Message-ID: <fb6dc88c-0829-11f8-b6f4-7eec3409e8bf@quicinc.com>
Date:   Mon, 16 May 2022 16:02:53 +0530
From:   Souradeep Chowdhury <quic_schowdhu@...cinc.com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
CC:     Andy Gross <agross@...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>,
        Sai Prakash Ranjan <quic_saipraka@...cinc.com>,
        Sibi Sankar <quic_sibis@...cinc.com>,
        Rajendra Nayak <quic_rjendra@...eaurora.org>,
        <vkoul@...nel.org>
Subject: Re: [PATCH V7 2/7] soc: qcom: dcc:Add driver support for Data Capture
 and Compare unit(DCC)


On 5/6/2022 8:23 AM, Bjorn Andersson wrote:
> On Thu 05 May 05:53 PDT 2022, Souradeep Chowdhury wrote:
>
>> On 5/3/2022 11:33 PM, Bjorn Andersson wrote:
>>> On Thu 03 Mar 00:27 CST 2022, Souradeep Chowdhury wrote:
>>>
>>>> The DCC is a DMA Engine designed to capture and store data
>>>> during system crash or software triggers. The DCC operates
>>>> based on user inputs via the sysfs interface. The user gives
>>>> addresses as inputs and these addresses are stored in the
>>>> dcc sram. In case of a system crash or a manual software
>>>> trigger by the user through the debugfs interface,
>>>> the dcc captures and stores the values at these addresses.
>>>> This patch contains the driver which has all the methods
>>>> pertaining to the debugfs interface, auxiliary functions to
>>>> support all the four fundamental operations of dcc namely
>>>> read, write, read/modify/write and loop. The probe method
>>>> here instantiates all the resources necessary for dcc to
>>>> operate mainly the dedicated dcc sram where it stores the
>>>> values. The DCC driver can be used for debugging purposes
>>>> without going for a reboot since it can perform software
>>>> triggers as well based on user inputs.
>>>>
>>>> Also added the documentation for debugfs entries and explained
>>>> the functionalities of each debugfs file that has been created
>>>> for dcc.
>>>>
>>>> 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.
>>>>
>>>> Signed-off-by: Souradeep Chowdhury <quic_schowdhu@...cinc.com>
>>>> ---
>>>>    Documentation/ABI/testing/debugfs-driver-dcc |  124 +++
>>>>    drivers/soc/qcom/Kconfig                     |    8 +
>>>>    drivers/soc/qcom/Makefile                    |    1 +
>>>>    drivers/soc/qcom/dcc.c                       | 1465 ++++++++++++++++++++++++++
>>>>    4 files changed, 1598 insertions(+)
>>>>    create mode 100644 Documentation/ABI/testing/debugfs-driver-dcc
>>>>    create mode 100644 drivers/soc/qcom/dcc.c
>>>>
>>>> diff --git a/Documentation/ABI/testing/debugfs-driver-dcc b/Documentation/ABI/testing/debugfs-driver-dcc
>>>> new file mode 100644
>>>> index 0000000..70029ab
>>>> --- /dev/null
>>>> +++ b/Documentation/ABI/testing/debugfs-driver-dcc
>>>> @@ -0,0 +1,124 @@
>>>> +What:          /sys/kernel/debug/dcc/.../trigger
>>>> +Date:          March 2022
>>>> +Contact:       Souradeep Chowdhury <quic_schowdhu@...cinc.com>
>>>> +Description:
>>>> +		This is the debugfs interface for manual software
>>>> +		triggers. The user can simply enter a 1 against
>>>> +		the debugfs file and enable a manual trigger.
>>>> +		Example:
>>>> +		echo  1 > /sys/kernel/debug/dcc/.../trigger
>>>> +
>>>> +What:          /sys/kernel/debug/dcc/.../enable
>>>> +Date:          March 2022
>>>> +Contact:       Souradeep Chowdhury <quic_schowdhu@...cinc.com>
>>>> +Description:
>>>> +		This debugfs interface is used for enabling the
>>>> +		the dcc hardware. On enabling the dcc, all the
>>>> +		addresses entered by the user is written into
>>>> +		dcc sram which is read by the dcc hardware on
>>>> +		manual or crash induced triggers.
>>>> +		Example:
>>>> +		echo  0 > /sys/bus/platform/devices/.../enable
>>>> +		(disable dcc)
>>>> +		echo  1 > /sys/bus/platform/devices/.../enable
