[PATCH v6 10/15] coresight: trbe: Workaround TRBE errata overwrite in FILL mode

[Suzuki K Poulose]

ARM Neoverse-N2 (#2139208) and Cortex-A710(##2119858) suffers from
an erratum, which when triggered, might cause the TRBE to overwrite
the trace data already collected in FILL mode, in the event of a WRAP.
i.e, the TRBE doesn't stop writing the data, instead wraps to the base
and could write upto 3 cache line size worth trace. Thus, this could
corrupt the trace at the "BASE" pointer.

The workaround is to program the write pointer 256bytes from the
base, such that if the erratum is triggered, it doesn't overwrite
the trace data that was captured. This skipped region could be
padded with ignore packets at the end of the session, so that
the decoder sees a continuous buffer with some padding at the
beginning. The trace data written at the base is considered
lost as the limit could have been in the middle of the perf
ring buffer, and jumping to the "base" is not acceptable.
We set the flags already to indicate that some amount of trace
was lost during the FILL event IRQ. So this is fine.

One important change with the work around is, we program the
TRBBASER_EL1 to current page where we are allowed to write.
Otherwise, it could overwrite a region that may be consumed
by the perf. Towards this, we always make sure that the
"handle->head" and thus the trbe_write is PAGE_SIZE aligned,
so that we can set the BASE to the PAGE base and move the
TRBPTR to the 256bytes offset.

Cc: Mike Leach <mike.leach@xxxxxxxxxx>
Cc: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
Cc: Leo Yan <leo.yan@xxxxxxxxxx>
Reviewed-by: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
---
Changes since v2:
 - Updated the ASCII art to include better description of
   all the steps in the work around
Change since v1:
 - Updated comment with ASCII art
 - Add _BYTES suffix for the space to skip for the work around.
---
 drivers/hwtracing/coresight/coresight-trbe.c | 168 +++++++++++++++++--
 1 file changed, 157 insertions(+), 11 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
index fdd60ff4fb3d..2d39e5ecb72c 100644
--- a/drivers/hwtracing/coresight/coresight-trbe.c
+++ b/drivers/hwtracing/coresight/coresight-trbe.c
@@ -89,14 +89,22 @@ struct trbe_buf {
  *   - Not duplicating the detection logic
  *   - Streamlined detection of erratum across the system
  */
+#define TRBE_WORKAROUND_OVERWRITE_FILL_MODE	0
 
 static int trbe_errata_cpucaps[] = {
+	[TRBE_WORKAROUND_OVERWRITE_FILL_MODE] = ARM64_WORKAROUND_TRBE_OVERWRITE_FILL_MODE,
 	-1,		/* Sentinel, must be the last entry */
 };
 
 /* The total number of listed errata in trbe_errata_cpucaps */
 #define TRBE_ERRATA_MAX			(ARRAY_SIZE(trbe_errata_cpucaps) - 1)
 
+/*
+ * Safe limit for the number of bytes that may be overwritten
+ * when ARM64_WORKAROUND_TRBE_OVERWRITE_FILL_MODE is triggered.
+ */
+#define TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP_BYTES	256
+
 /*
  * struct trbe_cpudata: TRBE instance specific data
  * @trbe_flag		- TRBE dirty/access flag support
@@ -147,6 +155,11 @@ static inline bool trbe_has_erratum(struct trbe_cpudata *cpudata, int i)
 	return (i < TRBE_ERRATA_MAX) && test_bit(i, cpudata->errata);
 }
 
+static inline bool trbe_may_overwrite_in_fill_mode(struct trbe_cpudata *cpudata)
+{
+	return trbe_has_erratum(cpudata, TRBE_WORKAROUND_OVERWRITE_FILL_MODE);
+}
+
 static int trbe_alloc_node(struct perf_event *event)
 {
 	if (event->cpu == -1)
@@ -550,10 +563,13 @@ static void trbe_enable_hw(struct trbe_buf *buf)
 	set_trbe_limit_pointer_enabled(buf->trbe_limit);
 }
 
-static enum trbe_fault_action trbe_get_fault_act(u64 trbsr)
+static enum trbe_fault_action trbe_get_fault_act(struct perf_output_handle *handle,
+						 u64 trbsr)
 {
 	int ec = get_trbe_ec(trbsr);
 	int bsc = get_trbe_bsc(trbsr);
+	struct trbe_buf *buf = etm_perf_sink_config(handle);
+	struct trbe_cpudata *cpudata = buf->cpudata;
 
 	WARN_ON(is_trbe_running(trbsr));
 	if (is_trbe_trg(trbsr) || is_trbe_abort(trbsr))
@@ -562,10 +578,16 @@ static enum trbe_fault_action trbe_get_fault_act(u64 trbsr)
 	if ((ec == TRBE_EC_STAGE1_ABORT) || (ec == TRBE_EC_STAGE2_ABORT))
 		return TRBE_FAULT_ACT_FATAL;
 
