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

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Date:   Thu, 20 Apr 2023 08:58:01 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Trilok Soni <quic_tsoni@...cinc.com>
Cc:     Souradeep Chowdhury <quic_schowdhu@...cinc.com>,
        Arnd Bergmann <arnd@...db.de>, 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 09:10:14AM -0700, Trilok Soni wrote:
> 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.

Then all of this should be documented in the driver, and in the
changelog please.

thanks,

greg k-h