[PATCH] drm/i915: only disable default vga device

[Ville Syrjälä]

On Tue, May 18, 2021 at 12:09:56PM +0100, Emil Velikov wrote:
> Hi Ville,
> 
> On Mon, 17 May 2021 at 18:24, Ville Syrjälä
> <ville.syrjala at linux.intel.com> wrote:
> >
> > On Sun, May 16, 2021 at 06:14:32PM +0100, Emil Velikov wrote:
> > > From: Vivek Das Mohapatra <vivek at collabora.com>
> > >
> > > This patch is to do with seamless handover, eg when the sequence is
> > > bootloader → plymouth → desktop.
> > >
> > > It switches the vga arbiter from the "other" GPU to the default one
> > > (intel in this case), so the driver can issue some io().
> >
> > I don't understand what this commit message is trying to say.
> >
> Bunch of context is lost due to the patch age, so I'm not 100% sure of
> the actual hardware setup where this occurs.
> Does the following make sense?
> 
> Currently on dual GPU systems, we do not get seamless handover as the
> output flickers during the transition bootloader -> plymouth ->
> desktop.
> This happens as a result of switching (via the VGA arbiter) from the
> "other" GPU back to the default i915 one and issuing io() commands.

Hmm. Does this work?

--- a/drivers/gpu/drm/i915/display/intel_vga.c
+++ b/drivers/gpu/drm/i915/display/intel_vga.c
@@ -29,6 +29,9 @@ void intel_vga_disable(struct drm_i915_private *dev_priv)
        i915_reg_t vga_reg = intel_vga_cntrl_reg(dev_priv);
        u8 sr1;
 
+       if (intel_de_read(dev_priv, vga_reg) & VGA_DISP_DISABLE)
+               return;
+
        /* WaEnableVGAAccessThroughIOPort:ctg,elk,ilk,snb,ivb,vlv,hsw */
        vga_get_uninterruptible(pdev, VGA_RSRC_LEGACY_IO);
        outb(SR01, VGA_SR_INDEX);

-- 
Ville Syrjälä
Intel