Date:   Fri, 7 Jan 2022 20:57:50 +0530
From:   Souradeep Chowdhury <quic_schowdhu@...cinc.com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
CC:     Souradeep Chowdhury <schowdhu@...eaurora.org>,
        Andy Gross <agross@...nel.org>,
        Rob Herring <robh+dt@...nel.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" <saiprakash.ranjan@...eaurora.org>,
        Sibi Sankar <sibis@...eaurora.org>,
        Rajendra Nayak <rnayak@...eaurora.org>, <vkoul@...nel.org>
Subject: Re: [PATCH V6 2/7] soc: qcom: dcc:Add driver support for Data Capture
 and Compare unit(DCC)


On 1/7/2022 5:31 AM, Bjorn Andersson wrote:
> On Wed 05 Jan 23:57 PST 2022, Souradeep Chowdhury wrote:
>
>> On 12/18/2021 1:41 AM, Bjorn Andersson wrote:
>>> On Tue 10 Aug 12:54 CDT 2021, Souradeep Chowdhury wrote:
>>>
>>>> The DCC is a DMA Engine designed to capture and store data
>>>> during system crash or software triggers.The DCC operates
>>> Please include a space after '.'
>> Ack
>>>> based on user inputs via the sysfs interface.The user gives
>>>> addresses as inputs and these addresses are stored in the
>>>> form of linkedlists.In case of a system crash or a manual
>>> I think the user configures the DCC hardware with "a sequence of
>>> operations to be performed as DCC is triggered".
>>>
>>> Afaict the sequence is stored just as a sequence of operations in SRAM,
>>> there's no linked list involved - except in your intermediate
>>> implementation.
>> The user just enters the addresses as input whereas the sequence of
>> operations takes
>>
>> place as per configuration code inside the driver. The end result is storage
>> of these
>>
>> addresses inside the DCC SRAM. The DCC hardware will capture the value at
>> these
>>
>> addresses on a crash or manual trigger by the user.
>>
>>>> software trigger by the user through the sysfs interface,
>>>> the dcc captures and stores the values at these addresses.
>>>> This patch contains the driver which has all the methods
>>>> pertaining to the sysfs interface, auxiliary functions to
>>>> support all the four fundamental operations of dcc namely
>>>> read, write, first read then write and loop.The probe method
>>> "first read then write" is called "read/modify/write"
>> Ack
>>>> 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 manual
>>>> triggers.
>>>>
>>>> Also added the documentation for sysfs entries
>>>> and explained the functionalities of each sysfs file that
>>>> has been created for dcc.
>>>>
>>>> The following is the justification of using sysfs interface
>>>> over the other alternatives like ioctls
>>>>
>>>> i) As can be seen from the sysfs attribute descriptions,
>>>> most of it does basic hardware manipulations like dcc_enable,
>>>> dcc_disable, config reset etc. As a result sysfs is preferred
>>>> over ioctl as we just need to enter a 0 or 1.
>>>>
>>> As I mentioned in our chat, using sysfs allows us to operate the
>>> interface using the shell without additional tools.
>>>
>>> But I don't think that it's easy to implement enable/disable/reset using
>>> sysfs is a strong argument. The difficult part of this ABI is the
>>> operations to manipulate the sequence of operations, so that's what you
>>> need to have a solid plan for.
>> The sysfs interface is being used to get the addresses values entered by the
>> user
>>
>> and to also go for manual triggers. The sequence of operations are kept as a
>> part of
>>
>> fixed driver code which is called when the user enters the data.
>>
> But does the hardware really just operate on "addresses values entered
> by the user". Given the various types of operations: read, write,
> read-modify-write and loop I get the feeling that the hardware
> "executes" a series of actions.
>
> I'm don't think the proposed sysfs interface best exposes this to the
> user and I don't think that "it's easy to implement enable/disable
> attributes in sysfs" is reason enough to go with that approach.

So the sysfs interface here has been introduced keeping in mind how the 
DCC_SRAM needs to be

programmed for the dcc hardware to work. We are maintaining a list here 
based on the address

entry. The 4 cases for the type of addresses are as follows-:

i) READ ADDRESSES

user enters something like "echo <addr> <len> > config"

DCC driver stores the <addr> along with the length information in the 
DCC_SRAM.

ii) WRITE ADDRESSES

User enters something like "echo <addr> <write_val> 1  > config_write"

DCC stores the <addr> first in sram followed by <write_val>.

