[PATCH v5 3/5] drm/msm/dp: set stream_pixel rate directly

[Dmitry Baryshkov]

On 08/03/2022 23:46, Stephen Boyd wrote:
> Quoting Dmitry Baryshkov (2022-03-03 23:58:58)
>> On Fri, 4 Mar 2022 at 07:31, Stephen Boyd <swboyd at chromium.org> wrote:
>>>
>>> Quoting Dmitry Baryshkov (2022-03-03 20:23:06)
>>>> On Fri, 4 Mar 2022 at 01:32, Stephen Boyd <swboyd at chromium.org> wrote:
>>>>>
>>>>> Quoting Dmitry Baryshkov (2022-02-16 21:55:27)
>>>>>> The only clock for which we set the rate is the "stream_pixel". Rather
>>>>>> than storing the rate and then setting it by looping over all the
>>>>>> clocks, set the clock rate directly.
>>>>>>
>>>>>> Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
>>>>> [...]
>>>>>> diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c
>>>>>> index 07f6bf7e1acb..8e6361dedd77 100644
>>>>>> --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
>>>>>> +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
>>>>>> @@ -1315,7 +1315,7 @@ static void dp_ctrl_set_clock_rate(struct dp_ctrl_private *ctrl,
>>>>>>          DRM_DEBUG_DP("setting rate=%lu on clk=%s\n", rate, name);
>>>>>>
>>>>>>          if (num)
>>>>>> -               cfg->rate = rate;
>>>>>> +               clk_set_rate(cfg->clk, rate);
>>>>>
>>>>> This looks bad. From what I can tell we set the rate of the pixel clk
>>>>> after enabling the phy and configuring it. See the order of operations
>>>>> in dp_ctrl_enable_mainlink_clocks() and note how dp_power_clk_enable()
>>>>> is the one that eventually sets a rate through dp_power_clk_set_rate()
>>>>>
>>>>>          dp_ctrl_set_clock_rate(ctrl, DP_CTRL_PM, "ctrl_link",
>>>>>                                          ctrl->link->link_params.rate * 1000);
>>>>>
>>>>>          phy_configure(phy, &dp_io->phy_opts);
>>>>>          phy_power_on(phy);
>>>>>
>>>>>          ret = dp_power_clk_enable(ctrl->power, DP_CTRL_PM, true);
>>>>
>>>> This code has been changed in the previous patch.
>>>>
>>>> Let's get back a bit.
>>>> Currently dp_ctrl_set_clock_rate() doesn't change the clock rate. It
>>>> just stores the rate in the config so that later the sequence of
>>>> dp_power_clk_enable() -> dp_power_clk_set_rate() ->
>>>> [dp_power_clk_set_link_rate() -> dev_pm_opp_set_rate() or
>>>> msm_dss_clk_set_rate() -> clk_set_rate()] will use that.
>>>>
>>>> There are only two users of dp_ctrl_set_clock_rate():
>>>> - dp_ctrl_enable_mainlink_clocks(), which you have quoted above.
>>>>    This case is handled in the patch 1 from this series. It makes
>>>
>>> Patch 1 form this series says DP is unaffected. Huh?
>>>
>>>> dp_ctrl_enable_mainlink_clocks() call dev_pm_opp_set_rate() directly
>>>> without storing (!) the rate in the config, calling
>>>> phy_configure()/phy_power_on() and then setting the opp via the
>>>> sequence of calls specified above
>>
>> Note, this handles the "ctrl_link" clock.
>>
>>>>
>>>> - dp_ctrl_enable_stream_clocks(), which calls dp_power_clk_enable()
>>>> immediately afterwards. This call would set the stream_pixel rate
>>>> while enabling stream clocks. As far as I can see, the stream_pixel is
>>>> the only stream clock. So this patch sets the clock rate without
>>>> storing in the interim configuration data.
>>>>
>>>> Could you please clarify, what exactly looks bad to you?
>>>>
>>
>> Note, this handles the "stream_pixel" clock.
>>
>>>
>>> I'm concerned about the order of operations changing between the
>>> phy being powered on and the pixel clk frequency being set. From what I
>>> recall the pixel clk rate operations depend on the phy frequency being
>>> set (which is done through phy_configure?) so if we call clk_set_rate()
>>> on the pixel clk before the phy is set then the clk frequency will be
>>> calculated badly and probably be incorrect.
>>
>> But the order of operations is mostly unchanged. The only major change
>> is that the opp point is now set before calling the
>> phy_configure()/phy_power_on()
> 
> Yes that's my concern. The qmp phy driver has a couple clk_set_rate()
> calls in the .configure_dp_phy callback. That is called from
> phy_power_on() (see qcom_qmp_phy_power_on() and qcom_qmp_phy_dp_ops).
> Looking at qcom_qmp_v3_phy_configure_dp_phy() it does
> 
>          clk_set_rate(dp_clks->dp_link_hw.clk, dp_opts->link_rate * 100000);
>          clk_set_rate(dp_clks->dp_pixel_hw.clk, pixel_freq);
> 
> and I believe the child of dp_pixel_hw is find_clock("stream_pixel").
> Looks like that is DISP_CC_MDSS_DP_PIXEL_CLK which is
> disp_cc_mdss_dp_pixel_clk_src for the rate settable part. That has
> clk_dp_ops which is clk_rcg2_dp_set_rate() for the set rate part. That
> wants the parent clk frequency to be something non-zero to use in
> rational_best_approximation(). If the clk_set_rate("stream_pixel") call
> is made before phy_power_on() then the parent_rate in
> clk_rcg2_dp_set_rate() won't be valid and the pixel clk frequency will
> be wrong.


Please excuse me, I didn't have time for this patchset up to now.

I double checked the previous patch (drm/msm/dp: "inline" 
dp_ctrl_set_clock_rate("ctrl_link")). Note, that the OPP is set _after_ 
the PHY is powered on and configured.

Does that cover your concerns?



-- 
With best wishes
Dmitry