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Subject[PATCH v1 1/2] drm/fb-helper: Bring back workaround for bugs of SDL 1.2
Date
SDL 1.2 sets all fields related to the pixel format to zero in some
cases[1]. Prior to commit db05c48197759 ("drm: fb-helper: Reject all
pixel format changing requests"), there was an unintentional workaround
for this that existed for more than a decade. First in device-specific DRM
drivers, then here in drm_fb_helper.c.

Previous code containing this workaround just ignores pixel format fields
from userspace code. Not a good thing either, as this way, driver may
silently use pixel format different from what client actually requested,
and this in turn will lead to displaying garbage on the screen. I think
that returning EINVAL to userspace in this particular case is the right
option, so I decided to left code from problematic commit untouched
instead of just reverting it entirely.

Here is the steps required to reproduce this problem exactly:
1) Compile fceux[2] with SDL 1.2.15 and without GTK or OpenGL
support. SDL should be compiled with fbdev support (which is
on by default).
2) Create /etc/fb.modes with following contents (values seems
not used, and just required to trigger problematic code in
SDL):

mode "test"
geometry 1 1 1 1 1
timings 1 1 1 1 1 1 1
endmode

3) Create ~/.fceux/fceux.cfg with following contents:

SDL.Hotkeys.Quit = 27
SDL.DoubleBuffering = 1

4) Ensure that screen resolution is at least 1280x960 (e.g.
append "video=Virtual-1:1280x960-32" to the kernel cmdline
for qemu/QXL).

5) Try to run fceux on VT with some ROM file[3]:

# ./fceux color_test.nes

[1] SDL 1.2.15 source code, src/video/fbcon/SDL_fbvideo.c,
FB_SetVideoMode()
[2] http://www.fceux.com
[3] Example ROM: https://github.com/bokuweb/rustynes/blob/master/roms/color_test.nes

Reported-by: saahriktu <mail@saahriktu.org>
Suggested-by: saahriktu <mail@saahriktu.org>
Cc: stable@vger.kernel.org
Fixes: db05c48197759 ("drm: fb-helper: Reject all pixel format changing requests")
Signed-off-by: Ivan Mironov <mironov.ivan@gmail.com>
---
drivers/gpu/drm/drm_fb_helper.c | 146 ++++++++++++++++++++------------
1 file changed, 93 insertions(+), 53 deletions(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index d3af098b0922..aff576c3c4fb 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -1621,6 +1621,64 @@ static bool drm_fb_pixel_format_equal(const struct fb_var_screeninfo *var_1,
var_1->transp.msb_right == var_2->transp.msb_right;
}

+static void drm_fb_helper_fill_pixel_fmt(struct fb_var_screeninfo *var,
+ u8 depth)
+{
+ switch (depth) {
+ case 8:
+ var->red.offset = 0;
+ var->green.offset = 0;
+ var->blue.offset = 0;
+ var->red.length = 8; /* 8bit DAC */
+ var->green.length = 8;
+ var->blue.length = 8;
+ var->transp.offset = 0;
+ var->transp.length = 0;
+ break;
+ case 15:
+ var->red.offset = 10;
+ var->green.offset = 5;
+ var->blue.offset = 0;
+ var->red.length = 5;
+ var->green.length = 5;
+ var->blue.length = 5;
+ var->transp.offset = 15;
+ var->transp.length = 1;
+ break;
+ case 16:
+ var->red.offset = 11;
+ var->green.offset = 5;
+ var->blue.offset = 0;
+ var->red.length = 5;
+ var->green.length = 6;
+ var->blue.length = 5;
+ var->transp.offset = 0;
+ break;
+ case 24:
+ var->red.offset = 16;
+ var->green.offset = 8;
+ var->blue.offset = 0;
+ var->red.length = 8;
+ var->green.length = 8;
+ var->blue.length = 8;
+ var->transp.offset = 0;
+ var->transp.length = 0;
+ break;
+ case 32:
+ var->red.offset = 16;
+ var->green.offset = 8;
+ var->blue.offset = 0;
+ var->red.length = 8;
+ var->green.length = 8;
+ var->blue.length = 8;
+ var->transp.offset = 24;
+ var->transp.length = 8;
+ break;
+ default:
+ break;
+ }
+}
+
/**
* drm_fb_helper_check_var - implementation for &fb_ops.fb_check_var
* @var: screeninfo to check
@@ -1654,6 +1712,40 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
return -EINVAL;
}

+ /*
+ * Workaround for SDL 1.2, which is known to be setting all pixel format
+ * fields values to zero in some cases. We treat this situation as a
+ * kind of "use some reasonable autodetected values".
+ */
+ if (!var->red.offset && !var->green.offset &&
+ !var->blue.offset && !var->transp.offset &&
+ !var->red.length && !var->green.length &&
+ !var->blue.length && !var->transp.length &&
+ !var->red.msb_right && !var->green.msb_right &&
+ !var->blue.msb_right && !var->transp.msb_right) {
+ u8 depth;
+
+ /*
+ * There is no way to guess the right value for depth when
+ * bpp is 16 or 32. So we just restore the behaviour previously
+ * introduced here by commit 785b93ef8c309. In fact, this was
+ * implemented even earlier in various device drivers.
+ */
+ switch (var->bits_per_pixel) {
+ case 16:
+ depth = 15;
+ break;
+ case 32:
+ depth = 24;
+ break;
+ default:
+ depth = var->bits_per_pixel;
+ break;
+ }
+
+ drm_fb_helper_fill_pixel_fmt(var, depth);
+ }
+
/*
* drm fbdev emulation doesn't support changing the pixel format at all,
* so reject all pixel format changing requests.
@@ -1967,59 +2059,7 @@ void drm_fb_helper_fill_var(struct fb_info *info, struct drm_fb_helper *fb_helpe
info->var.yoffset = 0;
info->var.activate = FB_ACTIVATE_NOW;

- switch (fb->format->depth) {
- case 8:
- info->var.red.offset = 0;
- info->var.green.offset = 0;
- info->var.blue.offset = 0;
- info->var.red.length = 8; /* 8bit DAC */
- info->var.green.length = 8;
- info->var.blue.length = 8;
- info->var.transp.offset = 0;
- info->var.transp.length = 0;
- break;
- case 15:
- info->var.red.offset = 10;
- info->var.green.offset = 5;
- info->var.blue.offset = 0;
- info->var.red.length = 5;
- info->var.green.length = 5;
- info->var.blue.length = 5;
- info->var.transp.offset = 15;
- info->var.transp.length = 1;
- break;
- case 16:
- info->var.red.offset = 11;
- info->var.green.offset = 5;
- info->var.blue.offset = 0;
- info->var.red.length = 5;
- info->var.green.length = 6;
- info->var.blue.length = 5;
- info->var.transp.offset = 0;
- break;
- case 24:
- info->var.red.offset = 16;
- info->var.green.offset = 8;
- info->var.blue.offset = 0;
- info->var.red.length = 8;
- info->var.green.length = 8;
- info->var.blue.length = 8;
- info->var.transp.offset = 0;
- info->var.transp.length = 0;
- break;
- case 32:
- info->var.red.offset = 16;
- info->var.green.offset = 8;
- info->var.blue.offset = 0;
- info->var.red.length = 8;
- info->var.green.length = 8;
- info->var.blue.length = 8;
- info->var.transp.offset = 24;
- info->var.transp.length = 8;
- break;
- default:
- break;
- }
+ drm_fb_helper_fill_pixel_fmt(&info->var, fb->format->depth);

info->var.xres = fb_width;
info->var.yres = fb_height;
--
2.20.1
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