Re: [Freedreno] [PATCH 03/23] drm/i915: Don't use struct drm_driver.get_scanout_position()

2020-01-10 Thread Ville Syrjälä
On Fri, Jan 10, 2020 at 03:56:06PM +0200, Jani Nikula wrote:
> On Fri, 10 Jan 2020, Thomas Zimmermann  wrote:
> > Hi
> >
> > Am 10.01.20 um 12:59 schrieb Jani Nikula:
> >> On Fri, 10 Jan 2020, Thomas Zimmermann  wrote:
> >>> The callback struct drm_driver.get_scanout_position() is deprecated in
> >>> favor of struct drm_crtc_helper_funcs.get_scanout_position().
> >>>
> >>> i915 doesn't use CRTC helpers. The patch duplicates the caller
> >>> drm_calc_vbltimestamp_from_scanoutpos() for i915, such that the callback
> >>> function is not needed.
> >>>
> >>> Signed-off-by: Thomas Zimmermann 
> >>> ---
> >>>  drivers/gpu/drm/i915/i915_drv.c |   3 +-
> >>>  drivers/gpu/drm/i915/i915_irq.c | 117 ++--
> >>>  drivers/gpu/drm/i915/i915_irq.h |   9 +--
> >>>  3 files changed, 119 insertions(+), 10 deletions(-)
> >> 
> >> Not really enthusiastic about the diffstat in a "cleanup" series.
> >
> > Well, the cleanup is about the content of drm_driver :)
> >
> >> 
> >> I wonder if you could add a generic helper version of
> >> drm_calc_vbltimestamp_from_scanoutpos where you pass the
> >> get_scanout_position function as a parameter. Both
> >> drm_calc_vbltimestamp_from_scanoutpos and the new
> >> i915_calc_vbltimestamp_from_scanoutpos would then be fairly thin
> >> wrappers passing in the relevant get_scanout_position function.
> >
> > Of course. Will be in v2 of the series.
> 
> Please give Ville (Cc'd) a moment before sending v2 in case he wants to
> chime in on this.

Passing the function pointer was one option I considered for this a while
back. Can't remeber what other solutions I condsidered. But I guess I
didn't like any of them enough to make an actual patch.

-- 
Ville Syrjälä
Intel
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Re: [Freedreno] [PATCH 03/23] drm/i915: Don't use struct drm_driver.get_scanout_position()

2020-01-10 Thread Jani Nikula
On Fri, 10 Jan 2020, Thomas Zimmermann  wrote:
> Hi
>
> Am 10.01.20 um 12:59 schrieb Jani Nikula:
>> On Fri, 10 Jan 2020, Thomas Zimmermann  wrote:
>>> The callback struct drm_driver.get_scanout_position() is deprecated in
>>> favor of struct drm_crtc_helper_funcs.get_scanout_position().
>>>
>>> i915 doesn't use CRTC helpers. The patch duplicates the caller
>>> drm_calc_vbltimestamp_from_scanoutpos() for i915, such that the callback
>>> function is not needed.
>>>
>>> Signed-off-by: Thomas Zimmermann 
>>> ---
>>>  drivers/gpu/drm/i915/i915_drv.c |   3 +-
>>>  drivers/gpu/drm/i915/i915_irq.c | 117 ++--
>>>  drivers/gpu/drm/i915/i915_irq.h |   9 +--
>>>  3 files changed, 119 insertions(+), 10 deletions(-)
>> 
>> Not really enthusiastic about the diffstat in a "cleanup" series.
>
> Well, the cleanup is about the content of drm_driver :)
>
>> 
>> I wonder if you could add a generic helper version of
>> drm_calc_vbltimestamp_from_scanoutpos where you pass the
>> get_scanout_position function as a parameter. Both
>> drm_calc_vbltimestamp_from_scanoutpos and the new
>> i915_calc_vbltimestamp_from_scanoutpos would then be fairly thin
>> wrappers passing in the relevant get_scanout_position function.
>
> Of course. Will be in v2 of the series.

Please give Ville (Cc'd) a moment before sending v2 in case he wants to
chime in on this.

Thanks,
Jani.


