Re: [PATCH 2/3] drm/i915/perf: Add support for report sizes that are not power of 2

[Lionel Landwerlin <lionel.g.landwerlin@xxxxxxxxx>]

On 16/09/2019 22:17, Umesh Nerlige Ramappa wrote:
> On Sun, Sep 15, 2019 at 02:24:41PM +0300, Lionel Landwerlin wrote:
> > On 14/09/2019 02:06, Umesh Nerlige Ramappa wrote:
> > > OA perf unit supports non-power of 2 report sizes. Enable support for
> > > these sizes in the driver.
> > >
> > > Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@xxxxxxxxx>
> > > ---
> > >  drivers/gpu/drm/i915/i915_perf.c | 59 ++++++++++++--------------------
> > >  1 file changed, 21 insertions(+), 38 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/i915/i915_perf.c 
> > > b/drivers/gpu/drm/i915/i915_perf.c
> > > index 50b6d154fd46..482fca3da7de 100644
> > > --- a/drivers/gpu/drm/i915/i915_perf.c
> > > +++ b/drivers/gpu/drm/i915/i915_perf.c
> > > @@ -450,7 +450,7 @@ static bool oa_buffer_check_unlocked(struct 
> > > i915_perf_stream *stream)
> > >      u32 gtt_offset = i915_ggtt_offset(stream->oa_buffer.vma);
> > >      int report_size = stream->oa_buffer.format_size;
> > >      unsigned long flags;
> > > -    u32 hw_tail;
> > > +    u32 hw_tail, aging_tail;
> > >      u64 now;
> > >      /* We have to consider the (unlikely) possibility that read() 
> > > errors
> > > @@ -459,16 +459,17 @@ static bool oa_buffer_check_unlocked(struct 
> > > i915_perf_stream *stream)
> > >       */
> > >      spin_lock_irqsave(&stream->oa_buffer.ptr_lock, flags);
> > > -    hw_tail = dev_priv->perf.ops.oa_hw_tail_read(stream);
> > > +    hw_tail = dev_priv->perf.ops.oa_hw_tail_read(stream) - gtt_offset;
> > > +    aging_tail = stream->oa_buffer.aging_tail - gtt_offset;
> > >      /* The tail pointer increases in 64 byte increments,
> > >       * not in report_size steps...
> > >       */
> > > -    hw_tail &= ~(report_size - 1);
> > > +    hw_tail = OA_TAKEN(hw_tail, (OA_TAKEN(hw_tail, aging_tail) % 
> > > report_size));
> >
> >
> > I'm struggling to parse this line above and I'm not 100% sure it's 
> > correct.
> >
> > Could add a comment to explain what is going on?
>
> The aging tail is always pointing to the boundary of a report whereas
> the hw_tail is advancing in 64 byte increments.
>
> The innermost OA_TAKEN is returning the number of bytes between the
> hw_tail and the aging_tail. The modulo is getting the size of the
> partial report (if any).
>
> The outermost OA_TAKEN is subtracting the size of partial report from
> the hw_tail to get a hw_tail that points to the boundary of the last
> full report.
>
> The value of hw_tail would be the same as from the deleted line of code
> above this line.
>
> Thanks,
> Umesh


Thanks, I ran a few tests locally to convince myself it's correct :)


It's still a bit difficult to parse, probably because OA_TAKEN() wasn't 
meant for this.

Could create a helper function that does this computation, something 
like this :


static inline u32 align_hw_tail_to_report_boundary(u32 hw_tail, u32 
last_aligned_tail)

{

    /* Compute potentially partially landed report in the OA buffer */

    u32 partial_report_size = OA_TAKEN(hw_tail, last_aligned_tail) % 
report_size;

    /* Substract that partial amount off the tail. */

    return (hw_tail - partial_report_size) % OA_BUFFER_SIZE;

