[v2] drm/i915: Avoid selecting unavailable BSD2 ring
diff mbox

Message ID 56CC5F25.6060303@linux.intel.com
State New
Headers show

Commit Message

Tvrtko Ursulin Feb. 23, 2016, 1:31 p.m. UTC
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>


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.

This patch makes tests requesting BSD1 skip when there is no BSD2
which I think is wrong in any case.

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.

Regards,

Tvrtko

Comments

Chris Wilson Feb. 23, 2016, 2:03 p.m. UTC | #1
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).
-Chris
Tvrtko Ursulin Feb. 23, 2016, 2:39 p.m. UTC | #2
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
Dave Gordon Feb. 23, 2016, 7:36 p.m. UTC | #3
On 23/02/16 14:39, Tvrtko Ursulin wrote:
>
> 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

Currently:

#define I915_EXEC_BSD         (2<<0)

/** Used for switching BSD rings on the platforms with two BSD rings */
#define I915_EXEC_BSD_SHIFT   (13)
#define I915_EXEC_BSD_MASK    (3 << I915_EXEC_BSD_SHIFT) /* default 
ping-pong mode */
#define I915_EXEC_BSD_DEFAULT (0 << I915_EXEC_BSD_SHIFT)
#define I915_EXEC_BSD_RING1   (1 << I915_EXEC_BSD_SHIFT)
#define I915_EXEC_BSD_RING2   (2 << I915_EXEC_BSD_SHIFT)

It makes sense to have the original "BSD" flag mean "the default BSD", 
and use different flags to mean specifically "BSD1" or "BSD2". Which 
appears to be what we've done; naive clients that don't set any of the 
new BSD bits will get default behaviour (execute on *any* BSD ring) with 
no control over the ping-pong mechanism (if any). Clients that specify a 
specific ring should get that one, and only that one; if it doesn't 
exist then it's an error.

Any program that's going to set these bits should first ask whether (or 
which) engines exist and select appropriately. So I think I'm with Chris 
here.

On the other hand, I think what Tvrtko said was "it should not be an 
error to select a specific ring [that exists] just because there are no 
other rings". Which I also agree with.

So the ring-select-checking code should:
1. reject the I915_EXEC_BSD_RING2 case if BSD2 does not exist
2. reject the I915_EXEC_BSD_RING1 case if BSD1 does not exist
      (for some future bizarre numbering scheme? or a
       partitioning system that reserves BSD1 for someone else?)
3. never reject the I915_EXEC_BSD_DEFAULT case
      (although it will have rejected the I915_EXEC_BSD flag
       before looking at these if there is no BSD ring at all)
4. for now (until we assign it a meaning) reject the case where
      BOTH BSD ring-select bits are set.

Therefore I disagree with Gabriel's patch which would reject trying to 
select BSD1 on a platform that only has the one BSD engine.

.Dave.
Feceoru, Gabriel Feb. 24, 2016, 3:27 p.m. UTC | #4
On 23.02.2016 21:36, Dave Gordon wrote:
> On 23/02/16 14:39, Tvrtko Ursulin wrote:
>>
>> 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
>
> Currently:
>
> #define I915_EXEC_BSD         (2<<0)
>
> /** Used for switching BSD rings on the platforms with two BSD rings */
> #define I915_EXEC_BSD_SHIFT   (13)
> #define I915_EXEC_BSD_MASK    (3 << I915_EXEC_BSD_SHIFT) /* default
> ping-pong mode */
> #define I915_EXEC_BSD_DEFAULT (0 << I915_EXEC_BSD_SHIFT)
> #define I915_EXEC_BSD_RING1   (1 << I915_EXEC_BSD_SHIFT)
> #define I915_EXEC_BSD_RING2   (2 << I915_EXEC_BSD_SHIFT)
>
> It makes sense to have the original "BSD" flag mean "the default BSD",
> and use different flags to mean specifically "BSD1" or "BSD2". Which
> appears to be what we've done; naive clients that don't set any of the
> new BSD bits will get default behaviour (execute on *any* BSD ring) with
> no control over the ping-pong mechanism (if any). Clients that specify a
> specific ring should get that one, and only that one; if it doesn't
> exist then it's an error.
>
> Any program that's going to set these bits should first ask whether (or
> which) engines exist and select appropriately. So I think I'm with Chris
> here.
>
> On the other hand, I think what Tvrtko said was "it should not be an
> error to select a specific ring [that exists] just because there are no
> other rings". Which I also agree with.
>
> So the ring-select-checking code should:
> 1. reject the I915_EXEC_BSD_RING2 case if BSD2 does not exist

This is one of the things the patch does, I guess everybody agrees on 
rejecting BSD2.

> 2. reject the I915_EXEC_BSD_RING1 case if BSD1 does not exist
>       (for some future bizarre numbering scheme? or a
>        partitioning system that reserves BSD1 for someone else?)
> 3. never reject the I915_EXEC_BSD_DEFAULT case
>       (although it will have rejected the I915_EXEC_BSD flag
>        before looking at these if there is no BSD ring at all)
> 4. for now (until we assign it a meaning) reject the case where
>       BOTH BSD ring-select bits are set.
>
> Therefore I disagree with Gabriel's patch which would reject trying to
> select BSD1 on a platform that only has the one BSD engine.


Depending on the BSD1 debate resolution I see two possible solutions:

1. Change the patch to reject I915_EXEC_BSD_RING2 only and change 
gem_require_ring to allow BSD1.

2. Keep the patch, eventually remove that check in gem_require_ring 
(which will become redundant) in igt

Thank you all for your input on this.
Gabriel.

>
> .Dave.

Patch
diff mbox

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 */