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

From: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
To: Ashutosh Dixit <ashutosh.dixit@intel.com>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 2/3] drm/i915/perf: Add support for report sizes that are not power of 2
Date: Tue, 17 Sep 2019 10:33:51 -0700	[thread overview]
Message-ID: <20190917173351.GA1279@orsosgc001.amr.corp.intel.com> (raw)
In-Reply-To: <87tv9b36k1.wl-ashutosh.dixit@intel.com>

On Mon, Sep 16, 2019 at 09:11:58PM -0700, Ashutosh Dixit wrote:
>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?

original expression assumes all report sizes are powers of 2 and hence 
does not need a reference (like aging_tail) to rounddown the hw_tail.

>
>    if (hw_tail < report_size)
>        hw_tail += OA_BUFFER_SIZE;

Assuming that this condition is detecting a report split across the OA 
buffer boundary, the above check will not catch the split in cases where 
there are multiple reports generated before we read the hw_tail.

>    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.

aging_tail is a cached copy of the hw_tail that advances only in report 
size increments.

Thanks,
Umesh
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