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SubjectRe: [PATCH v1 2/2] drm/fb-helper: Ignore the value of fb_var_screeninfo.pixclock
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On Fri, 2018-12-28 at 13:06 +0100, Daniel Vetter wrote:
> On Fri, Dec 28, 2018 at 04:13:08AM +0500, Ivan Mironov wrote:
> > 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.
> >
> > Feel free to NAK this patch if you think that it causes breakage of
> > user-space =).
>
> It's a regression, we don't nack regression fixes :-)
>
> > 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>
>
> I thought this is also a regression fix, so also needs Fixes: and cc:
> stable?
> -Daniel

This particular patch fixes a bug that existed for 10 years unnoticed.
Are "Fixes:" and "Cc: stable" really required?

"pixclock != 0" check was introduced in this file by 785b93ef8c309
("drm/kms: move driver specific fb common code to helper functions
(v2)"). But actually similar code existed in the device-specific
drivers even earlier.

>
> > ---
> > 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;
> > --
> > 2.20.1
> >

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