>>>> +		(enable dcc)
>>>> +
>>>> +What:          /sys/kernel/debug/dcc/.../config_read
>>> As mentioned last time, I don't like this interface of having 6 files
>>> that the user can write to in order to append items in the currently
>>> selected linked list.
>>>
>>> Why can't this be a single "config" which takes a multiline string of
>>> operations? (Bonus point for supporting appending to the list).
>>>
>>>
>>> This would also serve as a natural place to dump the linked list back to
>>> the user for inspection.
>> Following is the justification of having multiple files in debugfs
>>
>> 1-> Since there are fundamentally 4 instructions for DCC, Read,Write,Read
>> and then Write and Loop,having separate debugfs files for the same makes it
>>
>> convenient for the user to use this tool and also to document.This
>> also is consistent with the design principles of debugfs as it supports
>> logical segregation of Debugfs files based on the user instructions.
>>
> So say the user of DCC wants to read a register 10 times, they know that
> the DCC operates on lists of operations, so they want to tell the
> computer "read X, loop 10 times".
>
> But the API is "write to loop file", "write to read file" and "write to
> loop file".
>
> You're achieving the same thing, but the user and the driver thinks in
> terms of lists of operations and inbetween is a API which provides
> "logical segregation" of the different parts of that list.
>
>> 2-> We are maintaining a common linkedlist inside the driver and it can be
>> viewed by the user through the "config_read" debugfs file. Will be adding
>> this to the documentation as well.
>>
> So I use a mix of config_* files to build the lists, and then
> config_read is used to look at the list?
>
> So some config_* files will when written append to the list, some
> config_* files will perform some action (e.g. reset) and reading some
> config_* files will return something useful.
>
>> Let me know your thoughts regarding the above.
>>
> I am not convinced that having multiple files provides a nice user
> interface. But I certain that the use of the word "config" in various
> different ways is wrong.
>
> There are files in the interface which purpose is to append items to the
> linked list, name them append_*. Reading the appended items on the list
> should not be overloaded on one of the "append" files.
>
>
> So perhaps:
>
> append_read
> append_write
> append_rmw
> append_loop
> config (to dump the current config)
> enable
> ready
> reset
> trigger
>
>
> But then looking at the append_* functions again and the examples below.
> You could easily have a single append which takes read, write, rmw and
> loop as a first keyword - and build a crude parser based on sscanf to
> decode the strings.
>
> Then all append_* becomes "append", which is a single file for adding to
> the list and "config" is a single point to read the current list.
> Perhaps you could name this file just "config", reading will dump the
> list, writing will append to the list.
>
> Here you would have a cleaner interface.
>
>
> But as you write your own fops you could differentiate between write
> and append, so you could make this slightly cleaner by manifesting the
> "append" part by allowing the user to do:
>
>    echo read 1, 2, 3 > config
>    echo read 4, 5, 6 >> config
>
> Which clearly shows that the first writes to the config and second
> appends to the current config. With this interface reset would become:
>
>    echo > config
>
> You can still require that only single operation is written to or
> appended to the list per write - to allow you to continue to rely on the
> crude sscanf based parser.
>
> With this your interface is reduced to:
>
> config
> enable
> ready
> trigger

