Re: [RFC PATCH 1/4] STM Ftrace: Adding generic buffer interface driver

[Alexander Shishkin]

Chunyan Zhang <zhang.chunyan@xxxxxxxxxx> writes:

> On Tue, Jun 7, 2016 at 6:25 PM, Alexander Shishkin
> <alexander.shishkin@xxxxxxxxxxxxxxx> wrote:
>> Chunyan Zhang <zhang.chunyan@xxxxxxxxxx> writes:
>>
>>> This patch adds a driver that models itself as an stm_source and
>>> who's sole purpose is to export an interface to the rest of the
>>> kernel.  Once the stm and stm_source have been linked via sysfs,
>>> everything that is passed to the interface will endup in the STM
>>> trace engine.
>>
>> STM core already provides this exact interface to the rest of the
>
> Can you point out 'this exact interface' to me?

Well, you're saying that this stm_source exports an interface to send
data to STM for the rest of the kernel. Whereas, stm_source already is
that interface.

>>> +config STM_FTRACE
>>> +     tristate "Redirect/copy the output from kernel Ftrace to STM engine"
>>> +     help
>>> +       This option can be used to redirect or copy the output from kernel Ftrace
>>> +       to STM engine. Enabling this option will introduce a slight timing effect.
>>
>> This creates an impression that STM_FTRACE will somehow make events
>> bypass the normal ftrace ring buffer.
>
> Ok, this name can be adjusted, do you have a better one for me :)

What I mean is: from the description it sounds like there is an option
to bypass ftrace ring buffer, but I don't think that's the case at the
moment. I'm also not sure if it's practical at all to do.

>>> +/**
>>> + * stm_ftrace_write() - write data to STM via 'stm_ftrace' source
>>> + * @buf:     buffer containing the data packet
>>> + * @len:     length of the data packet
>>> + * @chan:    offset above the start channel number allocated to 'stm_ftrace'
>>> + */
>>> +void notrace stm_ftrace_write(const char *buf, unsigned int len,
>>> +                           unsigned int chan)
>>> +{
>>> +     stm_source_write(&stm_ftrace_data, chan, buf, len);
>>> +}
>>> +EXPORT_SYMBOL_GPL(stm_ftrace_write);
>>
>> An extra wrapper around stm_source_write().
>
> Yes, I think it's not good to expose the stm_source to ftrace_stm_func().

I understand, but wrapping it into an intermediary function doesn't
really solve it either.

>> So basically when ftrace is compiled in, it will pull in stm core
>> through this.
>
> Sorry I cannot get you here.  Could you please explain you concern further?

Well, if you plug the stm_source driver into the ftrace core (via a
wrapper or directly), you will end up with a link dependency. In other
words, stm_source and by association stm_core will have to be statically
linked.

Look at the way stm_console is done, for example: it registers with both
stm_source class and the console layer dynamically, so that it can be
dynamically loaded/unloaded.

Regards,
--
Alex