>
> Best regards
> Thomas
>
>> 
>> This would reduce the almost identical duplication of the function in
>> i915.
>> 
>> BR,
>> Jani.
>> 
>>>
>>> diff --git a/drivers/gpu/drm/i915/i915_drv.c 
>>> b/drivers/gpu/drm/i915/i915_drv.c
>>> index f7385abdd74b..4a0a7fb85c53 100644
>>> --- a/drivers/gpu/drm/i915/i915_drv.c
>>> +++ b/drivers/gpu/drm/i915/i915_drv.c
>>> @@ -2769,8 +2769,7 @@ static struct drm_driver driver = {
>>> .gem_prime_export = i915_gem_prime_export,
>>> .gem_prime_import = i915_gem_prime_import,
>>>  
>>> -   .get_vblank_timestamp = drm_calc_vbltimestamp_from_scanoutpos,
>>> -   .get_scanout_position = i915_get_crtc_scanoutpos,
>>> +   .get_vblank_timestamp = i915_calc_vbltimestamp_from_scanoutpos,
>>>  
>>> .dumb_create = i915_gem_dumb_create,
>>> .dumb_map_offset = i915_gem_dumb_mmap_offset,
>>> diff --git a/drivers/gpu/drm/i915/i915_irq.c 
>>> b/drivers/gpu/drm/i915/i915_irq.c
>>> index afc6aad9bf8c..99d0c3b0feae 100644
>>> --- a/drivers/gpu/drm/i915/i915_irq.c
>>> +++ b/drivers/gpu/drm/i915/i915_irq.c
>>> @@ -52,6 +52,11 @@
>>>  #include "i915_trace.h"
>>>  #include "intel_pm.h"
>>>  
>>> +/* Retry timestamp calculation up to 3 times to satisfy
>>> + * drm_timestamp_precision before giving up.
>>> + */
>>> +#define I915_TIMESTAMP_MAXRETRIES 3
>>> +
>>>  /**
>>>   * DOC: interrupt handling
>>>   *
>>> @@ -762,10 +767,11 @@ static int __intel_get_crtc_scanline(struct 
>>> intel_crtc *crtc)
>>> return (position + crtc->scanline_offset) % vtotal;
>>>  }
>>>  
>>> -bool i915_get_crtc_scanoutpos(struct drm_device *dev, unsigned int index,
>>> - bool in_vblank_irq, int *vpos, int *hpos,
>>> - ktime_t *stime, ktime_t *etime,
>>> - const struct drm_display_mode *mode)
>>> +static bool i915_get_crtc_scanoutpos(struct drm_device *dev,
>>> +unsigned int index, bool in_vblank_irq,
>>> +int *vpos, int *hpos,
>>> +ktime_t *stime, ktime_t *etime,
>>> +const struct drm_display_mode *mode)
>>>  {
>>> struct drm_i915_private *dev_priv = to_i915(dev);
>>> struct intel_crtc *crtc = to_intel_crtc(drm_crtc_from_index(dev, 
>>> index));
>>> @@ -879,6 +885,109 @@ bool i915_get_crtc_scanoutpos(struct drm_device *dev, 
>>> unsigned int index,
>>> return true;
>>>  }
>>>  
>>> +bool i915_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
>>> +   unsigned int pipe,
>>> +   int *max_error,
>>> +   ktime_t *vblank_time,
>>> +   bool in_vblank_irq)
>>> +{
>>> +   struct timespec64 ts_etime, ts_vblank_time;
>>> +   ktime_t stime, etime;
>>> +   bool vbl_status;
>>> +   struct drm_crtc *crtc;
>>> +   const struct drm_display_mode *mode;
>>> +   struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
>>> +   int vpos, hpos, i;
>>> +   int delta_ns, duration_ns;
>>> +
>>> +   crtc = drm_crtc_from_index(dev, pipe);
>>> +
>>> +   if (pipe >= dev->num_crtcs || !crtc) {
>>> +   DRM_ERROR("Invalid crtc %u\n", pipe);
>>> +   return false;
>>> +   }
>>> +
>>> +   if (drm_drv_uses_atomic_modeset(dev))
>>> +   mode = &vblank->hwmode;
>>> +   else
>>> +   mode = &crtc->hwmode;
>>> +
>>> +   /* If mode timing undefined, just return as no-op:
>>> +* Happens during initial modesetting of a crtc.
>>> +*/
>>> +   if (mode->crtc_clock == 0) {
>>> +   DRM_DE

