Re: [PATCH V4 5/7] clk: qcom: Add NSS clock Controller driver for IPQ9574

[Manikanta Mylavarapu]

On 10/4/2024 7:32 PM, Andrew Lunn wrote:
> On Fri, Oct 04, 2024 at 01:25:52PM +0530, Manikanta Mylavarapu wrote:
>>
>>
>> On 6/26/2024 8:09 PM, Devi Priya wrote:
>>>
>>>
>>> On 6/25/2024 8:09 PM, Andrew Lunn wrote:
>>>>> +static struct clk_alpha_pll ubi32_pll_main = {
>>>>> +    .offset = 0x28000,
>>>>> +    .regs = clk_alpha_pll_regs[CLK_ALPHA_PLL_TYPE_NSS_HUAYRA],
>>>>> +    .flags = SUPPORTS_DYNAMIC_UPDATE,
>>>>> +    .clkr = {
>>>>> +        .hw.init = &(const struct clk_init_data) {
>>>>> +            .name = "ubi32_pll_main",
>>>>> +            .parent_data = &(const struct clk_parent_data) {
>>>>> +                .index = DT_XO,
>>>>> +            },
>>>>> +            .num_parents = 1,
>>>>> +            .ops = &clk_alpha_pll_huayra_ops,
>>>>> +        },
>>>>> +    },
>>>>> +};
>>>>> +
>>>>> +static struct clk_alpha_pll_postdiv ubi32_pll = {
>>>>> +    .offset = 0x28000,
>>>>> +    .regs = clk_alpha_pll_regs[CLK_ALPHA_PLL_TYPE_NSS_HUAYRA],
>>>>> +    .width = 2,
>>>>> +    .clkr.hw.init = &(const struct clk_init_data) {
>>>>> +        .name = "ubi32_pll",
>>>>> +        .parent_hws = (const struct clk_hw *[]) {
>>>>> +            &ubi32_pll_main.clkr.hw
>>>>> +        },
>>>>> +        .num_parents = 1,
>>>>> +        .ops = &clk_alpha_pll_postdiv_ro_ops,
>>>>> +        .flags = CLK_SET_RATE_PARENT,
>>>>> +    },
>>>>> +};
>>>>
>>>> Can these structures be made const? You have quite a few different
>>>> structures in this driver, some of which are const, and some which are
>>>> not.
>>>>
>>> Sure, will check and update this in V6
>>>
>>> Thanks,
>>> Devi Priya
>>>>     Andrew
>>>>
>>
>> Hi Andrew,
>>
>> Sorry for the delayed response.
>>
>> The ubi32_pll_main structure should be passed to clk_alpha_pll_configure() API to configure UBI32 PLL. clk_alpha_pll_configure() API expects a non-const structure. Declaring it as const will result in the following compilation warning
>>
>> drivers/clk/qcom/nsscc-ipq9574.c:3067:26: warning: passing argument 1 of ‘clk_alpha_pll_configure’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
>>   clk_alpha_pll_configure(&ubi32_pll_main, regmap, &ubi32_pll_config);
>>                           ^
>> In file included from drivers/clk/qcom/nsscc-ipq9574.c:22:0:
>> drivers/clk/qcom/clk-alpha-pll.h:200:6: note: expected ‘struct clk_alpha_pll *’ but argument is of type ‘const struct clk_alpha_pll *’
>>  void clk_alpha_pll_configure(struct clk_alpha_pll *pll, struct regmap *regmap,
>>       ^~~~~~~~~~~~~~~~~~~~~~~
> 
> As far as i can see, clk_alpha_pll_configure() does not modify pll.
> 
> https://elixir.bootlin.com/linux/v6.12-rc1/source/drivers/clk/qcom/clk-alpha-pll.c#L391
> 
> So you can add a const there as well.
> 

It appears that multiple targets are invoking clk_alpha_pll_configure().
I will make changes in a separate series after this.

>> The ubi32_pll is the source for nss_cc_ubi0_clk_src, nss_cc_ubi1_clk_src, nss_cc_ubi2_clk_src, nss_cc_ubi3_clk_src. Therefore, to register ubi32_pll with clock framework, it should be assigned to UBI32_PLL index of nss_cc_ipq9574_clocks array. This assignment will result in the following compilation warning if the ubi32_pll structure is declared as const.
>>
>> drivers/clk/qcom/nsscc-ipq9574.c:2893:16: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
>>   [UBI32_PLL] = &ubi32_pll.clkr,
> 
> Which suggests you are missing a const somewhere else.
> 
> Getting these structures const correct has a few benefits. It makes
> you code smaller, since at the moment at load time it needs to copy
> these structures in to the BSS so they are writable, rather than
> keeping them in the .rodata segment. Also, by making them const, you
> avoid a few security issues, they cannot be overwritten, the MMU will
> protect them. The compiler can also make some optimisations, since it
> knows the values cannot change.
> 
> Now, it could be getting this all const correct needs lots of patches,
> because it has knock-on effects in other places. If so, then don't
> bother. But if it is simple to do, please spend a little time to get
> this right.
> 
> 	Andrew

We can make these structures const. However, it will require exhaustive changes. Below is the rationale.

To declare the ubi32_pll and ubi32_pll_main (since it is also passed to nss_cc_ipq9574_clocks[] array) structures as const,
the 'clks' member of 'struct qcom_cc_desc' needs to be const.

The following compilation errors are observed after making the 'clks' member of 'struct qcom_cc_desc' as const:
drivers/clk/qcom/common.c: In function ‘qcom_cc_icc_register’:
drivers/clk/qcom/common.c:274:7: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
hws = &desc->clks[desc->icc_hws[i].clk_id]->hw;
       ^

drivers/clk/qcom/common.c: In function ‘qcom_cc_really_probe’:
drivers/clk/qcom/common.c:294:30: error: initialization from incompatible pointer type [-Werror=incompatible-pointer-types]
struct clk_regmap **rclks = desc->clks;
                              ^~~~

The common.c code is designed in such a way that the 'clks' member needs to be assigned to non-const variables/pointers.

We need to update common.c and the drivers that utilize 'struct qcom_cc_desc'.
I will make changes in a separate series after this.

Thanks & Regards,
Manikanta.