[Intel-gfx] [PATCH i-g-t] tests/gem_flink_basic: Add documentation for subtests

Wed Aug 30 17:49:20 UTC 2017

On 8/30/2017 4:12 AM, Michał Winiarski wrote:
> On Tue, Aug 29, 2017 at 02:25:19PM -0700, Vinay Belgaumkar wrote:
>> Added the missing IGT_TEST_DESCRIPTION and some subtest
>> descriptions.
>>
>> Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar at intel.com>
>> ---
>>   tests/gem_flink_basic.c | 36 ++++++++++++++++++++++++++++++++++++
>>   1 file changed, 36 insertions(+)
>>
>> diff --git a/tests/gem_flink_basic.c b/tests/gem_flink_basic.c
>> index 26ae7d6..8761e0d 100644
>> --- a/tests/gem_flink_basic.c
>> +++ b/tests/gem_flink_basic.c
>> @@ -36,6 +36,8 @@
>>   #include <sys/ioctl.h>
>>   #include "drm.h"
>>   
>> +IGT_TEST_DESCRIPTION("Tests for flink - a way to export a gem object by name");
>> +
>>   static void
>>   test_flink(int fd)
>>   {
>> @@ -155,14 +157,48 @@ igt_main
>>   	igt_fixture
>>   		fd = drm_open_driver(DRIVER_INTEL);
>>   
>> +	/* basic:
>> +	This subtest creates a gem object, and then creates
>> +	a flink. It tests that we can gain access to the gem
>> +	object using the flink name.
>> +
>> +	Test fails if flink creation/open fails.
>> +	**/
> Please use kernel coding style.
> This is not the format we're using for multiline comments.
>
> /*
>   *
>   */
> ^^^ This is the format we're using.

Agreed. Will change it to match that style. The multi-line comments in 
/lib directory actually use this-
/**
  * <name of function>
  */

>
> And on the documentation itself, let's take a quote from the kernel coding
> style:
> "Comments are good, but there is also a danger of over-commenting.  NEVER
> try to explain HOW your code works in a comment: it's much better to
> write the code so that the **working** is obvious, and it's a waste of
> time to explain badly written code."
>
> Now, let's try to match the tests with the comments:
> 	/* This subtest creates a gem object */
> 	ret = ioctl(fd, DRM_IOCTL_I915_GEM_CREATE, &create);
> 	igt_assert_eq(ret, 0);
>
> 	/* and then creates a flink */
> 	flink.handle = create.handle;
> 	ret = ioctl(fd, DRM_IOCTL_GEM_FLINK, &flink);
> 	igt_assert_eq(ret, 0);
>
> 	/* It tests that we can gain access to the gem object using the flink
> 	 * name
> 	 */
> Well... not really, we're not accessing the object in any way.

Yes, but we are trying to open the flink in this line of the test-
         open_struct.name = flink.name;
         ret = ioctl(fd, DRM_IOCTL_GEM_OPEN, &open_struct);
         igt_assert_eq(ret, 0);
         igt_assert(open_struct.handle != 0);

I will change it to "open the flink" instead of "access the gem object".

>
> 	/* Test fails if flink creation/open fails. */
> 	open_struct.name = flink.name;
> 	ret = ioctl(fd, DRM_IOCTL_GEM_OPEN, &open_struct);
> 	igt_assert_eq(ret, 0);
> 	igt_assert(open_struct.handle != 0);
>
>>   	igt_subtest("basic")
>>   		test_flink(fd);
>> +
>> +	/* double-flink:
>> +	This test checks if it is possible to create 2 flinks
>> +	for the same gem object.
>> +
>> +	Test fails if 2 flink objects cannot be created.
>> +	**/
> 	/* This test checks if it is possible to create 2 flinks for the same
> 	 * gem object
> 	 */
> 	
> 	flink.handle = create.handle;
> 	ret = ioctl(fd, DRM_IOCTL_GEM_FLINK, &flink);
> 	igt_assert_eq(ret, 0);
>
> 	flink2.handle = create.handle;
> 	ret = ioctl(fd, DRM_IOCTL_GEM_FLINK, &flink2);
> 	igt_assert_eq(ret, 0);
>
> 	/* Test fails if 2 flink objects cannot be created. */
> Well - this is handled by the asserts above.
> You ignored this assumption in your description for some reason though:
> 	igt_assert(flink2.name == flink.name);

Agreed. Also need to add that comment saying the name remains the same 
across the two
applications opening the same gem object.