For the above 2 type of addresses there won't be much difference if we 
use IOCTL.

However, for the next 2 type of addresses

iii) LOOP ADDRESSES

user has to enter something like below

echo 9 > loop
echo 0x01741010 1 > config
echo 0x01741014 1 > config
echo 1 > loop

The DCC SRAM will be programmed precisely like the above entries where 
the loop count will be stored first

followed by loop addresses and then again a "echo 1 > loop " marks the 
end of loop addresses.

in DCC_SRAM.

iv) READ_WRITE ADDRESSES

User has to enter something like below

echo <addr> > /sys/bus/platform/devices/../config

echo <mask> <val> > /sys/bus/platform/devices/../rd_mod_wr

Here first the  <addr> is stored in DCC_SRAM followed by <mask> and then 
the <val>.

The above representation to the user space is consistent with the dcc 
hardware in terms of

the way the sequence of values are programmed in the DCC SRAM . Moving 
to IOCTL will

only change the way the READ_WRITE address is represented although user 
will have to enter

multiple parameters at once, let me know if we still need to go for the 
same.


>>>> ii) Existing similar debug hardwares are there for which drivers
>>>> have been written using sysfs interface.One such example is the
>>>> coresight-etm-trace driver.
>>> Afaict the etm interface has operations to enable and disable, I don't
>>> see anything that's similar to the interface for defining the sequence
>>> of operations.
>> Here I have just drawn analogy with an existing sysfs interface. The
>> argument for
>>
>> using sysfs instead of other interfaces can be as folllows
>>
>> 1)
>>
>> Debugfs interface is used by drivers to expose debugging information additional to
>> the function they do. But the sole usage of this driver depends on the configuration
>> exported through the attributes and therefore it is an ABI as suggested by Mani.
>>
>> 2)
>>
>> Debugfs is disabled in production so this will not give the user facility to use DCC.
>>
>> 3)
>>
>> As you mentioned sysfs can be used with the shell without any additional tools.
>>
>> 4)
>>
>> Alternatives like NETLINK has also been suggested although in this case by using
>> NETLINK we won't be able to exploit most it's features like duplex connection,
>> asychrony and bulk data transfers. We are not showing any stats here to the user
>> as such and also sysfs is considered to be a bit more reliable.
>>
> I'm not sure why netlink would preferred for this; to me sysfs is much
> preferred as it allows us to use DCC from scripts etc without having to
> bundle additional binaries. If we can't express the configuration
> appropriately in a sysfs (debugfs would be the same) interface, I think
> an ioctl interface on /dev/dcc would be a reasonable alternative.
>
>> Please let me your thoughts regarding this.
>>
>>>> A closer analog can also be the watchdog
>>>> subsystems though it is ioctls based.
>>>>
>>> I don't think this adds value to the argument for using a sysfs based
>>> interface.
>> Ack
>>>> Signed-off-by: Souradeep Chowdhury<schowdhu@...eaurora.org>
>>>> ---
>>>>    Documentation/ABI/testing/sysfs-driver-dcc |  114 ++
>>>>    drivers/soc/qcom/Kconfig                   |    8 +
>>>>    drivers/soc/qcom/Makefile                  |    1 +
>>>>    drivers/soc/qcom/dcc.c                     | 1549 ++++++++++++++++++++++++++++
>>>>    4 files changed, 1672 insertions(+)
>>>>    create mode 100644 Documentation/ABI/testing/sysfs-driver-dcc
>>>>    create mode 100644 drivers/soc/qcom/dcc.c
>>>>
>>>> diff --git a/Documentation/ABI/testing/sysfs-driver-dcc b/Documentation/ABI/testing/sysfs-driver-dcc
> [..]
>>>> +What:           /sys/bus/platform/devices/.../rd_mod_wr
>>>> +Date:           March 2021
>>>> +Contact:        Souradeep Chowdhury<schowdhu@...eaurora.org>
>>>> +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 in config.
>>> It's not clear from this description how to use this operation. E.g. is
>>> it appended to the same list of operations? When will this operation
>>> happen?
>>>
>>> Looking at the implementation I believe that "config", "config_write",
>>> "loop" and "rd_mod_wr" all appends operations to the same list.
>>>
>>> I think it would be much better to configure this with a single file
