[PATCH] drm/msm/dsi: Stash away calculated vco frequency on recalc

[Dmitry Baryshkov]

Hi Stephen,

On 08/06/2021 22:55, Stephen Boyd wrote:
> A problem was reported on CoachZ devices where the display wouldn't come
> up, or it would be distorted. It turns out that the PLL code here wasn't
> getting called once dsi_pll_10nm_vco_recalc_rate() started returning the
> same exact frequency, down to the Hz, that the bootloader was setting
> instead of 0 when the clk was registered with the clk framework.
> 
> After commit 001d8dc33875 ("drm/msm/dsi: remove temp data from global
> pll structure") we use a hardcoded value for the parent clk frequency,
> i.e.  VCO_REF_CLK_RATE, and we also hardcode the value for FRAC_BITS,
> instead of getting it from the config structure. This combination of
> changes to the recalc function allows us to properly calculate the
> frequency of the PLL regardless of whether or not the PLL has been
> clk_prepare()d or clk_set_rate()d. That's a good improvement.
> 
> Unfortunately, this means that now we won't call down into the PLL clk
> driver when we call clk_set_rate() because the frequency calculated in
> the framework matches the frequency that is set in hardware. If the rate
> is the same as what we want it should be OK to not call the set_rate PLL
> op. The real problem is that the prepare op in this driver uses a
> private struct member to stash away the vco frequency so that it can
> call the set_rate op directly during prepare. Once the set_rate op is
> never called because recalc_rate told us the rate is the same, we don't
> set this private struct member before the prepare op runs, so we try to
> call the set_rate function directly with a frequency of 0. This
> effectively kills the PLL and configures it for a rate that won't work.
> Calling set_rate from prepare is really quite bad and will confuse any
> downstream clks about what the rate actually is of their parent. Fixing
> that will be a rather large change though so we leave that to later.
> 
> For now, let's stash away the rate we calculate during recalc so that
> the prepare op knows what frequency to set, instead of 0. This way
> things keep working and the display can enable the PLL properly. In the
> future, we should remove that code from the prepare op so that it
> doesn't even try to call the set rate function.
> 
> Cc: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
> Cc: Abhinav Kumar <abhinavk at codeaurora.org>
> Fixes: 001d8dc33875 ("drm/msm/dsi: remove temp data from global pll structure")
> Signed-off-by: Stephen Boyd <swboyd at chromium.org>

Thank you for the lengthy explanation. May I suggest another solution:
  - Apply 
https://lore.kernel.org/linux-arm-msm/010101750064e17e-3db0087e-fc37-494d-aac9-2c2b9b0a7c5b-000000@us-west-2.amazonses.com/

  - And make save_state for 7nm and 10nm cache vco freq (like 14nm does).

What do you think?


> ---
> 
> I didn't update the 14nm file as the caching logic looks different. But
> between the 7nm and 10nm files it looks practically the same.
> 
>   drivers/gpu/drm/msm/dsi/phy/dsi_phy_10nm.c | 1 +
>   drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c  | 1 +
>   2 files changed, 2 insertions(+)
> 
> diff --git a/drivers/gpu/drm/msm/dsi/phy/dsi_phy_10nm.c b/drivers/gpu/drm/msm/dsi/phy/dsi_phy_10nm.c
> index 34bc93548fcf..657778889d35 100644
> --- a/drivers/gpu/drm/msm/dsi/phy/dsi_phy_10nm.c
> +++ b/drivers/gpu/drm/msm/dsi/phy/dsi_phy_10nm.c
> @@ -432,6 +432,7 @@ static unsigned long dsi_pll_10nm_vco_recalc_rate(struct clk_hw *hw,
>   	pll_freq += div_u64(tmp64, multiplier);
>   
>   	vco_rate = pll_freq;
> +	pll_10nm->vco_current_rate = vco_rate;
>   
>   	DBG("DSI PLL%d returning vco rate = %lu, dec = %x, frac = %x",
>   	    pll_10nm->phy->id, (unsigned long)vco_rate, dec, frac);
> diff --git a/drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c b/drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c
> index e76ce40a12ab..6f96fbac8282 100644
> --- a/drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c
> +++ b/drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c
> @@ -460,6 +460,7 @@ static unsigned long dsi_pll_7nm_vco_recalc_rate(struct clk_hw *hw,
>   	pll_freq += div_u64(tmp64, multiplier);
>   
>   	vco_rate = pll_freq;
> +	pll_7nm->vco_current_rate = vco_rate;
>   
>   	DBG("DSI PLL%d returning vco rate = %lu, dec = %x, frac = %x",
>   	    pll_7nm->phy->id, (unsigned long)vco_rate, dec, frac);
> 
> base-commit: 8124c8a6b35386f73523d27eacb71b5364a68c4c
> 


-- 
With best wishes
Dmitry