Date:   Wed, 15 Mar 2023 20:53:31 +0530
From:   Mukesh Ojha <quic_mojha@...cinc.com>
To:     Konrad Dybcio <konrad.dybcio@...aro.org>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        <agross@...nel.org>, <andersson@...nel.org>,
        <keescook@...omium.org>, <tony.luck@...el.com>,
        <gpiccoli@...lia.com>, <catalin.marinas@....com>, <will@...nel.org>
CC:     <linux-arm-msm@...r.kernel.org>,
        <linux-remoteproc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-hardening@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 3/6] soc: qcom: Add Qualcomm minidump kernel driver

Thanks for the review and comment.

On 3/9/2023 2:20 AM, Konrad Dybcio wrote:
> 
> 
> On 8.03.2023 21:22, Srinivas Kandagatla wrote:
>>
>>
>> On 21/02/2023 11:25, Mukesh Ojha wrote:
>>> Minidump is a best effort mechanism to collect useful and predefined
>>> data for first level of debugging on end user devices running on
>>> Qualcomm SoCs. It is built on the premise that System on Chip (SoC)
>>> or subsystem part of SoC crashes, due to a range of hardware and
>>> software bugs. Hence, the ability to collect accurate data is only
>>> a best-effort. The data collected could be invalid or corrupted,
>>> data collection itself could fail, and so on.
>>>
>>> Qualcomm devices in engineering mode provides a mechanism for
>>> generating full system ramdumps for post mortem debugging. But in some
>>> cases it's however not feasible to capture the entire content of RAM.
>>> The minidump mechanism provides the means for selecting region should
>>> be included in the ramdump. The solution supports extracting the
>>> ramdump/minidump produced either over USB or stored to an attached
>>> storage device.
>>>
>>> The core of minidump feature is part of Qualcomm's boot firmware code.
>>> It initializes shared memory(SMEM), which is a part of DDR and
>>> allocates a small section of it to minidump table i.e also called
>>> global table of content (G-ToC). Each subsystem (APSS, ADSP, ...) has
>>> their own table of segments to be included in the minidump, all
>>> references from a descriptor in SMEM (G-ToC). Each segment/region has
>>> some details like name, physical address and it's size etc. and it
>>> could be anywhere scattered in the DDR.
>>>
>>> Minidump kernel driver adds the capability to add linux region to be
>>> dumped as part of ram dump collection. It provides appropriate symbol
>>> to check its enablement and register client regions.
>>>
>>> To simplify post mortem debugging, it creates and maintain an ELF
>>> header as first region that gets updated with upon registration
>>> of a new region.
>>>
>>> Signed-off-by: Mukesh Ojha <quic_mojha@...cinc.com>
>>> ---
> [...]
> 
>>> +int qcom_minidump_ready(void)
>>> +{
>>> +    void *ptr;
>>> +    struct device_node *np;
>>> +    static bool is_smem_available = true;
>>> +
>>> +    if (!is_smem_available || !(np = of_find_compatible_node(NULL, NULL, "qcom,smem"))) {
>>
>> just check for dt node here does not mean that smem device is available, you should probably check if the device is avaliable aswell using of_device_is_available()
>>
>>
>> We should proabably return -EPROBEDEFER incase the node is present and device is not present.
> qcom_smem_get() seems to handle -EPROBE_DEFER internally, so this check
> may be entirely redundant.

The main idea behind checking qcom,smem availability is to not stop 
client(core kernel) probe if the smem dt node is not present at all
and qcom_smem_get() will always return  -EPROBE_DEFER for such cases.

For e.g: Again i am taking ramoops example which seems relevant for
this case, ramoops driver should still probe and not defer forever
if the smem node is not available at all for non-qcom device tree.

-Mukesh
> 
> Konrad

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