Re: [Freedreno] [PATCH 03/23] drm/i915: Don't use struct drm_driver.get_scanout_position()

2020-01-10 Thread Thomas Zimmermann
Hi

Am 10.01.20 um 12:59 schrieb Jani Nikula:
> On Fri, 10 Jan 2020, Thomas Zimmermann  wrote:
>> The callback struct drm_driver.get_scanout_position() is deprecated in
>> favor of struct drm_crtc_helper_funcs.get_scanout_position().
>>
>> i915 doesn't use CRTC helpers. The patch duplicates the caller
>> drm_calc_vbltimestamp_from_scanoutpos() for i915, such that the callback
>> function is not needed.
>>
>> Signed-off-by: Thomas Zimmermann 
>> ---
>>  drivers/gpu/drm/i915/i915_drv.c |   3 +-
>>  drivers/gpu/drm/i915/i915_irq.c | 117 ++--
>>  drivers/gpu/drm/i915/i915_irq.h |   9 +--
>>  3 files changed, 119 insertions(+), 10 deletions(-)
> 
> Not really enthusiastic about the diffstat in a "cleanup" series.

Well, the cleanup is about the content of drm_driver :)

> 
> I wonder if you could add a generic helper version of
> drm_calc_vbltimestamp_from_scanoutpos where you pass the
> get_scanout_position function as a parameter. Both
> drm_calc_vbltimestamp_from_scanoutpos and the new
> i915_calc_vbltimestamp_from_scanoutpos would then be fairly thin
> wrappers passing in the relevant get_scanout_position function.

Of course. Will be in v2 of the series.

Best regards
Thomas

> 
> This would reduce the almost identical duplication of the function in
> i915.
> 
> BR,
> Jani.
> 
>>
>> diff --git a/drivers/gpu/drm/i915/i915_drv.c 
>> b/drivers/gpu/drm/i915/i915_drv.c
>> index f7385abdd74b..4a0a7fb85c53 100644
>> --- a/drivers/gpu/drm/i915/i915_drv.c
>> +++ b/drivers/gpu/drm/i915/i915_drv.c
>> @@ -2769,8 +2769,7 @@ static struct drm_driver driver = {
>>  .gem_prime_export = i915_gem_prime_export,
>>  .gem_prime_import = i915_gem_prime_import,
>>  
>> -.get_vblank_timestamp = drm_calc_vbltimestamp_from_scanoutpos,
>> -.get_scanout_position = i915_get_crtc_scanoutpos,
>> +.get_vblank_timestamp = i915_calc_vbltimestamp_from_scanoutpos,
>>  
>>  .dumb_create = i915_gem_dumb_create,
>>  .dumb_map_offset = i915_gem_dumb_mmap_offset,
>> diff --git a/drivers/gpu/drm/i915/i915_irq.c 
>> b/drivers/gpu/drm/i915/i915_irq.c
>> index afc6aad9bf8c..99d0c3b0feae 100644
>> --- a/drivers/gpu/drm/i915/i915_irq.c
>> +++ b/drivers/gpu/drm/i915/i915_irq.c
>> @@ -52,6 +52,11 @@
>>  #include "i915_trace.h"
>>  #include "intel_pm.h"
>>  
>> +/* Retry timestamp calculation up to 3 times to satisfy
>> + * drm_timestamp_precision before giving up.
>> + */
>> +#define I915_TIMESTAMP_MAXRETRIES 3
>> +
>>  /**
>>   * DOC: interrupt handling
>>   *
>> @@ -762,10 +767,11 @@ static int __intel_get_crtc_scanline(struct intel_crtc 
>> *crtc)
>>  return (position + crtc->scanline_offset) % vtotal;
>>  }
>>  
>> -bool i915_get_crtc_scanoutpos(struct drm_device *dev, unsigned int index,
>> -  bool in_vblank_irq, int *vpos, int *hpos,
>> -  ktime_t *stime, ktime_t *etime,
>> -  const struct drm_display_mode *mode)
>> +static bool i915_get_crtc_scanoutpos(struct drm_device *dev,
>> + unsigned int index, bool in_vblank_irq,
>> + int *vpos, int *hpos,
>> + ktime_t *stime, ktime_t *etime,
>> + const struct drm_display_mode *mode)
>>  {
>>  struct drm_i915_private *dev_priv = to_i915(dev);
>>  struct intel_crtc *crtc = to_intel_crtc(drm_crtc_from_index(dev, 
>> index));
>> @@ -879,6 +885,109 @@ bool i915_get_crtc_scanoutpos(struct drm_device *dev, 
>> unsigned int index,
>>  return true;
>>  }
>>  
>> +bool i915_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
>> +unsigned int pipe,
>> +int *max_error,
>> +ktime_t *vblank_time,
>> +bool in_vblank_irq)
>> +{
>> +struct timespec64 ts_etime, ts_vblank_time;
>> +ktime_t stime, etime;
>> +bool vbl_status;
>> +struct drm_crtc *crtc;
>> +const struct drm_display_mode *mode;
>> +struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
>> +int vpos, hpos, i;
>> +int delta_ns, duration_ns;
>> +
>> +crtc = drm_crtc_from_index(dev, pipe);
>> +
>> +if (pipe >= dev->num_crtcs || !crtc) {
>> +DRM_ERROR("Invalid crtc %u\n", pipe);
>> +return false;
>> +}
>> +
>> +if (drm_drv_uses_atomic_modeset(dev))
>> +mode = &vblank->hwmode;
>> +else
>> +mode = &crtc->hwmode;
>> +
>> +/* If mode timing undefined, just return as no-op:
>> + * Happens during initial modesetting of a crtc.
>> + */
>> +if (mode->crtc_clock == 0) {
>> +DRM_DEBUG("crtc %u: Noop due to uninitialized mode.\n", pipe);
>> +WARN_ON_ONCE(drm_drv_uses_atomic_modeset(dev));
>> +
>> +return false;
>> +}
>> +
>> +/* Get current scanout position with system timestamp.
>> + * Re

