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(a)saahriktu.org>
Suggested-by: saahriktu <mail(a)saahriktu.org>
Cc: stable(a)vger.kernel.org
Fixes: db05c48197759 ("drm: fb-helper: Reject all pixel format changing requests")
Signed-off-by: Ivan Mironov <mironov.ivan(a)gmail.com>
---
drivers/gpu/drm/drm_fb_helper.c | 142 ++++++++++++++++++++------------
1 file changed, 89 insertions(+), 53 deletions(-)
diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index d3af098b0922..ed7e91423258 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 +1712,36 @@ 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) {
+ /*
+ * There is no way to guess the right value for depth when
+ * bits_per_pixel is 16 or 32. Instead of restoring the
+ * behaviour previously introduced here by commit 785b93ef8c309,
+ * we decided to just use the current depth and do not perform
+ * any guessing.
+ *
+ * Also, if requested bits_per_pixel differs from current,
+ * the next call to drm_fb_pixel_format_equal() will fail
+ * resulting in EINVAL error from ioctl().
+ *
+ * However, this still leaves the theoretical possibility of
+ * situation when userspace app requests different pixel format
+ * and ioctl() call silently succeeds. Garbage will be displayed
+ * in this case.
+ */
+ 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 +2055,7 @@ 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
Strict requirement of pixclock to be zero breaks support of SDL 1.2
which contains hardcoded table of supported video modes with non-zero
pixclock values[1].
To better understand which pixclock values are considered valid and how
driver should handle these values, I briefly examined few existing fbdev
drivers and documentation in Documentation/fb/. And it looks like there
are no strict rules on that and actual behaviour varies:
* some drivers treat (pixclock == 0) as "use defaults" (uvesafb.c);
* some treat (pixclock == 0) as invalid value which leads to
-EINVAL (clps711x-fb.c);
* some pass converted pixclock value to hardware (uvesafb.c);
* some are trying to find nearest value from predefined table
(vga16fb.c, video_gx.c).
Given this, I believe that it should be safe to just ignore this value if
changing is not supported. It seems that any portable fbdev application
which was not written only for one specific device working under one
specific kernel version should not rely on any particular behaviour of
pixclock anyway.
However, while enabling SDL1 applications to work out of the box when
there is no /etc/fb.modes with valid settings, this change affects the
video mode choosing logic in SDL. Depending on current screen
resolution, contents of /etc/fb.modes and resolution requested by
application, this may lead to user-visible difference (not always):
image will be displayed in a right way, but it will be aligned to the
left instead of center. There is no "right behaviour" here as well, as
emulated fbdev, opposing to old fbdev drivers, simply ignores any
requsts of video mode changes with resolutions smaller than current.
The easiest way to reproduce this problem is to install sdl-sopwith[2],
remove /etc/fb.modes file if it exists, and then try to run sopwith
from console without X. At least in Fedora 29, sopwith may be simply
installed from standard repositories.
[1] SDL 1.2.15 source code, src/video/fbcon/SDL_fbvideo.c, vesa_timings
[2] http://sdl-sopwith.sourceforge.net/
Signed-off-by: Ivan Mironov <mironov.ivan(a)gmail.com>
Cc: stable(a)vger.kernel.org
Fixes: 79e539453b34e ("DRM: i915: add mode setting support")
Fixes: 771fe6b912fca ("drm/radeon: introduce kernel modesetting for radeon hardware")
Fixes: 785b93ef8c309 ("drm/kms: move driver specific fb common code to helper functions (v2)")
---
drivers/gpu/drm/drm_fb_helper.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index ed7e91423258..2d4c2b38508e 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -1690,9 +1690,14 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
struct drm_fb_helper *fb_helper = info->par;
struct drm_framebuffer *fb = fb_helper->fb;
- if (var->pixclock != 0 || in_dbg_master())
+ if (in_dbg_master())
return -EINVAL;
+ if (var->pixclock != 0) {
+ DRM_DEBUG("fbdev emulation doesn't support changing the pixel clock, value of pixclock is ignored\n");
+ var->pixclock = 0;
+ }
+
if ((drm_format_info_block_width(fb->format, 0) > 1) ||
(drm_format_info_block_height(fb->format, 0) > 1))
return -EINVAL;
--
2.20.1