[Intel-gfx] [PATCH] drm/i915: Don't use the second dbuf slice on icl

[Mahesh Kumar]

Hi,

On Mon, Jan 21, 2019 at 9:01 PM Ville Syrjala
<ville.syrjala at linux.intel.com> wrote:
>
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
>
> The code managing the dbuf slices is borked and needs some
> real work to fix. In the meantime let's just stop using the
> second slice.
>
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_pm.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index 8b63afa3a221..1e41c899ffe2 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -3618,7 +3618,8 @@ static u8 intel_enabled_dbuf_slices_num(struct drm_i915_private *dev_priv)
>         enabled_slices = 1;
>
>         /* Gen prior to GEN11 have only one DBuf slice */
> -       if (INTEL_GEN(dev_priv) < 11)
> +       /* FIXME dbuf slice code is broken: see intel_get_ddb_size() */
> +       if (1 || INTEL_GEN(dev_priv) < 11)
>                 return enabled_slices;

IMHO we may not need this, If we return from above we'll never disable
second slice in case it's enabled by bios.
Anyhow code changes in intel_get_ddb_size will take care of enabling
only one slice.

>
>         if (I915_READ(DBUF_CTL_S2) & DBUF_POWER_STATE)
> @@ -3827,8 +3828,13 @@ static u16 intel_get_ddb_size(struct drm_i915_private *dev_priv,
>
>         /*
>          * 12GB/s is maximum BW supported by single DBuf slice.
> +        *
> +        * FIXME dbuf slice code is broken:
> +        * - must wait for planes to stop using the slice before powering it off

AFAIR we're  already doing it and disabling slice only after
update_crtcs, and skl_update_crtcs code is taking care of waiting for
vblank in case it's required.

> +        * - plane straddling both slices is illegal in multi-pipe scenarios

This is something new :)

although this change introduce a major limitation with number and size
of planes we can display, yet
As code is broken and mentioned conditions need to be taken care of,
This change should be ok until proper fix.

~Mahesh

> +        * - should validate we stay within the hw bandwidth limits
>          */
> -       if (num_active > 1 || total_data_bw >= GBps(12)) {
> +       if (0 && (num_active > 1 || total_data_bw >= GBps(12))) {
>                 ddb->enabled_slices = 2;
>         } else {
>                 ddb->enabled_slices = 1;
> --
> 2.19.2
>
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