Re: [Freedreno] [PATCH 03/23] drm/i915: Don't use struct drm_driver.get_scanout_position()

2020-01-10 Thread Jani Nikula
On Fri, 10 Jan 2020, Thomas Zimmermann  wrote:
> The callback struct drm_driver.get_scanout_position() is deprecated in
> favor of struct drm_crtc_helper_funcs.get_scanout_position().
>
> i915 doesn't use CRTC helpers. The patch duplicates the caller
> drm_calc_vbltimestamp_from_scanoutpos() for i915, such that the callback
> function is not needed.
>
> Signed-off-by: Thomas Zimmermann 
> ---
>  drivers/gpu/drm/i915/i915_drv.c |   3 +-
>  drivers/gpu/drm/i915/i915_irq.c | 117 ++--
>  drivers/gpu/drm/i915/i915_irq.h |   9 +--
>  3 files changed, 119 insertions(+), 10 deletions(-)

Not really enthusiastic about the diffstat in a "cleanup" series.

I wonder if you could add a generic helper version of
drm_calc_vbltimestamp_from_scanoutpos where you pass the
get_scanout_position function as a parameter. Both
drm_calc_vbltimestamp_from_scanoutpos and the new
i915_calc_vbltimestamp_from_scanoutpos would then be fairly thin
wrappers passing in the relevant get_scanout_position function.

This would reduce the almost identical duplication of the function in
i915.

BR,
Jani.