-	if (is_trbe_wrap(trbsr) && (ec == TRBE_EC_OTHERS) && (bsc == TRBE_BSC_FILLED)) {
-		if (get_trbe_write_pointer() == get_trbe_base_pointer())
-			return TRBE_FAULT_ACT_WRAP;
-	}
+	/*
+	 * If the trbe is affected by TRBE_WORKAROUND_OVERWRITE_FILL_MODE,
+	 * it might write data after a WRAP event in the fill mode.
+	 * Thus the check TRBPTR == TRBBASER will not be honored.
+	 */
+	if ((is_trbe_wrap(trbsr) && (ec == TRBE_EC_OTHERS) && (bsc == TRBE_BSC_FILLED)) &&
+	    (trbe_may_overwrite_in_fill_mode(cpudata) ||
+	     get_trbe_write_pointer() == get_trbe_base_pointer()))
+		return TRBE_FAULT_ACT_WRAP;
+
 	return TRBE_FAULT_ACT_SPURIOUS;
 }
 
@@ -574,6 +596,8 @@ static unsigned long trbe_get_trace_size(struct perf_output_handle *handle,
 {
 	u64 write;
 	u64 start_off, end_off;
+	u64 size;
+	u64 overwrite_skip = TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP_BYTES;
 
 	/*
 	 * If the TRBE has wrapped around the write pointer has
@@ -594,7 +618,18 @@ static unsigned long trbe_get_trace_size(struct perf_output_handle *handle,
 
 	if (WARN_ON_ONCE(end_off < start_off))
 		return 0;
-	return (end_off - start_off);
+
+	size = end_off - start_off;
+	/*
+	 * If the TRBE is affected by the following erratum, we must fill
+	 * the space we skipped with IGNORE packets. And we are always
+	 * guaranteed to have at least a PAGE_SIZE space in the buffer.
+	 */
+	if (trbe_has_erratum(buf->cpudata, TRBE_WORKAROUND_OVERWRITE_FILL_MODE) &&
+	    !WARN_ON(size < overwrite_skip))
+		__trbe_pad_buf(buf, start_off, overwrite_skip);
+
+	return size;
 }
 
 static void *arm_trbe_alloc_buffer(struct coresight_device *csdev,
@@ -713,7 +748,7 @@ static unsigned long arm_trbe_update_buffer(struct coresight_device *csdev,
 		clr_trbe_irq();
 		isb();
 
-		act = trbe_get_fault_act(status);
+		act = trbe_get_fault_act(handle, status);
 		/*
 		 * If this was not due to a WRAP event, we have some
 		 * errors and as such buffer is empty.
@@ -737,21 +772,117 @@ static unsigned long arm_trbe_update_buffer(struct coresight_device *csdev,
 	return size;
 }
 
