Re: [PATCH] staging: sm750fb: do not program the PLL from an uninitialized value

[Dan Carpenter <error27@gmail.com>]

On Mon, Aug 17, 2026 at 05:13:53PM +0800, Junrui Luo via B4 Relay wrote:
> From: Junrui Luo <moonafterrain@outlook.com>
> 
> sm750_calc_pll_value() writes pll->M, N, OD and POD only when its search
> loop finds a divider combination with 0 < M < 256, and returns 0 when
> there is none. ddk750_set_mode_timing() discards that return value and
> calls program_mode_registers() regardless, so sm750_format_pll_reg()
> reads the four members uninitialized and pokes them into PANEL_PLL_CTRL
> or CRT_PLL_CTRL. Nothing bounds var->pixclock on the way in, so a mode
> set can ask for a clock the loop cannot represent.
> 
> Consume the return value and reject the mode; hw_sm750_crtc_set_mode()
> already propagates a non-zero return. Initialize the structure as well:
> sm750_calc_pll_value() returns early for SM750LE without writing the
> members, and returns non-zero on that path.
> 
> Fixes: 81dee67e215b ("staging: sm750fb: add sm750 to staging")
> Reported-by: Yuhao Jiang <danisjiang@gmail.com>
> Assisted-by: Claude:claude-opus-5
> Cc: stable@vger.kernel.org
> Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
> ---

Greg is not taking AI patches unless they can be tested.
https://lore.kernel.org/all/2026080354-skater-urgent-31b2@gregkh/

I kind of hate AI commit messages...  They are so verbose, confident
and reasonable sounding.  But they don't answer any of the real
questions I want to know.  How did Yuhao Jiang find this bug?  What
did the symptoms look like to a user?  Are there ways we could
improve our QC process to prevent this sort of bug in the future?

Probably the answer is that the bug was detected with AI and we
have no idea what the symptoms look like.  Everyone sane automatically
initializes variables to zero so probably there are no symptoms.

So the problem is that the user inputs invalid var->pixclock, and
it leads to an uninitialized variable usage.  This patch addresses
it by initializing he variable to zero and checking if
sm750_calc_pll_value() returns an error code.  Either approach on
its own would would fix the problem, hopefully right?  So it's a belt
and suspenders approach.  But isn't the real solution to reject
invalid pixclocks in lynxfb_ops_check_var()?

We're not going to apply this patch because it hasn't been tested.
Probably we should invent a new tag so we can create a TODO list
of rejected AI patches.

KTODO: investigate unintialized variables in sm750fb found by AI

regards,
dan carpenter