>
> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
> index f7385abdd74b..4a0a7fb85c53 100644
> --- a/drivers/gpu/drm/i915/i915_drv.c
> +++ b/drivers/gpu/drm/i915/i915_drv.c
> @@ -2769,8 +2769,7 @@ static struct drm_driver driver = {
>   .gem_prime_export = i915_gem_prime_export,
>   .gem_prime_import = i915_gem_prime_import,
>  
> - .get_vblank_timestamp = drm_calc_vbltimestamp_from_scanoutpos,
> - .get_scanout_position = i915_get_crtc_scanoutpos,
> + .get_vblank_timestamp = i915_calc_vbltimestamp_from_scanoutpos,
>  
>   .dumb_create = i915_gem_dumb_create,
>   .dumb_map_offset = i915_gem_dumb_mmap_offset,
> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> index afc6aad9bf8c..99d0c3b0feae 100644
> --- a/drivers/gpu/drm/i915/i915_irq.c
> +++ b/drivers/gpu/drm/i915/i915_irq.c
> @@ -52,6 +52,11 @@
>  #include "i915_trace.h"
>  #include "intel_pm.h"
>  
> +/* Retry timestamp calculation up to 3 times to satisfy
> + * drm_timestamp_precision before giving up.
> + */
> +#define I915_TIMESTAMP_MAXRETRIES 3
> +
>  /**
>   * DOC: interrupt handling
>   *
> @@ -762,10 +767,11 @@ static int __intel_get_crtc_scanline(struct intel_crtc 
> *crtc)
>   return (position + crtc->scanline_offset) % vtotal;
>  }
>  
> -bool i915_get_crtc_scanoutpos(struct drm_device *dev, unsigned int index,
> -   bool in_vblank_irq, int *vpos, int *hpos,
> -   ktime_t *stime, ktime_t *etime,
> -   const struct drm_display_mode *mode)
> +static bool i915_get_crtc_scanoutpos(struct drm_device *dev,
> +  unsigned int index, bool in_vblank_irq,
> +  int *vpos, int *hpos,
> +  ktime_t *stime, ktime_t *etime,
> +  const struct drm_display_mode *mode)
>  {
>   struct drm_i915_private *dev_priv = to_i915(dev);
>   struct intel_crtc *crtc = to_intel_crtc(drm_crtc_from_index(dev, 
> index));
> @@ -879,6 +885,109 @@ bool i915_get_crtc_scanoutpos(struct drm_device *dev, 
> unsigned int index,
>   return true;
>  }
>  
> +bool i915_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
> + unsigned int pipe,
> + int *max_error,
> + ktime_t *vblank_time,
> + bool in_vblank_irq)
> +{
> + struct timespec64 ts_etime, ts_vblank_time;
> + ktime_t stime, etime;
> + bool vbl_status;
> + struct drm_crtc *crtc;
> + const struct drm_display_mode *mode;
> + struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
> + int vpos, hpos, i;
> + int delta_ns, duration_ns;
> +
> + crtc = drm_crtc_from_index(dev, pipe);
> +
> + if (pipe >= dev->num_crtcs || !crtc) {
> + DRM_ERROR("Invalid crtc %u\n", pipe);
> + return false;
> + }
> +
> + if (drm_drv_uses_atomic_modeset(dev))
> + mode = &vblank->hwmode;
> + else
> + mode = &crtc->hwmode;
> +
> + /* If mode timing undefined, just return as no-op:
> +  * Happens during initial modesetting of a crtc.
> +  */
> + if (mode->crtc_clock == 0) {
> + DRM_DEBUG("crtc %u: Noop due to uninitialized mode.\n", pipe);
> + WARN_ON_ONCE(drm_drv_uses_atomic_modeset(dev));
> +
> + return false;
> + }
> +
> + /* Get current scanout position with system timestamp.
> +  * Repeat query up to DRM_TIMESTAMP_MAXRETRIES times
> +  * if single query takes longer than max_error nanoseconds.
> +  *
> +  * This guarantees a tight bound on maximum error if
> +  * code gets preempted or delayed for some reason.
> +  */
> +

[Freedreno] [PATCH 03/23] drm/i915: Don't use struct drm_driver.get_scanout_position()

2020-01-10 Thread Thomas Zimmermann
The callback struct drm_driver.get_scanout_position() is deprecated in
favor of struct drm_crtc_helper_funcs.get_scanout_position().

i915 doesn't use CRTC helpers. The patch duplicates the caller
drm_calc_vbltimestamp_from_scanoutpos() for i915, such that the callback
function is not needed.

