On Fri, Jan 12, 2018 at 10:08:49PM +0100, Daniel Vetter wrote:
> On Fri, Jan 12, 2018 at 3:08 PM, Sean Paul <seanpaul@xxxxxxxxxxxx> wrote:
> > On Fri, Jan 12, 2018 at 4:48 AM, Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
> >> [Fair warning: This is pure conjecture right now.]
> >>
> >> In
> >>
> >> commit b8e2b0199cc377617dc238f5106352c06dcd3fa2
> >> Author: Peter Rosin <peda@xxxxxxxxxx>
> >> Date: Tue Jul 4 12:36:57 2017 +0200
> >>
> >> drm/fb-helper: factor out pseudo-palette
> >>
> >> Peter extracted the pseudo palette computation, but seems to have done
> >> an off-by-one. I spotted that +1, but then noticed that we've passed
> >> start++ to (now gone) setcolreg function, so it seemed to all match
> >> up. Except post vs. pre-increment ftw.
> >>
> >> Result is that the palette is off-by-one, and the forground color
> >> (slot 0) ends up black, rendering a fairly unreadable console.
> >>
> >> What baffles me is that on some systems it still seems to work
> >> somehow, which lead us all down a wild goose chase trying to add
> >> load_lut calls back in in various places (which was also intentionally
> >> removed, but really doesn't seem to be the real root cause).
> >>
> >> Fixes: b8e2b0199cc3 ("drm/fb-helper: factor out pseudo-palette")
> >> Cc: Peter Rosin <peda@xxxxxxxxxx>
> >> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> >> Cc: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> >> Cc: Gustavo Padovan <gustavo@xxxxxxxxxxx>
> >> Cc: Sean Paul <seanpaul@xxxxxxxxxxxx>
> >> Cc: David Airlie <airlied@xxxxxxxx>
> >> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> >> Cc: <stable@xxxxxxxxxxxxxxx> # v4.14+
> >> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=198123
> >> Reported-by: Deposite Pirate <dpirate@xxxxxxxxxxxxxxx>
> >> Reported-by: Bill Fraser <bill.fraser@xxxxxxxxx>
> >> Cc: Deposite Pirate <dpirate@xxxxxxxxxxxxxxx>
> >> Cc: Bill Fraser <bill.fraser@xxxxxxxxx>
> >> Cc: Michel Dänzer <michel@xxxxxxxxxxx>
> >> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> >> ---
> >> drivers/gpu/drm/drm_fb_helper.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> >> index 035784ddd133..1c3a200c4a10 100644
> >> --- a/drivers/gpu/drm/drm_fb_helper.c
> >> +++ b/drivers/gpu/drm/drm_fb_helper.c
> >> @@ -1295,7 +1295,7 @@ static int setcmap_pseudo_palette(struct fb_cmap *cmap, struct fb_info *info)
> >> mask <<= info->var.transp.offset;
> >> value |= mask;
> >> }
> >> - palette[cmap->start + i] = value;
> >> + palette[cmap->start] = value;
> >
> > I don't think this is equivalent to what was there before. Before
> > there we set palette[cmap->start]->palette[cmap->start + cmap->len -
> > 1], now we're only setting palette[cmap->start].
> >
> > In the previous version (before Peter's change), we wrote
> > palette[cmap->start] twice. So while there is an off-by-one bug, I
> > don't think this fixes the issue.
>
> Well this patch is complete nonsense, no idea what I was thinking ...
[Sending this out since it's in my tree and I'll forget soon]
Equivalent behavior would be:
diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 035784ddd133..df6709768723 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -1295,7 +1295,8 @@ static int setcmap_pseudo_palette(struct fb_cmap *cmap, struct fb_info *info)
mask <<= info->var.transp.offset;
value |= mask;
}
- palette[cmap->start + i] = value;
+ if (i > 0)
+ palette[cmap->start + i - 1] = value;
}
return 0;
> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
--
Sean Paul, Software Engineer, Google / Chromium OS
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