Re: [PATCH] staging: fbtft: Optimize partial write()

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Fri, Feb 13, 2026 at 09:59:46AM +0100, Nam Cao wrote:
> When user write() only to part of the screen, the driver still updates the
> entire screen. That wastes CPU cycles.
> 
> Optimize by updating only the changed lines. Improvement is measured by a
> pair of trace_printk() at the beginning of fb_write() and at the end of
> fbtft_deferred_io().

Can you elaborate on the HW (the exact model of the panel and [sub]driver)
in use?

> Update type         Before     After
> ====================================
> full screen         196ms      200ms
> half screen         200ms      124ms
> quarter screen      193ms       81ms
> one pixle           199ms       43ms
> 
> It is interesting to note that if the deferred IO's delay time (40ms) is
> subtracted, then the time amount scales linearly with the write size.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Some questions below, but they probably won't affect the decision on this change.

...

> -	/* special case, needed ? */

Do you know the history of this comment? What is "special case" and when does it appear?

> -	if (y == -1) {
> -		y = 0;
> -		height = info->var.yres;
> -	}

...

>  static void fbtft_ops_damage_range(struct fb_info *info, off_t off, size_t len)
>  {
>  	struct fbtft_par *par = info->par;
> +	u32 start, end;
> +
> +	start = off / info->fix.line_length;
> +	end = (off + len - 1) / info->fix.line_length;
>  
> -	/* TODO: only mark changed area update all for now */

Ah, this change actually targets the TODO!

> -	par->fbtftops.mkdirty(info, -1, 0);
> +	par->fbtftops.mkdirty(info, start, end - start + 1);
>  }

...

In light of the last TODO line I think that the change is desired by
the original author. Nevertheless I am wondering about these side effects
that might be (very unlikely?) considered if this code (partial refresh)
runs for a significant period of time:

- some bits of the screen (on non-updated part) might be flipped
- the static picture may lead to wearing of (decolourisation) the panel
- what is the thermal distribution (probably with backlight off)?

Maybe it makes sense to refresh a full screen from time to time, but
TBH my knowledge of TFT panels from last decade is quite limited. I only
think from the point of the general physics and common sense. That's
why this long footnote rather philosophical, but I would be glad to
learn the state of affairs of the modern TFT panels in terms of the
physical parameters lasting over the time under different application
cases.

-- 
With Best Regards,
Andy Shevchenko