Re: [PATCH v2] drm/i915: Avoid selecting unavailable BSD2 ring

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 23/02/16 14:03, Chris Wilson wrote:
> On Tue, Feb 23, 2016 at 01:31:17PM +0000, Tvrtko Ursulin wrote:
>>
>> On 23/02/16 13:06, Gabriel Feceoru wrote:
>>>
>>>
>>> On 23.02.2016 13:05, Tvrtko Ursulin wrote:
>>>>
>>>> Hi,
>>>>
>>>> On 23/02/16 10:52, Gabriel Feceoru wrote:
>>>>> Return error when I915_EXEC_BSD_RING2 flag is set but BSD2 ring
>>>>> is not available in the HW.
>>>>
>>>> What is the reasoning behind this? So far kernel was allowing userspace
>>>> to select these bits and execute on the first engine. With this patch it
>>>> would start failing potentially breaking userspace. Is it not too late
>>>> to make such change?
>>>
>>> I noticed some inconsistencies in igt with regards to bsd and bsd1.
>>> For instance, if bsd2 is not available, gem_sync@basic-bsd1 is skipped,
>>> but it's skipped because of the 2nd check gem_has_bsd2 (see
>>> gem_require_ring). Surprisingly gem_has_ring() didn't complain about bsd1.
>>>
>>> This fix will make gem_has_ring() return false.
>>>
>>> I'm not aware about legacy/compatibility issue - if that's the case,
>>> please disregard this.
>>
>> Hmmm.. Chris, what is the reasoning behind:
>>
>> commit eaa03678b00179da89f194113c0740c033857c1c
>> Author: Chris Wilson <chris@chris-wilson.co.uk>
>> Date:   Thu Jan 28 13:44:19 2016 +0000
>>
>>      lib: Hide BSD1/BSD2 rings on hardware without BSD2
>>
>>      The kernel happily lets us run on I915_EXEC_BSD2 even with such hardware
>>      existing. Sigh.
>>
>>      Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>
>> diff --git a/lib/ioctl_wrappers.c b/lib/ioctl_wrappers.c
>> index 9dfa9b2603ce..fa44080e5902 100644
>> --- a/lib/ioctl_wrappers.c
>> +++ b/lib/ioctl_wrappers.c
>> @@ -1341,6 +1341,12 @@ static int gem_has_ring(int fd, int ring)
>>   void gem_require_ring(int fd, int ring_id)
>>   {
>>          igt_require(gem_has_ring(fd, ring_id));
>> +
>> +       /* silly ABI, the kernel thinks everyone who has BSD also has BSD2 */
>> +       if ((ring_id & ~(3<<13)) == I915_EXEC_BSD) {
>> +               if (ring_id & (3 << 13))
>> +                       igt_require(gem_has_bsd2(fd));
>> +       }
>>   }
>>
>>   /* prime */
>>
>> ABI was (and still is) that specifying BSD1 or BSD2 explicitly is
>> silently ignored by the kernel when BSD2 is not preset, defaulting
>> to BSD1.
>
> Thereby pretending that BSD, BSD1, BSD2 exist.
>
>> This patch makes tests requesting BSD1 skip when there is no BSD2
>> which I think is wrong in any case.
>
> BSD 1/2 selection only makes sense when we have multiple BSD rings.
> Running tests on BSD default, BSD1 and BSD2 is pointless if they all
> are equivalent. Using the BSD ping-pong when we have BSD1 and BSD2 is
> questionable as the ping-pong nature is uncontrolled, but nevertheless
> the code path needs to be tested.
>
>> If we want to (and can) change the ABI it should only reject the
>> non-existent ring and not limit the selection mechanism to
>> hardware which has BSD2.
>
> I disagree, we have a ring selection mechanism. If the extension doesn't
> exist, trying to use it should be an error. The extension was not only
> an ABI mistake but undesired (it is now defunct).

Not sure that I understand what you meant here. Nothing as far as I can 
tell is now defunct. Neither the selection mechanism, or the existence 
of BSD2.

To be absolutely clear, you are, or you are not, in favour of Gabriel's 
patch to start failing execbuf with fine grained BSD selection flags 
when BSD2 is not present?

Regards,

Tvrtko
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