+
+static int trbe_apply_work_around_before_enable(struct trbe_buf *buf)
+{
+	/*
+	 * TRBE_WORKAROUND_OVERWRITE_FILL_MODE causes the TRBE to overwrite a few cache
+	 * line size from the "TRBBASER_EL1" in the event of a "FILL".
+	 * Thus, we could loose some amount of the trace at the base.
+	 *
+	 * Before Fix:
+	 *
+	 *  normal-BASE     head (normal-TRBPTR)         tail (normal-LIMIT)
+	 *  |                   \/                       /
+	 *   -------------------------------------------------------------
+	 *  |   Pg0      |   Pg1       |           |          |  PgN     |
+	 *   -------------------------------------------------------------
+	 *
+	 * In the normal course of action, we would set the TRBBASER to the
+	 * beginning of the ring-buffer (normal-BASE). But with the erratum,
+	 * the TRBE could overwrite the contents at the "normal-BASE", after
+	 * hitting the "normal-LIMIT", since it doesn't stop as expected. And
+	 * this is wrong. This could result in overwriting trace collected in
+	 * one of the previous runs, being consumed by the user. So we must
+	 * always make sure that the TRBBASER is within the region
+	 * [head, head+size]. Note that TRBBASER must be PAGE aligned,
+	 *
+	 *  After moving the BASE:
+	 *
+	 *  normal-BASE     head (normal-TRBPTR)         tail (normal-LIMIT)
+	 *  |                   \/                       /
+	 *   -------------------------------------------------------------
+	 *  |         |          |xyzdef.     |..   tuvw|                |
+	 *   -------------------------------------------------------------
+	 *                      /
+	 *              New-BASER
+	 *
+	 * Also, we would set the TRBPTR to head (after adjusting for
+	 * alignment) at normal-PTR. This would mean that the last few bytes
+	 * of the trace (say, "xyz") might overwrite the first few bytes of
+	 * trace written ("abc"). More importantly they will appear in what
+	 * userspace sees as the beginning of the trace, which is wrong. We may
+	 * not always have space to move the latest trace "xyz" to the correct
+	 * order as it must appear beyond the LIMIT. (i.e, [head..head+size]).
+	 * Thus it is easier to ignore those bytes than to complicate the
+	 * driver to move it, assuming that the erratum was triggered and
+	 * doing additional checks to see if there is indeed allowed space at
+	 * TRBLIMITR.LIMIT.
+	 *
+	 *  Thus the full workaround will move the BASE and the PTR and would
+	 *  look like (after padding at the skipped bytes at the end of
+	 *  session) :
+	 *
+	 *  normal-BASE     head (normal-TRBPTR)         tail (normal-LIMIT)
+	 *  |                   \/                       /
+	 *   -------------------------------------------------------------
+	 *  |         |          |///abc..     |..  rst|                |
+	 *   -------------------------------------------------------------
+	 *                      /    |
+	 *              New-BASER    New-TRBPTR
+	 *
+	 * To summarize, with the work around:
+	 *
+	 *  - We always align the offset for the next session to PAGE_SIZE
+	 *    (This is to ensure we can program the TRBBASER to this offset
+	 *    within the region [head...head+size]).
+	 *
+	 *  - At TRBE enable:
+	 *     - Set the TRBBASER to the page aligned offset of the current
+	 *       proposed write offset. (which is guaranteed to be aligned
+	 *       as above)
+	 *     - Move the TRBPTR to skip first 256bytes (that might be
+	 *       overwritten with the erratum). This ensures that the trace
+	 *       generated in the session is not re-written.
+	 *
+	 *  - At trace collection:
+	 *     - Pad the 256bytes skipped above again with IGNORE packets.
+	 */
+	if (trbe_has_erratum(buf->cpudata, TRBE_WORKAROUND_OVERWRITE_FILL_MODE)) {
+		if (WARN_ON(!IS_ALIGNED(buf->trbe_write, PAGE_SIZE)))
+			return -EINVAL;
+		buf->trbe_hw_base = buf->trbe_write;
+		buf->trbe_write += TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP_BYTES;
+	}
+
+	return 0;
+}
+
 static int __arm_trbe_enable(struct trbe_buf *buf,
 			     struct perf_output_handle *handle)
 {
+	int ret = 0;
+
 	perf_aux_output_flag(handle, PERF_AUX_FLAG_CORESIGHT_FORMAT_RAW);
 	buf->trbe_limit = compute_trbe_buffer_limit(handle);
 	buf->trbe_write = buf->trbe_base + PERF_IDX2OFF(handle->head, buf);
 	if (buf->trbe_limit == buf->trbe_base) {
-		trbe_stop_and_truncate_event(handle);
-		return -ENOSPC;
+		ret = -ENOSPC;
+		goto err;
 	}
 	/* Set the base of the TRBE to the buffer base */
 	buf->trbe_hw_base = buf->trbe_base;
+
+	ret = trbe_apply_work_around_before_enable(buf);
+	if (ret)
+		goto err;
+
 	*this_cpu_ptr(buf->cpudata->drvdata->handle) = handle;
 	trbe_enable_hw(buf);
 	return 0;
+err:
+	trbe_stop_and_truncate_event(handle);
+	return ret;
 }
 
 static int arm_trbe_enable(struct coresight_device *csdev, u32 mode, void *data)
@@ -891,7 +1022,7 @@ static irqreturn_t arm_trbe_irq_handler(int irq, void *dev)
 	if (!is_perf_trbe(handle))
 		return IRQ_NONE;
 
-	act = trbe_get_fault_act(status);
+	act = trbe_get_fault_act(handle, status);
 	switch (act) {
 	case TRBE_FAULT_ACT_WRAP:
 		truncated = !!trbe_handle_overflow(handle);
@@ -1043,7 +1174,22 @@ static void arm_trbe_probe_cpu(void *info)
 	 */
 	trbe_check_errata(cpudata);
 
-	cpudata->trbe_align = cpudata->trbe_hw_align;
+	/*
+	 * If the TRBE is affected by erratum TRBE_WORKAROUND_OVERWRITE_FILL_MODE,
+	 * we must always program the TBRPTR_EL1, 256bytes from a page
+	 * boundary, with TRBBASER_EL1 set to the page, to prevent
+	 * TRBE over-writing 256bytes at TRBBASER_EL1 on FILL event.
+	 *
+	 * Thus make sure we always align our write pointer to a PAGE_SIZE,
+	 * which also guarantees that we have at least a PAGE_SIZE space in
+	 * the buffer (TRBLIMITR is PAGE aligned) and thus we can skip
+	 * the required bytes at the base.
+	 */
+	if (trbe_may_overwrite_in_fill_mode(cpudata))
+		cpudata->trbe_align = PAGE_SIZE;
+	else
+		cpudata->trbe_align = cpudata->trbe_hw_align;
+
 	cpudata->trbe_flag = get_trbe_flag_update(trbidr);
 	cpudata->cpu = cpu;
 	cpudata->drvdata = drvdata;
-- 
2.25.4