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

[Ashutosh Dixit <ashutosh.dixit@intel.com>]

On Mon, 16 Sep 2019 12:17:54 -0700, 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@intel.com>
> >> ---
> >>  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.

Looks like aging_tail should not be needed (it is complicating the
expression and is not there in the original expression). All we need is a
"circular modulus". For example would the following work?

    if (hw_tail < report_size)
        hw_tail += OA_BUFFER_SIZE;
    hw_tail = rounddown(hw_tail, report_size);

Another way (if we want to avoid the division) would be to maintain a
cached copy of hw_tail in SW which is successively (and circularly)
incremented by report_size till it catches up with hw_tail read from
HW. But probably the first method above is simpler.
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