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.
Sean
> }
>
> return 0;
> --
> 2.15.1
>
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