>>> that can either be written to '>' or appended to '>>' and you would feed
>>> it a sequence of the operations to be performed.
>>>
>>> That said, I'm afraid that might no longer be a sysfs attribute.
>>>
>>> Something like:
>>>     # echo 'r 0x80000010 0x10' > config
>>>     # echo 'w 0x80000000 0xff' >> config
>>>     # echo 'rmw 0x80000000 0xf 0xa' >> config
>>>     # echo 'l 0x80000010 10' >> config
>>>
>>> and:
>>>     # cat config
>>>     r 0x80000010 0x10
>>>     w 0x80000000 0xff
>>>     rmw 0x80000000 0xf 0xa
>>>     l 0x80000010 10
>>>
>>> (Or read/write/modify/loop as keywords...)
>>>
>>>
>>> reset could be done by just: echo '' > config
>>>
>>> This would make it quite similar to several of the files in the tracing
>>> framework.
>> Currently this is being implemented as follows
>>
>> Step 1
>>
>> echo 0x80000000 > /sys/bus/platform/devices/../config
>>
>> Step 2
>>
>> echo 0xF 0xA > /sys/bus/platform/devices/../rd_mod_wr
>>
>> So the particular address is tagged as rd_mod_wr type and therefore
>>
>> while the value at the address 0x80000000 is captured, 0xA is also
>>
>> written to it's last 4 bits. Will be updating this in the documentation
>>
>> along with details for loop and config_write as well.
>>
> But can you help me confirm what is actually written in SRAM when you
> run these two steps?
Explained as above.
>
>>>> +		The address argument should
>>>> +		be given of the form <mask> <value>.For debugging
>>>> +		purposes sometimes we need to first read from a register
>>>> +		and then set some values to the register.
>>>> +		Example:
>>>> +		echo 0x80000000 > /sys/bus/platform/devices/.../config
>>>> +		(Set the address in config file)
>>>> +		echo 0xF 0xA > /sys/bus/platform/devices/.../rd_mod_wr
>>>> +		(Provide the mask and the value to write)
>>>> +
>>>> +What:           /sys/bus/platform/devices/.../ready
>>>> +Date:           March 2021
>>>> +Contact:        Souradeep Chowdhury<schowdhu@...eaurora.org>
>>>> +Description:
>>>> +		This file is used to check the status of the dcc
>>>> +		hardware if it's ready to take the inputs.
>>> When will this read "false"?
>> This will give false if the DCC hardware is not in an operational state.
>>
>> Will update accordingly.
>>
>>>> +		Example:
>>>> +		cat /sys/bus/platform/devices/.../ready
>>>> +
>>>> +What:		/sys/bus/platform/devices/.../curr_list
>>>> +Date:		February 2021
>>>> +Contact:	Souradeep Chowdhury<schowdhu@...eaurora.org>
>>>> +Description:
>>>> +		This attribute is used to enter the linklist to be
>>> I think it would be more appropriate to use the verb "select" here and
>>> afaict it's a "list" as the "linked" part only relates to your
>>> implementation).
>>>
>>> But that said, I don't like this ABI. I think it would be cleaner if you
>>> had specific attributes for each of the lists. That way it would be
>>> clear that you have N lists and they can be configured and enabled
>>> independently, and there's no possible race conditions.
>> So we do have attributes for independent lists in this case. The user is
>> given the option
>>
>> to configure multiple lists at one go. For example I can do
>>
>> echo 1 > curr_list
>>
>> echo 0x18000010 1 > config
>> echo 0x18000024 1 > config
>>
>> Then followed by
>>
>> echo 2 > curr_list
>>
>> echo 0x18010038 6 > config
>> echo 0x18020010 1 > config
>>
>> We will get the output in terms of two separate list of registers values.
>>
> I understand that this will define two lists of operations and that we
> will get 2 and 7 registers dumped, respectively. Perhaps unlikely, but
> what happens if you try to do these two operations concurrently?
>
>
> What I'm suggesting here is that if you have N contexts, you should have
> N interfaces to modify each one independently - simply because that's
> generally a very good thing.

Not sure if there will ever be a concurrency issue in this case. This is 
just about programming the DCC SRAM

from the user entries sequentially. The curr_list number is nothing but 
some register writes done in the dcc

so that the dcc_hardware knows the beginning and end of a particular 
list and can dump the captured data

according. Even if an user chooses multiple curr_list entries, it will 
be programmed sequentially in DCC_SRAM.

