[PATCH] drm/omap: fix max fclk divider for omap36xx

[Adam Ford]

On Wed, Oct 2, 2019 at 7:25 AM Tomi Valkeinen <tomi.valkeinen at ti.com> wrote:
>
> The OMAP36xx and AM/DM37x TRMs say that the maximum divider for DSS fclk
> (in CM_CLKSEL_DSS) is 32. Experimentation shows that this is not
> correct, and using divider of 32 breaks DSS with a flood or underflows
> and sync losts. Dividers up to 31 seem to work fine.
>
> There is another patch to the DT files to limit the divider correctly,
> but as the DSS driver also needs to know the maximum divider to be able
> to iteratively find good rates, we also need to do the fix in the DSS
> driver.
>

Tomi,

Is there any way you can do a patch for the FB version for the older
4.9 and 4.14 kernels?  I think they are still defaulting to the omapfb
instead of DRM, so the underflow issue still appears by default and
the patch only impacts the DRM version of the driver.  If not, do you
have any objections if I submit a patch to stable for those two LTS
branches?

thanks,

adam
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen at ti.com>
> Cc: Adam Ford <aford173 at gmail.com>
> Cc: stable at vger.kernel.org
> ---
>  drivers/gpu/drm/omapdrm/dss/dss.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/omapdrm/dss/dss.c b/drivers/gpu/drm/omapdrm/dss/dss.c
> index e226324adb69..4bdd63b57100 100644
> --- a/drivers/gpu/drm/omapdrm/dss/dss.c
> +++ b/drivers/gpu/drm/omapdrm/dss/dss.c
> @@ -1083,7 +1083,7 @@ 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            =       31,
>         .fck_freq_max           =       173000000,
>         .dss_fck_multiplier     =       1,
>         .parent_clk_name        =       "dpll4_ck",
> --
> Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
> Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
>