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

From: Souradeep Chowdhury <quic_schowdhu@quicinc.com>
To: Arnd Bergmann <arnd@arndb.de>,
	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 15:50:04 +0530	[thread overview]
Message-ID: <44834c75-4db7-ec8a-9367-c6b83fa96b22@quicinc.com> (raw)
In-Reply-To: <50844899-b047-42fd-807a-db7136e5e590@app.fastmail.com>

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.

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

The dcc_sram is a dedicated io-memory for exclusive usage by dcc.
The register addresses are programmed in the dcc_sram corresponding
to a particular list and the start and end location on dcc_sram for
a particular list is updated in dcc hardware registers. So no two
lists can overlap. This is ensured by the driver code as follows:-

/* 3. Program DCC_RAM_CFG reg */
dcc_list_writel(drvdata, ram_cfg_base +
                        drvdata->ram_offset / 4, curr_list, DCC_LL_BASE);
dcc_list_writel(drvdata, drvdata->ram_start +
                         drvdata->ram_offset / 4, curr_list, DCC_FD_BASE);

> 
>      Arnd

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