>
>>>> +		used while appending addresses.The range of values
>>>> +		for this can be from 0 to 3.This feature is given in
>>>> +		order to use certain linkedlist for certain debugging
>>>> +		purposes.
>>>> +		Example:
>>>> +		echo 0 > /sys/bus/platform/devices/10a2000.dcc/curr_list
>>>> +
> [..]
>>>> diff --git a/drivers/soc/qcom/dcc.c b/drivers/soc/qcom/dcc.c
> [..]
>>>> +static int dcc_valid_list(struct dcc_drvdata *drvdata, int curr_list)
>>>> +{
>>>> +	u32 lock_reg;
>>>> +
>>>> +	if (list_empty(&drvdata->cfg_head[curr_list]))
>>>> +		return -EINVAL;
>>>> +
>>>> +	if (drvdata->enable[curr_list]) {
>>>> +		dev_err(drvdata->dev, "List %d is already enabled\n",
>>>> +				curr_list);
>>>> +		return -EINVAL;
>>>> +	}
>>>> +
>>>> +	lock_reg = dcc_readl(drvdata, DCC_LL_LOCK(curr_list));
>>> Under what circumstances would this differ from
>>> drvdata->enable[curr_list}?
>> So locking the list is done on the register as soon as the user enters the
>> curr_list entry whereas
>>
>> the list is marked as enabled only on successfully programming the SRAM
>> contents. So a list can
>>
>> be locked and not marked enabled in certain cases. The first is used so that
>> the user doesn't
>>
>> mistakenly enter the same curr_list twice whereas the later is used to mark
>> that the list has been
>>
>> successfully configured.
>>
> So this will mark the list as "actively in use, but disabled"? Why is
> this kept in the hardware? When is this not the same as the list of
> operations for that list being non-empty?

So this is in accordance with the dcc hardware configuration 
requirement. We have to lock the list first and

after that proceed with the subsequent writes . As per the driver code below

                /* 1. Take ownership of the list */
                 dcc_writel(drvdata, BIT(0), DCC_LL_LOCK(list));

                 /* 2. Program linked-list in the SRAM */
                 ram_cfg_base = drvdata->ram_cfg;
                 ret = __dcc_ll_cfg(drvdata, list);
                 if (ret) {
                         dcc_writel(drvdata, 0, DCC_LL_LOCK(list));
                         goto err;
                 }

                 /* 3. program DCC_RAM_CFG reg */
                 dcc_writel(drvdata, ram_cfg_base +
                         drvdata->ram_offset/4, DCC_LL_BASE(list));
                 dcc_writel(drvdata, drvdata->ram_start +
                         drvdata->ram_offset/4, DCC_FD_BASE(list));
                 dcc_writel(drvdata, 0xFFF, DCC_LL_TIMEOUT(list));

                 /* 4. Clears interrupt status register */
                 dcc_writel(drvdata, 0, DCC_LL_INT_ENABLE(list));
                 dcc_writel(drvdata, (BIT(0) | BIT(1) | BIT(2)),
                                         DCC_LL_INT_STATUS(list));

In case of any errors we again unlock the list before exiting.

