Re: [Intel-xe] [PATCH v7 1/2] drm/xe/buddy: add visible tracking

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 21 Mar 2023, Matthew Auld <matthew.auld@intel.com> wrote:
> On 21/03/2023 11:15, Jani Nikula wrote:
>> On Tue, 21 Mar 2023, Matthew Auld <matthew.auld@intel.com> wrote:
>>> +	XE_BUG_ON(min_page_size < mm->chunk_size);
>>> +	XE_BUG_ON(min_page_size > SZ_2G); /* FIXME: sg limit */
>>> +	XE_BUG_ON(size > SZ_2G &&
>>> +		  (vres->base.placement & TTM_PL_FLAG_CONTIGUOUS));
>>> +	XE_BUG_ON(!IS_ALIGNED(size, min_page_size));
>> 
>> Hijacking the thread, sorry. ;)
>> 
>> If we are supposed to not replicate i915-ism, why do we have
>> XE_BUG_ON()?
>> 
>> I think it's an anti-pattern.
>> 
>> Currently XE_BUG_ON() is just BUG_ON(), which should not be used for any
>> reason. Not even CI. The plan in i915 is to migrate all of them to
>> WARN_ON()'s and use panic_on_warn in CI.
>> 
>> The other thing is that having BUG_ON() in the name gives you the false
>> impression that it's sufficient for input (or any precondition)
>> validation. In i915 it gets conditionally built for CI only. So we catch
>> errors in CI but not in actual production use. You just plunge on.
>> 
>> Part of it is just naming. The BUG_ON() association is pretty strong for
>> any kernel developers. If it were called XE_ASSERT() it would have an
>> association with assert() that does what XE_BUG_ON() actually is: code
>> is only generated for debug builds.
>> 
>> In i915 it has become habitual to sprinkle GEM_BUG_ON() all over the
>> place, just in case, belt and suspenders. IMO there's not enough
>> discretion about what can and can't actually happen, and what can and
>> can't happen in CI vs. production.
>
> Fair enough. The above probably should have just been WARN_ON(), along 
> with returning a proper error. Will fix, and keep in mind for the future.

I'll emphasize that this was not about your patch in particular (see the
"hijack" part) but a more general observation about Xe adding
XE_BUG_ON().

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center