linux-kernel - Re: [PATCH V22 2/3] misc: dcc: Add driver support for Data Capture and Compare unit(DCC)

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Date:   Wed, 19 Apr 2023 09:10:14 -0700
From:   Trilok Soni <quic_tsoni@...cinc.com>
To:     Souradeep Chowdhury <quic_schowdhu@...cinc.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Rob Herring <robh+dt@...nel.org>, Alex Elder <elder@...e.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
        <devicetree@...r.kernel.org>, Sibi Sankar <quic_sibis@...cinc.com>,
        Rajendra Nayak <quic_rjendra@...cinc.com>
Subject: Re: [PATCH V22 2/3] misc: dcc: Add driver support for Data Capture
 and Compare unit(DCC)

On 4/19/2023 9:08 AM, Trilok Soni wrote:
> On 4/19/2023 3:20 AM, Souradeep Chowdhury wrote:
>>
>>
>> On 4/19/2023 1:00 PM, Arnd Bergmann wrote:
>>> On Wed, Apr 19, 2023, at 09:00, Souradeep Chowdhury wrote:
>>>> On 4/18/2023 9:15 PM, Greg Kroah-Hartman wrote:
>>>>>
>>>>>> The following is the justification of using debugfs interface over 
>>>>>> the
>>>>>> other alternatives like sysfs/ioctls
>>>>>>
>>>>>> i) As can be seen from the debugfs attribute descriptions, some of 
>>>>>> the
>>>>>> debugfs attribute files here contains multiple arguments which 
>>>>>> needs to
>>>>>> be accepted from the user. This goes against the design style of 
>>>>>> sysfs.
>>>>>>
>>>>>> ii) The user input patterns have been made simple and convenient 
>>>>>> in this
>>>>>> case with the use of debugfs interface as user doesn't need to 
>>>>>> shuffle
>>>>>> between different files to execute one instruction as was the case on
>>>>>> using other alternatives.
>>>>>
>>>>> Why do you have debugfs and also a misc device?  How are they related?
>>>>> Why both?  Why not just one?  What userspace tools are going to use
>>>>> either of these interfaces and where are they published to show how 
>>>>> this
>>>>> all was tested?
>>>>
>>>> DCC has two fundamental steps of usage:-
>>>>
>>>> 1.Configuring the register addresses on the dcc_sram which is done by
>>>> user through the debugfs interface. For example:-
>>>>
>>>> echo R 0x10c004 > /sys/kernel/debug/dcc/../3/config
>>>>
>>>> Here we are configuring the register addresses for list 3, the 'R'
>>>> indicates a read operation, so this register value will be read
>>>> in case of a software trigger or kernel panic/watchdog bite and
>>>> dumped into the dcc_sram.
>>>
>>> Can you describe why the register location needs to be
>>> runtime configurable? I would have expected this type of setting
>>> to be part of the devicetree, which already describes other
>>> parts that interact with sram devices.
>>
>> Register addresses are made runtime configurable to give the user the
>> option of going for a software trigger. So the user can debug issues
>> during run-time as well. These register locations are arbitrary
>> and is configured by the user for debugging purposes and is not 
>> related to the DCC hardware itself.
> 
> Please note that we don't want to recompile the devicetree for new 
> settings since these registers can be set by team of engineers who are 
> debugging system level issues with various IPs across the SOCs. You 
> don't want to recompile the images while reproducing the system hangs/IP 
> watchdogs etc;

...and also these registers list is not fixed, it will vary based on the 
problem you are seeing and debugging on the SOC across the IPs.

---Trilok Soni