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

[Trilok Soni]

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