>
>>>> +	if (lock_reg & 0x1) {
>>>> +		dev_err(drvdata->dev, "List %d is already locked\n",
>>>> +				curr_list);
>>>> +		return -EINVAL;
>>>> +	}
>>>> +
>>>> +	dev_err(drvdata->dev, "DCC list passed %d\n", curr_list);
>>> This is noise, please drop it.
>> Ack
>>>> +	return 0;
>>>> +}
>>>> +
>>>> +static bool is_dcc_enabled(struct dcc_drvdata *drvdata)
>>>> +{
>>>> +	bool dcc_enable = false;
>>>> +	int list;
>>>> +
>>>> +	for (list = 0; list < DCC_MAX_LINK_LIST; list++) {
>>>> +		if (drvdata->enable[list]) {
>>> 			return true;
>>>
>>>> +			dcc_enable = true;
>>>> +			break;
>>>> +		}
>>>> +	}
>>>> +
>>> 	return false;
>>>
>>>> +	return dcc_enable;
>>>> +}
>>>> +
>>>> +static int dcc_enable(struct dcc_drvdata *drvdata)
>>>> +{
>>>> +	int ret = 0;
>>>> +	int list;
>>>> +	u32 ram_cfg_base;
>>>> +
>>>> +	mutex_lock(&drvdata->mutex);
>>>> +
>>>> +	if (!is_dcc_enabled(drvdata)) {
>>>> +		memset_io(drvdata->ram_base,
>>>> +			0xDE, drvdata->ram_size);
>>> No need to wrap this line, and please use lowercase hex digits.
>> Ack
>>>> +	}
>>>> +
>>>> +	for (list = 0; list < drvdata->nr_link_list; list++) {
>>>> +
>>> Please drop the empty line.
>> Ack
>>>> +		if (dcc_valid_list(drvdata, list))
>>>> +			continue;
>>>> +
>>>> +		/* 1. Take ownership of the list */
>>>> +		dcc_writel(drvdata, BIT(0), DCC_LL_LOCK(list));
>>> Can we have a define for BIT(0)? Is it really about ownership or just
>>> that we "enable" it?
>>>
>>> If ownership, who's the other contenders for the ownership?
>> This is done to mark that the user has already entered the curr_list and
>> therefore
>>
>> cannot enter it again.
>>
> What does "entered" mean here? Do you mean that the list of operations
> that the driver kept track of has been flushed out to SRAM and we're not
> not allowed to modify it?
>
> What does this actually mean? Why is this done in the hardware and not
> simply with a bool in the driver?
That is correct. As explained above this is as per dcc hardware 
configuration requirement.
>
>>>> +
>>>> +		/* 2. Program linked-list in the SRAM */
>>>> +		ram_cfg_base = drvdata->ram_cfg;
>>>> +		ret = __dcc_ll_cfg(drvdata, list);
>>>> +		if (ret) {
>>>> +			dcc_writel(drvdata, 0, DCC_LL_LOCK(list));
>>>> +			goto err;
>>>> +		}
>>>> +
>>>> +		/* 3. program DCC_RAM_CFG reg */
>>>> +		dcc_writel(drvdata, ram_cfg_base +
>>>> +			drvdata->ram_offset/4, DCC_LL_BASE(list));
>>>> +		dcc_writel(drvdata, drvdata->ram_start +
>>>> +			drvdata->ram_offset/4, DCC_FD_BASE(list));
>>>> +		dcc_writel(drvdata, 0xFFF, DCC_LL_TIMEOUT(list));
>>>> +
>>>> +		/* 4. Clears interrupt status register */
>>>> +		dcc_writel(drvdata, 0, DCC_LL_INT_ENABLE(list));
>>>> +		dcc_writel(drvdata, (BIT(0) | BIT(1) | BIT(2)),
>>>> +					DCC_LL_INT_STATUS(list));
>>>> +
>>>> +		drvdata->enable[list] = true;
>>>> +
>>>> +		/* 5. Configure trigger */
>>>> +		dcc_writel(drvdata, BIT(9), DCC_LL_CFG(list));
>>>> +	}
>>>> +
>>>> +err:
>>>> +	mutex_unlock(&drvdata->mutex);
>>>> +	return ret;
>>>> +}
>>>> +
>>>> +static void dcc_disable(struct dcc_drvdata *drvdata)
>>>> +{
>>>> +	int curr_list;
>>>> +
>>>> +	mutex_lock(&drvdata->mutex);
>>>> +
>>>> +	for (curr_list = 0; curr_list < drvdata->nr_link_list; curr_list++) {
>>>> +		if (!drvdata->enable[curr_list])
>>>> +			continue;
>>>> +		dcc_writel(drvdata, 0, DCC_LL_CFG(curr_list));
>>>> +		dcc_writel(drvdata, 0, DCC_LL_BASE(curr_list));
>>>> +		dcc_writel(drvdata, 0, DCC_FD_BASE(curr_list));
>>>> +		dcc_writel(drvdata, 0, DCC_LL_LOCK(curr_list));
>>>> +		drvdata->enable[curr_list] = false;
>>>> +	}
>>>> +	memset_io(drvdata->ram_base, 0, drvdata->ram_size);
>>> Is there any reason why DCC is filled with 0xde during initialization
>>> but 0 when disabled?
>> Yes this poison value is required to distinguish between bus hang on
>> register accesses
>>
>> and a zero value for the registers. In case of the former dcc returns zero
>> value on registers
>>
>> which causes ambiguity.
>>
> That explains why you poison ram_base with 0xde during dcc_enable(). But
> why do you overwrite it with 0s in dcc_disable()?
>
> You should be able to either just leave it as it will be poisoned again
> next time dcc_enable() is called, or if you want to ensure things are
> cleared you should be able to poison it with the same poison - to
> distinguish it from a bunch of read 0s?
Ack
>
> Regards,
> Bjorn

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