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Message-ID: <7777c016-4875-a6c9-cd5e-78c2ac686448@linaro.org>
Date:   Tue, 9 May 2023 09:11:32 +0200
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Subject: Re: [PATCH v3 04/18] soc: qcom: Add Qualcomm minidump kernel driver
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To:     Mukesh Ojha <quic_mojha@quicinc.com>, agross@kernel.org,
        andersson@kernel.org, konrad.dybcio@linaro.org, corbet@lwn.net,
        keescook@chromium.org, tony.luck@intel.com, gpiccoli@igalia.com,
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        krzysztof.kozlowski+dt@linaro.org, robh+dt@kernel.org,
        linus.walleij@linaro.org, linux-gpio@vger.kernel.org,
        srinivas.kandagatla@linaro.org
Cc:     linux-arm-msm@vger.kernel.org, linux-remoteproc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org
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From:   Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
In-Reply-To: <e714566e-39b7-d46b-13bd-3c0e20e9f944@quicinc.com>
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On 08/05/2023 09:10, Mukesh Ojha wrote:
> 
> 
> On 5/4/2023 10:04 PM, Krzysztof Kozlowski wrote:
>> On 04/05/2023 17:21, Krzysztof Kozlowski wrote:
>>>>>
>>>>>> +	ret = qcom_minidump_init_apss_subsystem(md);
>>>>>> +	if (ret) {
>>>>>> +		dev_err(&pdev->dev, "apss minidump initialization failed: %d\n", ret);
>>>>>> +		goto unlock;
>>>>>> +	}
>>>>>> +
>>>>>> +	__md = md;
>>>>>
>>>>> No. This is a platform device, so it can have multiple instances.
>>>>
>>>> It can have only one instance that is created from SMEM driver probe.
>>>
>>> Anyone can instantiate more of them.... how did you solve it?
>>
>> To clarify - sprinkling more of singletons makes everything tightly
>> coupled, difficult to debug and non-portable. You cannot have two
>> instances, you have to control concurrent initialization by yourself in
>> each of such singletons.
>>
>> I understand sometimes they are unavoidable, for example when this does
>> not map to hardware property. However here you have the parent - smem -
>> which can return you valid instance. Thus you avoid entire problem of
>> file-scope variables.
> 
> I get your point, why one's should avoid file scope variables.
> 
> 
> This is infrastructure driver and will not have multiple instances and 
> even if it happens could be avoided with with the help of global mutex 
> and protect below function which i am already doing at the moment and 

But we do not want global mutexes... so incorrect design is being
improved by more incorrect design.

> fail the other probe if it is already initialized with proper logging..e.g
> 
> "already initialized..."
> 
> 
> ret = qcom_minidump_init_apss_subsystem(md);
> 
> 
> And this will be in-lined with
> 
> /* Pointer to the one and only smem handle */
> static struct qcom_smem *__smem;
> 
> Let me know if you still disagree...and have some other way ?

Why the parent - smem - cannot return every consumer the instance it
has? There will be one smem having only one minidump, so all problems
solved?

Best regards,
Krzysztof