Re: [PATCH] drm/omap: Migrate minimum FCK/PCK ratio from Kconfig to dts

[Adam Ford]

On Mon, Sep 30, 2019 at 8:39 AM H. Nikolaus Schaller <hns@xxxxxxxxxxxxx> wrote:
>
>
> > Am 30.09.2019 um 10:53 schrieb Tero Kristo <t-kristo@xxxxxx>:
> >
> > The best action here is probably to drop the max-div value for this clock to 16. Can someone check this with their display setup and see what happens? Attached patch should do the trick.
>
> I have checked on GTA04 and OpenPandora (DM3730 resp. OMAP3430) and did not notice a negative effect.
>
> (Well, we never see the problem that is discussed here and have built with CONFIG_OMAP2_DSS_MIN_FCK_PER_PCK=0).

I have never been able to use CONFIG_OMAP2_DSS_MIN_FCK_PER_PCK=0, but
I assume it's either a function of pck or a combination of pck with
the resolution.

Based on Tomi's comment, I assume he's working on the following.  Can
you also try:

diff --git a/drivers/gpu/drm/omapdrm/dss/dss.c
b/drivers/gpu/drm/omapdrm/dss/dss.c
index 5711b7a720e6..5e584f32ea6a 100644
--- a/drivers/gpu/drm/omapdrm/dss/dss.c
+++ b/drivers/gpu/drm/omapdrm/dss/dss.c
@@ -1090,7 +1090,7 @@ static const struct dss_features omap34xx_dss_feats = {

 static const struct dss_features omap3630_dss_feats = {
        .model                  =       DSS_MODEL_OMAP3,
-       .fck_div_max            =       32,
+       .fck_div_max            =       16,
        .fck_freq_max           =       173000000,
        .dss_fck_multiplier     =       1,
        .parent_clk_name        =       "dpll4_ck",


Hopefully it doesn't break the 3630 for you, but it fixed my issue
with no back trace:

[    9.915588] DSS: set fck to 54000000
[    9.915618] DISPC: lck = 54000000 (1)
[    9.915649] DISPC: pck = 9000000 (6)
[    9.917633] DISPC: channel 0 xres 480 yres 272
[    9.917663] DISPC: pck 9000000

I do wonder, however if there is a divider that is higher than 16, but
lower than 32.
I was able to run fck at 36MHz before with divide by 4 to 9MHz, so I
am hoping that by running at 54MHz / 6 doesn't draw more power.  I was
reading through the datasheet, but I could not find any reference to
the max divider.

adam
>
> BR,
> Nikolaus
>