Message-ID: <iqg6jpq4i3olwugnlnsczisbrbysxzik6otby3pgkv5uqsez3f@diwpjgf26mk3>
Date: Tue, 13 Jan 2026 18:29:04 +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, 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.

-- 
With best wishes
Dmitry

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