linux-kernel - Re: [PATCH v6 9/9] drm/amd/display: Mark dc_fixpt_from

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Message-ID: <73c297f1-05a7-e786-557c-b267b125b6d5@loongson.cn>
Date: Wed, 25 Dec 2024 17:43:54 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Xi Ruoyao <xry111@...111.site>, Peter Zijlstra <peterz@...radead.org>,
 Alex Deucher <alexdeucher@...il.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Huacai Chen <chenhuacai@...nel.org>, loongarch@...ts.linux.dev,
 amd-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 Nick Desaulniers <ndesaulniers@...gle.com>, llvm@...ts.linux.dev
Subject: Re: [PATCH v6 9/9] drm/amd/display: Mark dc_fixpt_from_fraction()
 noinline

On 12/24/2024 05:46 AM, Nathan Chancellor wrote:
> On Sun, Dec 22, 2024 at 12:27:47PM +0800, Tiezhu Yang wrote:

...

>> With the above changes, there is no "falls through" objtool warning
>> compiled with both clang 19 and the latest mainline clang 20.
>
> I am somewhat surprised that changes anything because the ASSERT is not
> stopping control flow so I would expect the same problem as before. I
> guess it does not happen perhaps due to inlining differences? I looked

It is weird and I think it is not the correct way.

> at this code briefly when I sent my initial message and I was not sure
> where such a check should exist. It does not look like these functions
> really do any sort of error handling.
>
>> If you are OK with it, I will send a separate formal patch to handle
>> this issue after doing some more testing.
>
> It may still be worth doing this to get some initial thoughts from the
> AMD DRM folks.

I think the correct way is:

Keep the current ASSERT for the aim of debugging, just add BUG() to
stop control flow if the divisor is zero.

--- >8 ---

diff --git a/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c 
b/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
index 88d3f9d7dd55..e15391e36b40 100644
--- a/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
+++ b/drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.c
@@ -52,6 +52,7 @@ static inline unsigned long long 
complete_integer_division_u64(
         unsigned long long result;

         ASSERT(divisor);
+       BUG_ON(!divisor);

         result = div64_u64_rem(dividend, divisor, remainder);

diff --git a/drivers/gpu/drm/amd/display/dc/spl/spl_fixpt31_32.c 
b/drivers/gpu/drm/amd/display/dc/spl/spl_fixpt31_32.c
index 131f1e3949d3..ce2036950808 100644
--- a/drivers/gpu/drm/amd/display/dc/spl/spl_fixpt31_32.c
+++ b/drivers/gpu/drm/amd/display/dc/spl/spl_fixpt31_32.c
@@ -30,6 +30,7 @@ static inline unsigned long long 
spl_complete_integer_division_u64(
         unsigned long long result;

         SPL_ASSERT(divisor);
+       BUG_ON(!divisor);

         result = spl_div64_u64_rem(dividend, divisor, remainder);

It looks reasonable and works well both on x86 and LoongArch, there are
no the following objtool warnings:

   dc_fixpt_recip() falls through to next function dc_fixpt_sinc()
   spl_fixpt_recip() falls through to next function spl_fixpt_sinc()

If no more comments, I will send a separate formal patch for your
review in the next week.

Thanks,
Tiezhu