Re: [PATCH 1/4] fbdev: make fb_find_mode look for a mode with thehighest refresh rate

From: Michal Januszewski
Date: Sat Jun 23 2007 - 17:26:42 EST


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@xxxxxxxxxx> 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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