Re: [PATCH v3 2/3] soc: qcom: ubwc: Get HBB from SMEM

[Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>]

On 1/13/26 5:29 PM, Dmitry Baryshkov wrote:
> On Tue, Jan 13, 2026 at 04:31:15PM +0100, Konrad Dybcio wrote:
>> On 1/10/26 11:45 AM, Dmitry Baryshkov wrote:
>>> On Fri, Jan 09, 2026 at 11:50:46AM -0600, Bjorn Andersson wrote:
>>>> On Fri, Jan 09, 2026 at 05:21:10AM +0200, Dmitry Baryshkov wrote:
>>>>> On Thu, Jan 08, 2026 at 11:49:54AM -0600, Bjorn Andersson wrote:
>>>>>> On Thu, Jan 08, 2026 at 04:45:49PM +0200, Dmitry Baryshkov wrote:
>>>>>>> On Thu, Jan 08, 2026 at 03:21:51PM +0100, Konrad Dybcio wrote:
>>>>>>>> From: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
>>>>>>>>
>>>>>>>> To make sure the correct settings for a given DRAM configuration get
>>>>>>>> applied, attempt to retrieve that data from SMEM (which happens to be
>>>>>>>> what the BSP kernel does, albeit with through convoluted means of the
>>>>>>>> bootloader altering the DT with this data).
>>>>>>>>
>>>>>>>> Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
>>>>>>>>
>>>>>>>> ---
>>>>>>>> I'm not sure about this approach - perhaps a global variable storing
>>>>>>>> the selected config, which would then be non-const would be better?
>>>>>>>
>>>>>>> I'd prefer if const data was const, split HBB to a separate API.
>>>>>>>
>>>>>>
>>>>>> I agree, but I'd prefer to avoid a separate API for it.
>>>>>>
>>>>>> Instead I'd like to either return the struct by value (after updating
>>>>>> the hbb), but we then loose the ability to return errors, or by changing
>>>>>> the signature to:
>>>>>>
>>>>>> int qcom_ubwc_config_get_data(struct qcom_ubwc_cfg_data *data)
>>>>>>
>>>>>> This costs us an additional 16 bytes in each client (as the pointer is
>>>>>> replaced with the data), but I think it's a cleaner API.
>>>>>
>>>>> What about:
>>>>>
>>>>> const struct qcom_ubwc_cfg_data qcom_ubwc_config_get_data(u32 *hbb)
>>>>>
>>>>> I really want to keep the data as const and, as important, use it as a
>>>>> const pointer.
>>>>>
>>>>
>>>> I guess the question is what are you actually trying to achive; my goal
>>>> was to keep the base data constant, but I'm guessing that you also want
>>>> to retain the "const" classifier in the client's context struct (e.g.
>>>> the "mdss" member in struct dpu_kms)
>>>>
>>>> If we're returning the data by value, there's no way for you to mark
>>>> it as "const" in the calling code's context object (as by definition you
>>>> shouldn't be able to change the value after initializing the object).
>>>
>>> And I, of course, misssed one star:
>>>
>>> const struct qcom_ubwc_cfg_data *qcom_ubwc_config_get_data(u32 *hbb)
>>>
>>> This leaks the knowledge that HBB is slightly different kind of property
>>> than the rest of UBWC data.
>>>
>>>>
>>>> You also can't return the data by value and then track it by reference -
>>>> as that value lives on the stack. This has the benefit of making the
>>>> lifecycle of that object clear (it lives in each client) - but perhaps
>>>> not a goal of ours... 
>>>>
>>>> How come the ubwc config is const but the hbb isn't?
>>>>
>>>>
>>>> If we want both the per-target data to remain const and data in the
>>>> client's context to be carrying the const qualifier, the one solution I
>>>> can see is:
>>>>
>>>> const struct qcom_ubwc_cfg_data *qcom_ubwc_config_get_data(void)
>>>> {
>>>>         const struct qcom_ubwc_cfg_data *data;
>>>>         static struct qcom_ubwc_cfg_data cfg;
>>>>         int hbb;
>>>>
>>>>         ...
>>>>
>>>>         data = of_machine_get_match_data(qcom_ubwc_configs);
>>>>         ...
>>>>
>>>>         hbb = qcom_smem_dram_get_hbb();
>>>> 	...
>>>>
>>>>         cfg = *data;
>>>>         cfg.highest_bank_bit = hbb;
>>>>
>>>>         return &cfg;
>>>> }
>>>>
>>>> But we'd need to deal with the race in cfg assignment...
>>>
>>> static struct qcom_ubwc_cfg_data *cfg;
>>> static DEFINE_MUTEX(cfg_mutex);
>>> const struct qcom_ubwc_cfg_data *qcom_ubwc_config_get_data(void)
>>> {
>>>         const struct qcom_ubwc_cfg_data *data;
>>>         int hbb;
>>>
>>> 	guard(mutex)(&cfg_mutex);
>>>
>>> 	if (cfg)
>>> 		return cfg;
>>>
>>>         data = of_machine_get_match_data(qcom_ubwc_configs);
>>> 	if (!data)
>>> 		return ERR_PTR(-ENOMEM);
>>>
>>>         hbb = qcom_smem_dram_get_hbb();
>>> 	if (hbb = -ENODATA)
>>> 		hbb = 15; /* I think it was default */
>>> 	else if (hbb < 0)
>>> 		return ERR_PTR(hbb);
>>>
>>>         cfg = kmemdup(data, sizeof(*data), GFP_KERNEL);
>>> 	if (!cfg)
>>> 		return ERR_PTR(-ENOMEM);
>>>
>>>         cfg->highest_bank_bit = hbb;
>>>
>>> 	return cfg;
>>> }
>>>
>>> This potentially leaks sizeof(*data) memory if the module gets removed.
>>> Granted that all users also use qcom_ubwc_config_get_data() symbol, it
>>> should be safe to kfree(cfg) on module removal.
>>
>> I really don't understand why you'd want a separate API for hbb, if
>> hbb is already available from the larger struct *and* if a driver needs
>> to know about the value of hbb, it really needs to know about all the
>> other values as well
> 
> Please take another look, qcom_ubwc_config_get_data() is the only public
> API, qcom_smem_dram_get_hbb() is an internal API.
> 
> My goal is to keep having UBWC db which keeps const data and which which
> also returns a const pointer.

Right

So if I understand correctly, this is almost exactly what I originally had
in mind in the under-"---" message (modulo having a static global ptr vs full
struct instance) but I failed to express that I wanted to keep returning a
const pointer to the consumers

So in the end it's

A) int qcom_ubwc_config_get_data(struct qcom_ubwc_cfg_data *data)

vs 

B) const struct qcom_ubwc_cfg_data *qcom_ubwc_config_get_data(void)

I think the latter is better since we won't have to store a separate copy
of the config in each consumer driver (which the SSOT rework was largely
sparked by), essentially removing the ability for any of these drivers to
mess with the config internally and make it out-of-sync with others again

Konrad