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

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

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

Konrad