Message-ID: <i74iyesupz2elaoefyn5s5r53op5aifftcmsgktk2pyghozvwx@usg4hp4wrtug>
Date: Wed, 18 Feb 2026 00:53:50 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>, Kees Cook <kees@...nel.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Rob Clark <robin.clark@....qualcomm.com>, Sean Paul <sean@...rly.run>,
        Akhil P Oommen <akhilpo@....qualcomm.com>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jesszhan0024@...il.com>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-hardening@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org
Subject: Re: [PATCH v3 2/3] soc: qcom: ubwc: Get HBB from SMEM

On Tue, Feb 17, 2026 at 01:59:48PM +0100, Konrad Dybcio wrote:
> 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@....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@....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

You have my vote for the latter option.

-- 
With best wishes
Dmitry

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