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@gmail.com Cc: stable@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;