On 12/16/2021 9:18 PM, Thara Gopinath wrote:
On 8/10/21 1:54 PM, Souradeep Chowdhury wrote:
DCC(Data Capture and Compare) is a DMA engine designed for debugging
purposes.In case of a system
crash or manual software triggers by the user the DCC hardware stores
the value at the register
addresses which can be used for debugging purposes.The DCC driver
provides the user with sysfs
interface to configure the register addresses.The options that the
DCC hardware provides include
reading from registers,writing to registers,first reading and then
writing to registers and looping
through the values of the same register.
In certain cases a register write needs to be executed for accessing
the rest of the registers,
also the user might want to record the changing values of a register
with time for which he has the
option to use the loop feature.
Hello Souradeep,
First of all, I think this is very a useful feature to have. I have
some generic design related queries/comments on driver and the
interface exposed to the user space. Also, I do not understand the h/w
well here, so feel free to correct me if I am wrong.
1. Linked list looks like a very internal feature to the h/w. It
really is not an info that user should be aware of. I tried reading
the code a bit. IUC, every time a s/w trigger is issued the configs in
all the enabled linked lists are executed. The final ram dump that you
get from /dev/dcc_sram is a dump of contents from all the enabled
list? Is this understanding correct ? And we are talking of at-most 4
linked list?
If yes, I think it might be better to have a folder per linked list
with config, config_write etc. Also if possible it will be better to
dump the results to a file in the specific folder instead of reading
from /dev/dcc_sram.
If no, there is no real need for user to know the linked list, right?
Choosing of linked list can be done by kernel driver in this case with
no input needed from user.
2. Now to the sysfs interface itself, I know lot of thought has gone
into sysfs vs debugfs considerations. But, have you considered using
netlink interface instead of sysfs. Netlink interface is used for
asynchronous communication between kernel and user space. In case of
DCC, the communication appears to be asynchronous, where in user asks
the kernel to capture some info and kernel can indicate back to user
when the info is captured. Also the entire mess surrounding echoing
addr / value / offset repeatedly into a sysfs entry can be avoided
using netlink interface.
Hello Thara,
Thanks for your review comments. Following are some points from my end
1) Each linked list represent a particular block of memory in DCC_SRAM
which is preserved for that particular list. That is why offset
calculation is done on the driver based on the linked list chosen by the
user.
This choice needs to be made by the user since the number for the
linked list chosen is specific to the registers used to debug a
particular component. Also we are giving the user flexibility to
configure multiple
linked lists at one go so that even if we don't have a separate
folder for it , the dumps are collected as a separate list of registers.
Also there are certain curr_list values which may be supported by the dcc
hardware but may not be accessible to the user and so the choice
cannot be made arbitrarily from the driver.
2) From opensource, I can see that Netlink has been used in most of the
cases where we need to notify stats to the user by taking the advantage
of asynchronous communication. In this case, that requirement is not
there since it is mostly one way communication from user to kernel.
Also since this is used for debugging purposes perhaps sysfs adds more
reliability than Netlink. In case of Netlink we have the additional
overhead of dealing with socket calls. Let me know otherwise.
Thanks,
Souradeep
