysis/master/docker/kernel-clang/Dockerfile && \
docker build -t kernel-clang . && \
docker run -v <your kernel source directory>:/linux/ kernel-clang /bin/sh -c "cd linux && make CC=clang-5.0 clean && make HOSTCC=clang-5.0 defconfig && make -j32 HOSTCC=clang-5.0 CC=clang-5.0"
 
While we were doing our analysis on 4.15, we noticed that you already 
resolved this warning on linux-next with your work in commit bb911536f07e 
("drm/i915: Eliminate pll->state usage from bxt_calc_pll_link()"). So, 
since it is resolved on linux-next and we expect that this commit will be 
merged in the merge window for 4.16, there is probably nothing further to 
do.

Linux 4.15 is shipped with this clang warning, but we don't see the 
crucial need to provide a backport commit to the stable branch for 4.15. 
We just wanted to inform you about our analysis of this clang warning. 
Ultimately the final call if you would like to address this clang warning 
in 4.15 is yours.

Best regards,

Ozan & Lukas