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SubjectRe: [Linux-fbdev-devel] fb: modedb uses wrong default_mode

> > > It seems that default_mode is always overwritten in
> > > fb_find_mode() if caller gives its own modedb; this
> > > patch should fix it.
>
> > Sigh.
> >
> > 2.6.19-rc5 has:
> >
> > if (!default_mode)
> > default_mode = &modedb[DEFAULT_MODEDB_INDEX];
> >
> > and Jordan changed it to
> >
> > if (!default_mode && db != modedb)
> > default_mode = &db[0];
> > else
> > default_mode = &modedb[DEFAULT_MODEDB_INDEX];
>
>
> > and you want to change it to
> >
> > if (!default_mode && db != modedb)
> > default_mode = &db[0];
> > else if (!default_mode)
> > default_mode = &modedb[DEFAULT_MODEDB_INDEX];
> >
> > which is actually a complicated way of doing
> >
> > if (!default_mode)
> > default_mode = &db[DEFAULT_MODEDB_INDEX];
>
> Unless DEFAULT_MODEDB_INDEX for some reason gets set to non-zero, then
> it could be dangerous. If we agree that the default entry should aways be
> at 0, then nuke the define and hard code the zero. That way, nobody will be
> tempted to change it.

Taking a look at modedb.c and neofb.c I noticed the bug is in the neofb.c
driver. The problem is the confusion with the fb_find_mode function
itself.

int fb_find_mode(struct fb_var_screeninfo *var,
struct fb_info *info, const char *mode_option,
const struct fb_videomode *db, unsigned int dbsize,
const struct fb_videomode *default_mode,
unsigned int default_bpp)

db is the database but default_mode is the mode we want to run. As you
can see neofb.c does

if (!fb_find_mode(&info->var, info, mode_option, NULL, 0,
info->monspecs.modedb, 16)) {
printk(KERN_ERR "neofb: Unable to find usable video mode.\n");
goto err_map_video;
}


it should be
if (!fb_find_mode(&info->var, info, mode_option, info->monspecs.modedb,
0, NULL, 16)) {

Who knows how many drivers get this wrong. BTW Jordan is right.
DEFAULT_MODEDB_INDEX is unless. Also we don't need dbsize anymore.
Jordan did point out a error in fb_find_mode. It should be

if (!db)
db = modedb;
dbsize = ARRAY_SIZE(modedb);

if (!default_mode)
default_mode = &db[DEFAULT_MODEDB_INDEX];
if (!default_bpp)
default_bpp = 8;

db will always be set.



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