On Wed, 05 Apr 2017, Madhav Chauhan <madhav.chauhan@xxxxxxxxx> wrote:
> As per BSPEC, valid cdclk values for glk are 79.2, 158.4, 316.8 Mhz.
> Practically we can achive only 99% of these cdclk values (HW team
> checking on this). So cdclk should be calculated for the given pixclk as
> per that otherwise it may lead to screen corruption, explained below:
> 1. For DSI AUO panel(1920x1200 @60) required pixclk is 157100 KHZ
> 2. glk_calc_cdclk returns 79200 KHZ for this pixclk, For 2PPC it
> will be 158400 KHZ
> 3. Practically 100% of the cdclk can’t be achieved, so 99% of 158400
> KHZ = 156816 which is less than the desired pixlclk and causes
> panel corruption.
>
> v2: Rebased to new CDLCK code framework
> v3: Addressed review comments from Ander/Jani
> - Add comment in code about 99% usage of CDCLK
> - Calculate max dot clock as well with 99% limit
> v4 by Jani:
> - drop superfluous whitespace change
> - rewrite code comments to clarify
> v5: Added details of non-working scenario in commit message
>
> Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@xxxxxxxxx>
> Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> Signed-off-by: Madhav Chauhan <madhav.chauhan@xxxxxxxxx>
> Signed-off-by: Jani Nikula <jani.nikula@xxxxxxxxx>
> Reviewed-by: Ander Conselvan de Oliveira <conselvan2@xxxxxxxxx>
Pushed to drm-intel-next-queued, thanks for the patch and review.
BR,
Jani.
> ---
> drivers/gpu/drm/i915/intel_cdclk.c | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_cdclk.c b/drivers/gpu/drm/i915/intel_cdclk.c
> index dd3ad52..763010f 100644
> --- a/drivers/gpu/drm/i915/intel_cdclk.c
> +++ b/drivers/gpu/drm/i915/intel_cdclk.c
> @@ -1071,9 +1071,15 @@ static int bxt_calc_cdclk(int max_pixclk)
>
> static int glk_calc_cdclk(int max_pixclk)
> {
> - if (max_pixclk > 2 * 158400)
> + /*
> + * FIXME: Avoid using a pixel clock that is more than 99% of the cdclk
> + * as a temporary workaround. Use a higher cdclk instead. (Note that
> + * intel_compute_max_dotclk() limits the max pixel clock to 99% of max
> + * cdclk.)
> + */
> + if (max_pixclk > DIV_ROUND_UP(2 * 158400 * 99, 100))
> return 316800;
> - else if (max_pixclk > 2 * 79200)
> + else if (max_pixclk > DIV_ROUND_UP(2 * 79200 * 99, 100))
> return 158400;
> else
> return 79200;
> @@ -1664,7 +1670,11 @@ static int intel_compute_max_dotclk(struct drm_i915_private *dev_priv)
> int max_cdclk_freq = dev_priv->max_cdclk_freq;
>
> if (IS_GEMINILAKE(dev_priv))
> - return 2 * max_cdclk_freq;
> + /*
> + * FIXME: Limiting to 99% as a temporary workaround. See
> + * glk_calc_cdclk() for details.
> + */
> + return 2 * max_cdclk_freq * 99 / 100;
> else if (INTEL_INFO(dev_priv)->gen >= 9 ||
> IS_HASWELL(dev_priv) || IS_BROADWELL(dev_priv))
> return max_cdclk_freq;
--
Jani Nikula, Intel Open Source Technology Center
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