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

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Message-ID: <nhgxvv5bm3bevq4qkqnew6gaqlwiaicrs7yjckg26fgwnffqj5@u2phabfukljx>
Date: Sat, 10 Jan 2026 12:45:54 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Bjorn Andersson <andersson@...nel.org>
Cc: 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,
        Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Subject: Re: [PATCH v3 2/3] soc: qcom: ubwc: Get HBB from SMEM

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.

-- 
With best wishes
Dmitry