drm/fb-helper: Partially bring back workaround for bugs of SDL 1.2

SDL 1.2 sets all fields related to the pixel format to zero in some
cases[1]. Prior to commit db05c481 ("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.nesReported-by: saahriktu <mail@saahriktu.org>
Suggested-by: saahriktu <mail@saahriktu.org>
Cc: stable@vger.kernel.org
Fixes: db05c481 ("drm: fb-helper: Reject all pixel format changing requests")
Signed-off-by: Ivan Mironov <mironov.ivan@gmail.com>
[danvet: Delete misleading comment.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190108072353.28078-2-mironov.ivan@gmail.com
Link: https://patchwork.freedesktop.org/patch/msgid/20190108072353.28078-2-mironov.ivan@gmail.com

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 +1712,20 @@ 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) {
+		drm_fb_helper_fill_pixel_fmt(var, fb->format->depth);
+	}
+
 	/*
 	 * drm fbdev emulation doesn't support changing the pixel format at all,
 	 * so reject all pixel format changing requests.
@@ -1967,59 +2039,7 @@ 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;