Re: [PATCH v2 1/2] staging: sm750fb: Replace busy-wait loop with udelay()

[Greg KH]

On Mon, Mar 16, 2026 at 03:42:04PM +0800, OaroraEtimis wrote:
> Hi Greg,
> 
> Thanks for the review. Sorry for dropping the historical comment in v2. My
> only goal was to fix the -Wunused-but-set-variable warning and prevent the
> loop from being optimized away by the compiler.
> 
> I will definitely restore the comment.
> 
> 
> On Mon, Mar 16, 2026 at 14:11, Greg KH wrote:
> 
> > How is "2" the same as this busy loop?
> 
> It was a rough estimation. A 600-iteration empty loop on older CPUs
> (~500MHz) took about 2 to 3 microseconds.
> 
> 
> > And why not fix this properly, as the comment states?
> 
> The comment suggests writing to VGA ports (0x3ce/0x3cf) to force a delay. I
> didn't implement this because I don't have the specific hardware or
> datasheets to test it.
> 
> I was afraid that introducing direct VGA I/O just to fix a compiler warning
> might cause unexpected hardware regressions *or compatibility issues across
> different platforms.*
> 
> Given that I can't test hardware I/O, how would you prefer I handle this in
> v3?

Don't do any logical change that testing without hardware can not be
verified (i.e. whitespace changes).

> 1. Keep the original loop but add cpu_relax() inside to prevent compiler
> optimization. (Safest for the hardware)

Are you sure?  Can you sleep at that point in time?  What does the
performance of the device then look like?

> 2. Use udelay(2) (or ndelay) and restore the historical comment.
> 3. Migrate the driver to the standard i2c-algo-bit framework (a much heavier
> refactoring).

Number 3 is the best way.

good luck!

greg k-h