[Intel-gfx] [PATCH 06/11] drm/i915: Fix unclaimed register access due to delayed VGA memroy disable

[Paulo Zanoni]

2013/9/16  <ville.syrjala at linux.intel.com>:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
>
> VGA registers live inside the power well on HSW, so in order to write
> the VGA MSR register we need the power well to be on.
>
> We really must write to the register to properly clear the
> VGA_MSR_MEM_EN enable bit, even if all VGA registers get zeroed when
> the power well is down. It seems that the implicit zeroing done by
> the power well is not enough to propagate the VGA_MSR_MEM_EN bit to
> whomever is actually responsible for the memory decode ranges.
>
> If we leave VGA memory decode enabled, and then turn off the power well,
> all VGA memory reads will return zeroes. But if we first disable VGA
> memory deocde and then turn off the power well, VGA memory reads
> return all ones, indicating that the access wasn't claimed by anyone.
> For the vga arbiter to function correctly the IGD must not claim the
> VGA memory accesses.
>
> Previously we were doing the VGA_MSR register access while the power well
> was excplicitly powered up during driver init. But ever since
>
>  commit 6e1b4fdad5157bb9e88777d525704aba24389bee
>  Author: Ville Syrjälä <ville.syrjala at linux.intel.com>
>  Date:   Thu Sep 5 20:40:52 2013 +0300
>
>     drm/i915: Delay disabling of VGA memory until vgacon->fbcon handoff is done
>
> we delay the VGA memory disable until fbcon has initialized, and so
> there's a possibility that the power well got turned off during the
> fbcon modeset. Also vgacon_save_screen() will need the power well to be
> on to be able to read the VGA memory.
>
> So immediately after enabling the power well during init grab a refence
> for VGA purposes, and after all the VGA handling is done, release it.
>
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_dma.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
> index e5c7b10..23f965d 100644
> --- a/drivers/gpu/drm/i915/i915_dma.c
> +++ b/drivers/gpu/drm/i915/i915_dma.c
> @@ -1326,6 +1326,9 @@ static int i915_load_modeset_init(struct drm_device *dev)
>
>         intel_init_power_well(dev);
>
> +       /* Keep VGA alive until i915_disable_vga_mem() */
> +       intel_display_power_get(dev, POWER_DOMAIN_VGA);
> +
>         intel_modeset_gem_init(dev);
>
>         /* Always safe in the mode setting case. */
> @@ -1358,6 +1361,7 @@ static int i915_load_modeset_init(struct drm_device *dev)
>          * vgacon_save_screen() works during the handover.
>          */
>         i915_disable_vga_mem(dev);
> +       intel_display_power_put(dev, POWER_DOMAIN_VGA);

There's a "return 0" between the get and the put. (I was already at
patch 8 when I realized that!)

While reviewing your series I started thinking: shouldn't we just
get/put POWER_DOMAIN_VGA directly inside the functions that touch the
VGA code? This shouldn't really be an expensive thing, and it will
make our code simpler and harder to break. While your series seems
correct, there is code that depends on state left by other code, which
IMHO increases the complexity of the already-too-complex init/resume
paths. This is just a suggestion, not a requirement :)

>
>         /* Only enable hotplug handling once the fbdev is fully set up. */
>         dev_priv->enable_hotplug_processing = true;
> --
> 1.8.1.5
>
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-- 
Paulo Zanoni