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

[Chunyan Zhang]

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?

> kernel. You need something that ftrace will call into to export its
> traces.
>
>>
>> Signed-off-by: Chunyan Zhang <zhang.chunyan@xxxxxxxxxx>
>> ---
>>  drivers/hwtracing/stm/Kconfig      |  9 +++++++
>>  drivers/hwtracing/stm/Makefile     |  2 ++
>>  drivers/hwtracing/stm/stm_ftrace.c | 54 ++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 65 insertions(+)
>>  create mode 100644 drivers/hwtracing/stm/stm_ftrace.c
>>
>> diff --git a/drivers/hwtracing/stm/Kconfig b/drivers/hwtracing/stm/Kconfig
>> index 847a39b..a36633a 100644
>> --- a/drivers/hwtracing/stm/Kconfig
>> +++ b/drivers/hwtracing/stm/Kconfig
>> @@ -39,4 +39,13 @@ config STM_SOURCE_HEARTBEAT
>>         If you want to send heartbeat messages over STM devices,
>>         say Y.
>>
>> +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 :)

>
>> +
>> +       If you want to send kernel Ftrace messages over STM devices,
>> +       say Y.
>> +
>>  endif
>> diff --git a/drivers/hwtracing/stm/Makefile b/drivers/hwtracing/stm/Makefile
>> index a9ce3d4..5eef041 100644
>> --- a/drivers/hwtracing/stm/Makefile
>> +++ b/drivers/hwtracing/stm/Makefile
>> @@ -9,3 +9,5 @@ obj-$(CONFIG_STM_SOURCE_HEARTBEAT)    += stm_heartbeat.o
>>
>>  stm_console-y                := console.o
>>  stm_heartbeat-y              := heartbeat.o
>> +
>> +obj-$(CONFIG_STM_FTRACE)     += stm_ftrace.o
>
> We don't have any other source files prefixed with "stm_" in
> drivers/hwtracing/stm, because the directory is already "stm".

Ok, I will remove 'stm_' prefix.

>
>> diff --git a/drivers/hwtracing/stm/stm_ftrace.c b/drivers/hwtracing/stm/stm_ftrace.c
>> new file mode 100644
>> index 0000000..0b4eb70
>> --- /dev/null
>> +++ b/drivers/hwtracing/stm/stm_ftrace.c
>> @@ -0,0 +1,54 @@
>> +/*
>> + * Simple kernel driver to link kernel Ftrace and an STM device
>> + * Copyright (c) 2016, Linaro Ltd.
>> + *
>> + * This program is free software; you can redistribute it and/or modify it
>> + * under the terms and conditions of the GNU General Public License,
>> + * version 2, as published by the Free Software Foundation.
>> + *
>> + * This program is distributed in the hope it will be useful, but WITHOUT
>> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
>> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
>> + * more details.
>> + */
>> +
>> +#include <linux/kernel.h>
>> +#include <linux/module.h>
>> +#include <linux/slab.h>
>> +#include <linux/console.h>
>> +#include <linux/stm.h>
>> +
>> +static struct stm_source_data stm_ftrace_data = {
>> +     .name           = "stm_ftrace",
>> +     .nr_chans       = 1,
>
> Unless you want to allocate one channel per cpu core, which might or
> might not be more efficient, but it would be nice to see at least a
> comment about it.
>
>> +};
>> +
>> +/**
>> + * 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().

>
>> +
>> +static int __init stm_ftrace_init(void)
>> +{
>> +     return stm_source_register_device(NULL, &stm_ftrace_data);
>> +}
>> +
>> +static void __exit stm_ftrace_exit(void)
>> +{
>> +     stm_source_unregister_device(&stm_ftrace_data);
>> +}
>
> 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?

Thanks,
Chunyan

>
> Regards,
> --
> Alex