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Subject: Re: [RFC PATCH] dynamic_debug: Add config option of
 DYNAMIC_DEBUG_CORE
To:     Andrew Morton <akpm@linux-foundation.org>,
        Changbin Du <changbin.du@intel.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Brendan Higgins <brendanhiggins@google.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Ard Biesheuvel <ardb@kernel.org>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        David Gow <davidgow@google.com>,
        Mark Rutland <mark.rutland@arm.com>, orsonzhai@gmail.com,
        linux-kernel@vger.kernel.org, kernel-team@android.com
References: <1584558186-23373-1-git-send-email-orson.unisoc@gmail.com>
 <51568376-da8b-3265-ddb3-6ddba74207dc@akamai.com>
 <20200319152820.GA2793@lenovo>
From:   Jason Baron <jbaron@akamai.com>
Message-ID: <d8474138-6f8f-8a99-351d-5e5b37999373@akamai.com>
Date:   Thu, 19 Mar 2020 16:19:49 -0400
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On 3/19/20 11:28 AM, Orson Zhai wrote:
> Hi Jason,
> 
> On Wed, Mar 18, 2020 at 05:18:43PM -0400, Jason Baron wrote:
>>
>>
>> On 3/18/20 3:03 PM, Orson Zhai wrote:
>>> There is the requirement from new Android that kernel image (GKI) and
>>> kernel modules are supposed to be built at differnet places. Some people
>>> want to enable dynamic debug for kernel modules only but not for kernel
>>> image itself with the consideration of binary size increased or more
>>> memory being used.
>>>
>>> By this patch, dynamic debug is divided into core part (the defination of
>>> functions) and macro replacement part. We can only have the core part to
>>> be built-in and do not have to activate the debug output from kenrel image.
>>>
>>> Signed-off-by: Orson Zhai <orson.unisoc@gmail.com>
>>
>> Hi Orson,
>>
>> I think this is a nice feature. Is the idea then that driver can do
>> something like:
>>
>> #if defined(CONFIG_DRIVER_FOO_DEBUG)
>> #define driver_foo_debug(fmt, ...) \
>>         dynamic_pr_debug(fmt, ##__VA_ARGS__)
>> #else
>> 	no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
>> #enif
>>
>> And then the Kconfig:
>>
>> config DYNAMIC_DRIVER_FOO_DEBUG
>> 	bool "Enable dynamic driver foo printk() support"
>> 	select DYNAMIC_DEBUG_CORE
>>
> I highly appreciate you for giving this good example to us.
> To be honest I did not really think of this kind of usage. :)
> But it makes much sense. I think dynamic debug might be a little
> bit high for requirement of memory. Every line of pr_debug will be
> added with a static data structure and malloc with an item in link table.
> It might be sensitive especially in embeded system.
> So this example shows how to avoid to turn on dynamci debug for whole
> system but part of it when being needed.
> 
>>
>> Or did you have something else in mind? Do you have an example
>> code for the drivers that you mention?
> 
> My motivation comes from new Andorid GKI release flow. Android kernel team will
> be in charge of GKI release. And SoC vendors will build their device driver as
> kernel modules which are diffrent from each vendor. End-users will get their phones
> installed with GKI plus some modules all together.
> 
> So at Google side, they can only set DYNAMIC_DEBUG_CORE in their defconfig to build
> out GKI without worrying about the kernel image size increased too much. Actually
> GKI is relatively stable as a common binary and there is no strong reason to do 
> dynamic debugging to it.
> 
> And at vendor side, they will use a local defconfig which is same with Google one but add 
> CONFIG_DYNAMIC_DEBUG to build their kenrel modules. As DYNAMIC_DEBUG enables only a
> set of macro expansion, so it has no impact to kernel ABI or the modversion.
> All modules will be compatible with GKI and with dynamic debug enabled.
> 
> Then the result will be that Google has his clean GKI and vendors have their dynamic-debug-powered modules.
>


static int __init dynamic_debug_init(void)
{
        struct _ddebug *iter, *iter_start;
        const char *modname = NULL;
        char *cmdline;
        int ret = 0;
        int n = 0, entries = 0, modct = 0;
        int verbose_bytes = 0;

        if (__start___verbose == __stop___verbose) {
                pr_warn("_ddebug table is empty in a CONFIG_DYNAMIC_DEBUG build\n");
                return 1;
        }

...

I wonder if we should just remove it now.

Thanks,

-Jason