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

[chandanu@codeaurora.org]

Hello Stephen,

On 2018-10-29 11:43, Stephen Boyd wrote:
> Quoting Taniya Das (2018-10-28 03:34:55)
>> Hello Stephen,
>> 
>> On 2018-10-19 16:04, Taniya Das wrote:
>> > Hello Stephen,
>> >

/snip

>> >>> +static struct clk_rcg2 disp_cc_mdss_dp_crypto_clk_src = {
>> >>> +       .cmd_rcgr = 0x2154,
>> >>> +       .mnd_width = 0,
>> >>> +       .hid_width = 5,
>> >>> +       .parent_map = disp_cc_parent_map_1,
>> >>> +       .freq_tbl = ftbl_disp_cc_mdss_dp_crypto_clk_src,
>> >>> +       .clkr.hw.init = &(struct clk_init_data){
>> >>> +               .name = "disp_cc_mdss_dp_crypto_clk_src",
>> >>> +               .parent_names = disp_cc_parent_names_1,
>> >>> +               .num_parents = 4,
>> >>> +               .flags = CLK_GET_RATE_NOCACHE,
>> >>
>> >> Why?
>> >>
>> >>> +               .ops = &clk_rcg2_ops,
>> >>> +       },
>> >>> +};
>> >>> +
>> >>> +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.
> 

/snip


>> >>
>> >>> +static struct clk_branch disp_cc_mdss_dp_link_intf_clk = {
>> >>> +       .halt_reg = 0x2044,
>> >>> +       .halt_check = BRANCH_HALT,
>> >>> +       .clkr = {
>> >>> +               .enable_reg = 0x2044,
>> >>> +               .enable_mask = BIT(0),
>> >>> +               .hw.init = &(struct clk_init_data){
>> >>> +                       .name = "disp_cc_mdss_dp_link_intf_clk",
>> >>> +                       .parent_names = (const char *[]){
>> >>> +                               "disp_cc_mdss_dp_link_clk_src",
>> >>> +                       },
>> >>> +                       .num_parents = 1,
>> >>> +                       .flags = CLK_GET_RATE_NOCACHE,
>> >>
>> >> Why?
>> >>
>> >
>> > It was a requirement, but let me get back on this too.
>> >
>> I had a discussion with the Display Port teams and below is the 
>> requirement,
>> 
>> This flag is required since we reset/power-down the PLL every time 
>> they
>> disconnect/connect the DP cable or during suspend/resume.
>> Only with this flag, the calls to the PLL driver properly.
> 
> Ok. So that explains the get rate nocache flag. Can you please add a
> comment that explains that these clk registers here are lost across
> suspend/resume of the display device? It really sounds like these
> display clks are inside of the display power domain and thus they lose
> their state across the display power domain power down. It would be
> better if we could properly implement suspend/restore for these clk
> registers across suspend/resume of the display device so that we don't
> need this nocache flag and the display code can work together with the
> clk code here to restore the frequency to the clk.


We already handle the suspend/restore for these clk registers
in Dp PLL domain. Without the "NOCACHE_FLAG", and if we are requesting 
the same clock rate
for any of the clocks, the set_rate call never reaches the DP PLL Ops.

I am not clear on what you are suggesting for removing the 
"NOCACHE_FLAG" for
the DisplayPort clocks. Are you suggesting design changes in DP PLL 
driver or in dispcc-driver?
Can you please provide more details?

thanks
Chandan

> 
> Is it really the case that the rcg here is always selecting a 
> particular
> PLL and doing a div-1? Because that is very simple then to just write
> that setting again on genpd restore.