Re: [PATCH V12 08/10] arm64/perf: Add struct brbe_regset helper functions

On 6/21/23 18:45, Mark Rutland wrote:

Hi Anshuman,

Thanks, this is looking much better; I just a have a couple of minor comments.

With those fixed up:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

On Thu, Jun 15, 2023 at 07:02:37PM +0530, Anshuman Khandual wrote:

quoted

The primary abstraction level for fetching branch records from BRBE HW has
been changed as 'struct brbe_regset', which contains storage for all three
BRBE registers i.e BRBSRC, BRBTGT, BRBINF. Whether branch record processing
happens in the task sched out path, or in the PMU IRQ handling path, these
registers need to be extracted from the HW. Afterwards both live and stored
sets need to be stitched together to create final branch records set. This
adds required helper functions for such operations.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Tested-by: James Clark <redacted>
Signed-off-by: Anshuman Khandual <redacted>
---
 drivers/perf/arm_brbe.c | 127 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 127 insertions(+)

diff --git a/drivers/perf/arm_brbe.c b/drivers/perf/arm_brbe.c
index 4729cb49282b..f6693699fade 100644
--- a/drivers/perf/arm_brbe.c
+++ b/drivers/perf/arm_brbe.c

@@ -44,6 +44,133 @@ static void select_brbe_bank(int bank)
 	isb();
 }
 
+static bool __read_brbe_regset(struct brbe_regset *entry, int idx)
+{
+	entry->brbinf = get_brbinf_reg(idx);
+
+	/*
+	 * There are no valid entries anymore on the buffer.
+	 * Abort the branch record processing to save some
+	 * cycles and also reduce the capture/process load
+	 * for the user space as well.
+	 */

This comment refers to the process of handling multiple entries, though it's
only handling one entry, and I don't think we need to mention saving cycles here.

Could we please delete this comment entirely? The comment above
capture_brbe_regset() already explains that we read until the first invalid
entry.

Sure, will drop the comment.

quoted

+	if (brbe_invalid(entry->brbinf))
+		return false;
+
+	entry->brbsrc = get_brbsrc_reg(idx);
+	entry->brbtgt = get_brbtgt_reg(idx);
+	return true;
+}
+
+/*
+ * This scans over BRBE register banks and captures individual branch records
+ * [BRBSRC, BRBTGT, BRBINF] into a pre-allocated 'struct brbe_regset' buffer,
+ * until an invalid one gets encountered. The caller for this function needs
+ * to ensure BRBE is an appropriate state before the records can be captured.
+ */

Could we simplify this to:

/*
 * Read all BRBE entries in HW until the first invalid entry.
 *
 * The caller must ensure that the BRBE is not concurrently modifying these
 * entries.
 */

Okay, will change the comment as suggested.

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