Message-ID: <CACSVV01D6u1B2d6ipd7cuomOhE_iGDBF84V8=v7jcm_5hJ87eQ@mail.gmail.com>
Date: Tue, 17 Feb 2026 15:08:29 -0800
From: Rob Clark <rob.clark@....qualcomm.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>, Kees Cook <kees@...nel.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        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 2:53 PM Dmitry Baryshkov
<dmitry.baryshkov@....qualcomm.com> wrote:
>
> 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.

same here, B pls

BR,
-R

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