[RFC PATCH 2/2] drm/fb-helper: Ignore the value of fb_var_screeninfo.pixclock

From: Ivan Mironov
Date: Wed Dec 26 2018 - 07:13:03 EST

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.

[1] SDL 1.2.15 source code, src/video/fbcon/SDL_fbvideo.c, vesa_timings

Signed-off-by: Ivan Mironov <mironov.ivan@xxxxxxxxx>
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 aff576c3c4fb..b95a0c23c7c8 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;