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

From: "Arnd Bergmann" <arnd@arndb.de>
To: "Souradeep Chowdhury" <quic_schowdhu@quicinc.com>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>
Cc: "Andy Gross" <agross@kernel.org>,
	"Konrad Dybcio" <konrad.dybcio@somainline.org>,
	"Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
	"Bjorn Andersson" <andersson@kernel.org>,
	"Rob Herring" <robh+dt@kernel.org>, "Alex Elder" <elder@ieee.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org,
	devicetree@vger.kernel.org,
	"Sibi Sankar" <quic_sibis@quicinc.com>,
	"Rajendra Nayak" <quic_rjendra@quicinc.com>
Subject: Re: [PATCH V22 2/3] misc: dcc: Add driver support for Data Capture and Compare unit(DCC)
Date: Wed, 19 Apr 2023 09:30:52 +0200	[thread overview]
Message-ID: <50844899-b047-42fd-807a-db7136e5e590@app.fastmail.com> (raw)
In-Reply-To: <f1456dd7-5dcf-d91a-459c-65efca4a3444@quicinc.com>

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

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