linux-kernel - Re: [PATCH 1/4] fbdev: make fb_find_mode look for a mode with the highest refresh rate

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Date:	Sat, 23 Jun 2007 23:26:32 +0200
From:	Michal Januszewski <spock@...too.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-fbdev-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] fbdev: make fb_find_mode look for a mode with the
	highest refresh rate

On Sat, Jun 23, 2007 at 11:04:24AM -0700, Andrew Morton wrote:
> On Sat, 23 Jun 2007 12:50:46 +0200 Michal Januszewski <spock@...too.org> wrote:
> 
> > +	if (!refresh_specified)
> > +		refresh = 200;
> >  	diff = refresh;
> >  	best = -1;
> >  	for (i = 0; i < dbsize; i++) {
> >  		if (name_matches(db[i], name, namelen) ||
> >  		    (res_specified && res_matches(db[i], xres, yres))) {
> >  			if(!fb_try_mode(var, info, &db[i], bpp)) {
> > -				if(!refresh_specified || db[i].refresh == refresh)
> > +				if (refresh_specified && db[i].refresh == refresh)
> >  					return 1;
> >  				else {
> > -					if(diff > abs(db[i].refresh - refresh)) {
> > +					if (diff > abs(db[i].refresh - refresh)) {
> >  						diff = abs(db[i].refresh - refresh);
> >  						best = i;
> >  					}
> > @@ -938,6 +940,7 @@ void fb_destroy_modelist(struct list_head *head)
> >  		kfree(pos);
> >  	}
> >  }
> > +EXPORT_SYMBOL_GPL(fb_destroy_modelist);
> >  
> 
> fbdev ignoramus asks: isn't this pretty risky?  People who were previously
> relying upon (or at least using) the kernel's default resolution will find
> their displays coming up in a quite different resolution.

The resolution will be unchanged (this part of the code is only executed
if either the name of the mode or the resolution are a match).

What can change is the refresh rate -- it can be set higher than what it
used to be before.  But since we're checking all modes with fb_try_mode(),
(which calls fb_check_var()), I think that this change should be safe.
 
To avoid any side effects we could also do the following:

	for (i = 0; i < dbsize; i++) {
		if (name_matches(db[i], name, namelen) ||
		    (res_specified && res_matches(db[i], xres, yres))) {
			if(!fb_try_mode(var, info, &db[i], bpp)) {
				if (refresh_specified && db[i].refresh == refresh)
					return 1;
				else {
					if (diff > abs(db[i].refresh - refresh)) {
						diff = abs(db[i].refresh - refresh);
						best = i;
					}
				}
			}
		}
	}

	if (best != -1) {
		fb_try_mode(var, info, &db[best], bpp);
-		return 2;
+ 		return (refresh_specified) ? 2 : 1;
	}

which would ensure that 1 is returned if the refresh rate is not
explicitly specified, just as it is done currently:

				if(!refresh_specified || db[i].refresh == refresh)
					return 1

I might be missing something here, so it would be nice if someone
involved in the fb development could comment on the proposed change.

> This change seems to be quite unrelated to the uvesafb stuff and should be
> in a separate patch from the export, which _is_ uvesafb-related.  I think. 
> If that's wrong then the changelog could do with some attention.

You're right, I'll split this into two patches in the next round.

Best regards,
Michal

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