}


Cheers,


-Lionel


> > Thanks,
> >
> >
> > -Lionel
> >
> >
> > >      now = ktime_get_mono_fast_ns();
> > > -    if (hw_tail == stream->oa_buffer.aging_tail) {
> > > +    if (hw_tail == aging_tail) {
> > >          /* If the HW tail hasn't move since the last check and the HW
> > >           * tail has been aging for long enough, declare it the new
> > >           * tail.
> > > @@ -486,8 +487,6 @@ static bool oa_buffer_check_unlocked(struct 
> > > i915_perf_stream *stream)
> > >           * a read() in progress.
> > >           */
> > >          head = stream->oa_buffer.head - gtt_offset;
> > > -
> > > -        hw_tail -= gtt_offset;
> > >          tail = hw_tail;
> > >          /* Walk the stream backward until we find at least 2 reports
> > > @@ -613,7 +612,18 @@ static int append_oa_sample(struct 
> > > i915_perf_stream *stream,
> > >      buf += sizeof(header);
> > >      if (sample_flags & SAMPLE_OA_REPORT) {
> > > -        if (copy_to_user(buf, report, report_size))
> > > +        u8 *oa_buf_end = stream->oa_buffer.vaddr + OA_BUFFER_SIZE;
> > > +        int report_size_partial = oa_buf_end - report;
> > > +
> > > +        if (report_size_partial < report_size) {
> > > +            if (copy_to_user(buf, report, report_size_partial))
> > > +                return -EFAULT;
> > > +            buf += report_size_partial;
> > > +
> > > +            if (copy_to_user(buf, stream->oa_buffer.vaddr,
> > > +                    report_size - report_size_partial))
> > > +                return -EFAULT;
> > > +        } else if (copy_to_user(buf, report, report_size))
> > >              return -EFAULT;
> > >      }
> > > @@ -682,8 +692,8 @@ static int gen8_append_oa_reports(struct 
> > > i915_perf_stream *stream,
> > >       * only be incremented by multiples of the report size (notably 
> > > also
> > >       * all a power of two).
> > >       */
> > > -    if (WARN_ONCE(head > OA_BUFFER_SIZE || head % report_size ||
> > > -              tail > OA_BUFFER_SIZE || tail % report_size,
> > > +    if (WARN_ONCE(head > OA_BUFFER_SIZE ||
> > > +              tail > OA_BUFFER_SIZE,
> > >                "Inconsistent OA buffer pointers: head = %u, tail = 
> > > %u\n",
> > >                head, tail))
> > >          return -EIO;
> > > @@ -697,20 +707,6 @@ static int gen8_append_oa_reports(struct 
> > > i915_perf_stream *stream,
> > >          u32 ctx_id;
> > >          u32 reason;
> > > -        /*
> > > -         * All the report sizes factor neatly into the buffer
> > > -         * size so we never expect to see a report split
> > > -         * between the beginning and end of the buffer.
> > > -         *
> > > -         * Given the initial alignment check a misalignment
> > > -         * here would imply a driver bug that would result
> > > -         * in an overrun.
> > > -         */
> > > -        if (WARN_ON((OA_BUFFER_SIZE - head) < report_size)) {
> > > -            DRM_ERROR("Spurious OA head ptr: non-integral report 
> > > offset\n");
> > > -            break;
> > > -        }
> > > -
> > >          /*
> > >           * The reason field includes flags identifying what
> > >           * triggered this specific report (mostly timer
> > > @@ -956,8 +952,8 @@ static int gen7_append_oa_reports(struct 
> > > i915_perf_stream *stream,
> > >       * only be incremented by multiples of the report size (notably 
> > > also
> > >       * all a power of two).
> > >       */
> > > -    if (WARN_ONCE(head > OA_BUFFER_SIZE || head % report_size ||
> > > -              tail > OA_BUFFER_SIZE || tail % report_size,
> > > +    if (WARN_ONCE(head > OA_BUFFER_SIZE ||
> > > +              tail > OA_BUFFER_SIZE,
> > >                "Inconsistent OA buffer pointers: head = %u, tail = 
> > > %u\n",
> > >                head, tail))
> > >          return -EIO;
> > > @@ -969,19 +965,6 @@ static int gen7_append_oa_reports(struct 
> > > i915_perf_stream *stream,
> > >          u8 *report = oa_buf_base + head;
> > >          u32 *report32 = (void *)report;
> > > -        /* All the report sizes factor neatly into the buffer
> > > -         * size so we never expect to see a report split
> > > -         * between the beginning and end of the buffer.
> > > -         *
> > > -         * Given the initial alignment check a misalignment
> > > -         * here would imply a driver bug that would result
> > > -         * in an overrun.
> > > -         */
> > > -        if (WARN_ON((OA_BUFFER_SIZE - head) < report_size)) {
> > > -            DRM_ERROR("Spurious OA head ptr: non-integral report 
> > > offset\n");
> > > -            break;
> > > -        }
> > > -
> > >          /* The report-ID field for periodic samples includes
> > >           * some undocumented flags related to what triggered
> > >           * the report and is never expected to be zero so we

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