>
>>   	igt_subtest("double-flink")
>>   		test_double_flink(fd);
>> +
>> +	/* bad-flink:
>> +	Use an invalid flink handle.
>> +
>> +	DRM_IOCTL_GEM_FLINK ioctl call should return failure.
>> +	**/
> 	ret = ioctl(fd, DRM_IOCTL_GEM_FLINK, &flink);
> 	igt_assert(ret == -1 && errno == ENOENT);
>
> There is also an igt_info message:
> 	igt_info("Testing error return on bad flink ioctl.\n");

True, there is some duplication in the comments at this point.

The documentation that I am adding before the subtest call will be 
rolled up by gtkdoc/Sphinx/doxygen, it likely
will not look at the text documentation in the actual code. When we look 
at the rolled up documentation, it
is good to have an idea of when a particular test will pass/fail without 
having to dig into code.

So, yes, there will be some duplication for existing tests. But if we 
start following this method for new tests,
we can have one place to describe what the test does/when does it fail, 
and then expand on anything that is
not very clear in the code itself.

>
>
>>   	igt_subtest("bad-flink")
>>   		test_bad_flink(fd);
>> +
>> +	/* bad-open:
>> +	Try to use an invalid flink name.
>> +
>> +	DRM_IOCTL_GEM_FLINK ioctl call should return failure.
>> +	**/
> 	open_struct.name = 0x10101010;
> 	ret = ioctl(fd, DRM_IOCTL_GEM_OPEN, &open_struct);
>
> 	igt_assert(ret == -1 && errno == ENOENT);
>
> Same as for bad flink:
> 	igt_info("Testing error return on bad open ioctl.\n");
>
>>   	igt_subtest("bad-open")
>>   		test_bad_open(fd);
>> +
>> +	/* flink-lifetime:
>> +	Check if a flink name can be used even after the drm
>> +	fd used to create it is closed.
>> +
>> +	Flink name should remain valid until the gem object
>> +	it points to has not been freed.
>> +	**/
> That's better, however...
> Why wasn't the object freed when we closed the drm fd (fd2) used to create it?
> (hint, it wasn't freed because we're doing OPEN using a different fd before
> closing fd2, and that changes the lifetime of an object since we're bumping the
> refcount this way, which perhaps could use a comment, not in the description
> but in the testcase itself).
> As for a one-line description, perhaps something more general would work better?
> Check if a flink name is valid for the whole duration of underlying gem object
> lifetime.

Agreed. In this case, it makes more sense to have this clarification in 
the actual test code itself.

>
> Overall - do you believe, that 1:1 from C to English translation is not a
> perfect example of "over-commenting"? Do we really need to take an approach
> where we're documenting even the simple ABI checks (e.g. invalid usage - error)?
> What value does such documentation have and for whom? I would expect developers
> to be able to consume C, are we trying to explain things for non-developers?
>
> -Michał

I am not suggesting we do a C to English translation. The point of this 
patch is to encourage
documenting of subtests so that anyone who is starting out with 
kernel/test development
can get a better idea of what the test is doing and why, before digging 
into the actual code.

Also, do we expect just kernel developers to use these tests? What about 
QA teams who
are usually focused on test execution? This might help them understand a 
little
better what they are using this test for, and give more information to 
debug the test
as well.

It's not possible even for kernel developers to be familiar with every 
feature of the driver.
Having some descriptive tests/comments can make it easier to ramp up to 
a new feature
and reduce debug time in some cases.

We are using gtkdoc today, the generated documentation that we share 
with every release is here -
https://01.org/linuxgraphics/gfx-docs/igt/

If you drill down to the subtest level for any of the tests, all we have 
is the name of the subtest
and a generic one line description of the entire binary. Not sure what 
value that really adds unless
we start describing the purpose of each subtest. There is "some" inline 
documentation in the test
code today, but not nearly enough to understand the purpose of writing 
that test. This was
clearly evidenced by the fact that I missed commenting about the second 
fd being used before
closing the first one. And this is one of the more straightforward tests 
in the IGT suite.

The part about linking this subtest documentation to the tool chain is 
being worked on by Petri,
but this effort is aimed at making our tests a little more easier to 
understand. I agree that we
should focus on the WHAT and WHY for every subtest and explain the HOW 
when needed.

I also agree that over documenting everything is not worthwhile, but 
majority of the IGT tests
today need a little more explanation of their purpose.

>
>>   	igt_subtest("flink-lifetime")
>>   		test_flink_lifetime(fd);
>>   }
>

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