Signed-off-by: Thomas Zimmermann 
---
 drivers/gpu/drm/i915/i915_drv.c |   3 +-
 drivers/gpu/drm/i915/i915_irq.c | 117 ++--
 drivers/gpu/drm/i915/i915_irq.h |   9 +--
 3 files changed, 119 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index f7385abdd74b..4a0a7fb85c53 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -2769,8 +2769,7 @@ static struct drm_driver driver = {
.gem_prime_export = i915_gem_prime_export,
.gem_prime_import = i915_gem_prime_import,
 
-   .get_vblank_timestamp = drm_calc_vbltimestamp_from_scanoutpos,
-   .get_scanout_position = i915_get_crtc_scanoutpos,
+   .get_vblank_timestamp = i915_calc_vbltimestamp_from_scanoutpos,
 
.dumb_create = i915_gem_dumb_create,
.dumb_map_offset = i915_gem_dumb_mmap_offset,
diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index afc6aad9bf8c..99d0c3b0feae 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -52,6 +52,11 @@
 #include "i915_trace.h"
 #include "intel_pm.h"
 
+/* Retry timestamp calculation up to 3 times to satisfy
+ * drm_timestamp_precision before giving up.
+ */
+#define I915_TIMESTAMP_MAXRETRIES 3
+
 /**
  * DOC: interrupt handling
  *
@@ -762,10 +767,11 @@ static int __intel_get_crtc_scanline(struct intel_crtc 
*crtc)
return (position + crtc->scanline_offset) % vtotal;
 }
 
-bool i915_get_crtc_scanoutpos(struct drm_device *dev, unsigned int index,
- bool in_vblank_irq, int *vpos, int *hpos,
- ktime_t *stime, ktime_t *etime,
- const struct drm_display_mode *mode)
+static bool i915_get_crtc_scanoutpos(struct drm_device *dev,
+unsigned int index, bool in_vblank_irq,
+int *vpos, int *hpos,
+ktime_t *stime, ktime_t *etime,
+const struct drm_display_mode *mode)
 {
struct drm_i915_private *dev_priv = to_i915(dev);
struct intel_crtc *crtc = to_intel_crtc(drm_crtc_from_index(dev, 
index));
@@ -879,6 +885,109 @@ bool i915_get_crtc_scanoutpos(struct drm_device *dev, 
unsigned int index,
return true;
 }
 
+bool i915_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
+   unsigned int pipe,
+   int *max_error,
+   ktime_t *vblank_time,
+   bool in_vblank_irq)
+{
+   struct timespec64 ts_etime, ts_vblank_time;
+   ktime_t stime, etime;
+   bool vbl_status;
+   struct drm_crtc *crtc;
+   const struct drm_display_mode *mode;
+   struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
+   int vpos, hpos, i;
+   int delta_ns, duration_ns;
+
+   crtc = drm_crtc_from_index(dev, pipe);
+
+   if (pipe >= dev->num_crtcs || !crtc) {
+   DRM_ERROR("Invalid crtc %u\n", pipe);
+   return false;
+   }
+
+   if (drm_drv_uses_atomic_modeset(dev))
+   mode = &vblank->hwmode;
+   else
+   mode = &crtc->hwmode;
+
+   /* If mode timing undefined, just return as no-op:
+* Happens during initial modesetting of a crtc.
+*/
+   if (mode->crtc_clock == 0) {
+   DRM_DEBUG("crtc %u: Noop due to uninitialized mode.\n", pipe);
+   WARN_ON_ONCE(drm_drv_uses_atomic_modeset(dev));
+
+   return false;
+   }
+
+   /* Get current scanout position with system timestamp.
+* Repeat query up to DRM_TIMESTAMP_MAXRETRIES times
+* if single query takes longer than max_error nanoseconds.
+*
+* This guarantees a tight bound on maximum error if
+* code gets preempted or delayed for some reason.
+*/
+   for (i = 0; i < I915_TIMESTAMP_MAXRETRIES; i++) {
+   /*
+* Get vertical and horizontal scanout position vpos, hpos,
+* and bounding timestamps stime, etime, pre/post query.
+*/
+   vbl_status = i915_get_crtc_scanoutpos(dev, pipe, in_vblank_irq,
+ &vpos, &hpos, &stime,
+ &etime, mode);
+   /* Return as no-op if scanout query unsupported or failed. */
+   if (!vbl_status) {
+   DRM_DEBUG("crtc %u : scanoutpos query failed.\n",
+