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 09:30:52 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Souradeep Chowdhury" <quic_schowdhu@...cinc.com>,
        "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 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.

How does a user ensure that the address they configure does
not overlap with some other use of the sram?

    Arnd