linux-kernel - Re: [PATCH v1 2/2] clk: qcom : dispcc: Add support for display port clocks

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Date:   Thu, 1 Nov 2018 10:32:22 +0530
From:   Taniya Das <tdas@...eaurora.org>
To:     Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        chandanu@...eaurora.org
Cc:     Andy Gross <andy.gross@...aro.org>,
        David Brown <david.brown@...aro.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, linux-soc@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        chandanu@...eaurora.org, linux-arm-msm-owner@...r.kernel.org
Subject: Re: [PATCH v1 2/2] clk: qcom : dispcc: Add support for display port
 clocks

+ Chandan from Display Port team,

On 10/30/2018 10:03 PM, Stephen Boyd wrote:
> Quoting Taniya Das (2018-10-29 23:01:44)
>> On 10/30/2018 12:13 AM, Stephen Boyd wrote:
>>> Quoting Taniya Das (2018-10-28 03:34:55)
>>>> On 2018-10-19 16:04, Taniya Das wrote:
>>>>> On 10/10/2018 2:04 AM, Stephen Boyd wrote:
>>>>>> Quoting Taniya Das (2018-10-09 06:57:47)
>>>>>>> diff --git a/drivers/clk/qcom/dispcc-sdm845.c
>>>>>>> b/drivers/clk/qcom/dispcc-sdm845.c
>>>>>>> index 0cc4909..6d3136a 100644
>>>>>>> --- a/drivers/clk/qcom/dispcc-sdm845.c
>>>>>>> +++ b/drivers/clk/qcom/dispcc-sdm845.c
>>>>>>> +       },
>>>>>>> +};
>>>>>>> +
>>>>>>> +static const struct freq_tbl ftbl_disp_cc_mdss_dp_link_clk_src[] = {
>>>>>>> +       F(162000, P_DP_PHY_PLL_LINK_CLK,   1,   0,   0),
>>>>>>> +       F(270000, P_DP_PHY_PLL_LINK_CLK,   1,   0,   0),
>>>>>>> +       F(540000, P_DP_PHY_PLL_LINK_CLK,   1,   0,   0),
>>>>>>> +       F(810000, P_DP_PHY_PLL_LINK_CLK,   1,   0,   0),
>>>>>>
>>>>>> Are these in kHz? They really look like it and that's bad. Why do we
>>>>>> need them at all? Just to make sure the display driver picks these
>>>>>> exact
>>>>>> frequencies? It seems like we could just pass whatever number comes in
>>>>>> up to the parent and see what it can do.
>>>>>>
>>>>>
>>>>> Let me check back the reason we had to make this change.
>>>>
>>>> We will need this flag since we reset/power-down the PLL every time we
>>>> disconnect/connect the DP cable or during suspend/resume. Only with this
>>>> flag, the calls to the PLL driver are properly called.
>>>
>>> What does this mean? I wanted to know about the weird frequencies listed
>>> above, and why it can't be done without a frequency table and direct
>>> rates passed up to the parent.
>>>
>>
>> OOps, my bad :(.
>>
>> We added these changes to handle higher clock rates. These rates when
>> greater than 4.3Ghz cannot be represented in 32bit variables. For DP, we
>> already have 5.4G and 8.1GHz freq for VCO clock. We will need these Khz
>> freq list in clock driver.
>>    Let me check if they can do something like the byte/pixel clocks of
>> display.
> 
> Well then we really should just throw away the freq table here and have
> rcg ops that pass the frequency up to the display PLL. Also, those
> numbers look like gigabits per second (Gbit/s) for the DP spec which
> isn't exactly the same as a clk frequency. What frequency does the PLL
> run at for these various DP link speeds?
> 
Could you please help with the above query from Stephen?
-- 
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