Ack. Will create a single config for this interface.

>
>>>> +Date:          March 2022
>>>> +Contact:       Souradeep Chowdhury <quic_schowdhu@...cinc.com>
>>>> +Description:
>>>> +		This stores the addresses of the registers which
>>>> +		needs to be read in case of a hardware crash or
>>>> +		manual software triggers. The address entered here
>>>> +		are considered under read type instruction.
>>>> +		Example:
>>>> +		echo <1> <2> <3> >/sys/kernel/debug/dcc/../config_read
>>>> +		1->Address to be considered for reading the value.
>>>> +		2->The word count of the addresses, read n words
>>>> +		   starting from address <1>.
>>>> +		3->Can be a 1 or 0 which indicates if it is apb or ahb
>>>> +		bus respectively.
>>>> +
>>>> +What:          /sys/kernel/debug/dcc/.../config_write
>>>> +Date:          March 2022
>>>> +Contact:       Souradeep Chowdhury <quic_schowdhu@...cinc.com>
>>>> +Description:
>>>> +		This file allows user to write a value to the register
>>>> +		address given as argument. The reason for this feature
>>>> +		of dcc is that for accessing certain registers it is
>>>> +		necessary to set some bits of some other register.
>>>> +		Example:
>>>> +		echo <1> <2> <3> > /sys/bus/platform/devices/.../config_write
>>>> +		1->Address to be considered for writing the value.
>>>> +		2->The value that needs to be written at the location.
>>>> +		3->Can be a 1 or 0 which indicates if it is apb or ahb
>>>> +		bus respectively.
>>>> +
>>>> +What:          /sys/kernel/debug/dcc/.../config_reset
>>>> +Date:          March 2022
>>>> +Contact:       Souradeep Chowdhury <quic_schowdhu@...cinc.com>
>>>> +Description:
>>>> +		This file is used to reset the configuration of
>>>> +		a dcc driver to the default configuration. This
>>>> +		means that all the previous addresses stored in
>>>> +		the driver gets removed and user needs to enter
>>>> +		the address values from the start.
>>>> +		Example:
>>>> +		echo  1 > /sys/bus/platform/devices/.../config_reset
>>>> +
>>>> +What:          /sys/kernel/debug/dcc/.../config_loop
>>>> +Date:		March 2022
>>>> +Contact:       Souradeep Chowdhury <quic_schowdhu@...cinc.com>
>>>> +Description:
>>>> +		This file is used to enter the loop type addresses for
>>>> +		dcc. DCC hardware provides feature to loop among multiple
>>>> +		addresses. For debugging purposes register values need to
>>>> +		be captured repeatedly in a loop. On giving the loop count
>>>> +		as n, the value at address will be captured n times in a
>>>> +		loop. At most 8 loop addresses can be configured at once.
>>>> +		Example:
>>>> +		echo <1> <2> <3> > /sys/kernel/debug/dcc/../config_loop
>>>> +		1->The loop count, the number of times the value of the
>>>> +		   addresses will be captured.
>>>> +		2->The address count, total number of addresses to be
>>>> +		   entered in this instruction.
>>>> +		3->The series of addresses to be entered separated by a
>>>> +		   space like <addr1> <addr2>... and so on.
>>>> +
>>>> +What:          /sys/kernel/debug/dcc/.../config_read_write
>>>> +Date:          March 2022
>>>> +Contact:       Souradeep Chowdhury <quic_schowdhu@...cinc.com>
>>>> +Description:
>>>> +		This file is used to read the value of the register
>>>> +		and then write the value given as an argument to the
>>>> +		register address. The address argument should be given
>>>> +		of the form <addr> <mask> <value>.For debugging purposes
>>>> +		sometimes we need to first read from a register and then
>>>> +		set some values to the register.
>>>> +		Example:
>>>> +		echo <1> <2> <3> > /sys/kernel/debug/dcc/.../config_read_write
>>>> +		1->The address which needs to be considered for read then write.
>>>> +		2->The value that needs to be written on the address.
>>>> +		3->The mask of the value to be written.
>>>> +
>>>> +What:		/sys/kernel/debug/dcc/.../ready
>>>> +Date:		March 2022
>>>> +Contact	Souradeep Chowdhury <quic_schowdhu@...cinc.com>
>>>> +Description:
>>>> +		This file is used to check the status of the dcc
>>>> +		hardware if it's ready to take the inputs. A 0
>>>> +		here indicates dcc is in a ready condition.
>>>> +		Example:
>>>> +		cat /sys/kernel/debug/dcc/.../ready
>>>> +
>>>> +What:		/sys/kernel/debug/dcc/.../curr_list
>>> I still don't like the idea of having a single set of files to interface
>>> with all N lists. I think you should discover how many lists you have
>>> and create N directories of files, each on operating on a given list.
>> As explained before there cannot be different files based on lists as
>> the number of lists to be used varies across platforms where DCC is
>> used.
> Isn't this dcc->nr_link_list?
Yes. This varies across different SoCs and also user may not have access 
to all the lists supported by DCC.
>
>> Also we are giving the user the flexibility to configure
>> multiple lists at one go whereas the dumps are collected in the form
>> of separate lists that are configured by the user.
>>
> I'm not sure I follow what you're trying to say here.
>
> If we determine that nr_link_list is 2, you create a directory named 0
> and one named 1, fill them with the interface files to operate on list 0
> and list 1 respectively. Then you still allow the user to configure and
> enable the 2 available lists?
>
> The difference is that it's clear how many lists you have and it's clear
> when you poke at 0/config_read that you're referring to the first list
> and poking 0/enable will mean the same list - there's no curr_list to
> mux between them in the interface.

So from DCC hardware perspective, it needs to be told that "list n 
begins at x and ends at y". That is why the limitation of DCC

hardware is that the lists can only be configured sequentially and not 
in a overlapping manner. So for example we need to enter

all the addresses to be stored for list n before jumping to any other 
lists. Seeing this limitation, will it be advantageous to have

different set of files for different lists as we will be configuring 
them sequentially anyway?

>
> Regards,
> Bjorn
>
>>>> +Date:		March 2022
>>>> +Contact:	Souradeep Chowdhury <quic_schowdhu@...cinc.com>
>>>> +Description:
>>>> +		This attribute is used to enter the linklist to be
>>>> +		used while appending addresses. The range of values
>>>> +		for this is advertised either by a register or is
>>>> +		predefined. Max value for this can be till 8.
>>>> +		Example:
>>>> +		echo 0 > /sys/kernel/debug/dcc/...curr_list
>>>> +
>>> Regards,
>>> Bjorn

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