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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 10 Nov 2025 11:48:21.7458 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 6f57ce87-070e-4f94-13a6-08de204f0c91 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AM4PEPF00025F99.EURPRD83.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM9PR08MB5891 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251110_034829_683698_6E2F7AAD X-CRM114-Status: GOOD ( 22.20 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Reviewed-by: Yeoreum Yun > As recommended in section 4.3.7 "Synchronization when using the > memory-mapped interface" of ARM IHI0064H.b: > > When using the memory-mapped interface to program the trace unit, the > trace analyzer must ensure that writes have completed, to ensure that > the trace unit is fully programmed and either enabled or disabled. > > To ensure writes have completed, the trace analyzer can do ... > > If the memory marked is as Device-nGnRE or stronger, read back the > value of any register in the trace unit. This relies on peripheral > coherence order defined in the Arm architecture. > > Polling TRCSTATR ensures the previous write has completed. Therefore, > removes the redundant DSB barrier in the enabling flow. > > Update the comment in the disable flow for consistency. > > Signed-off-by: Leo Yan > --- > drivers/hwtracing/coresight/coresight-etm4x-core.c | 23 +++++++++++++++++----- > 1 file changed, 18 insertions(+), 5 deletions(-) > > diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c > index fdda924a2c7117a6828e5325d78586b09e75febd..bdf5ab85b2213dfd601391fdcf50960667fa7cea 100644 > --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c > +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c > @@ -475,10 +475,16 @@ static int etm4_enable_trace_unit(struct etmv4_drvdata *drvdata) > } > > /* > - * As recommended by section 4.3.7 ("Synchronization when using the > - * memory-mapped interface") of ARM IHI 0064D > + * As recommended in section 4.3.7 (Synchronization of register updates) > + * of ARM IHI 0064H.b, the self-hosted trace analyzer always executes an > + * ISB instruction after programming the trace unit registers. > + * > + * For the memory-mapped interface, the registers are mapped as Device > + * type (Device-nGnRE). Reading back the value of any register in the > + * trace unit ensures that all writes have completed. Therefore, polling > + * on TRCSTATR guarantees that the writing TRCPRGCTLR is complete, and > + * no explicit dsb() is required at here. > */ > - dsb(sy); > isb(); > > return 0; > @@ -974,8 +980,15 @@ static void etm4_disable_trace_unit(struct etmv4_drvdata *drvdata) > dev_err(etm_dev, > "timeout while waiting for PM stable Trace Status\n"); > /* > - * As recommended by section 4.3.7 (Synchronization of register updates) > - * of ARM IHI 0064H.b. > + * As recommended in section 4.3.7 (Synchronization of register updates) > + * of ARM IHI 0064H.b, the self-hosted trace analyzer always executes an > + * ISB instruction after programming the trace unit registers. > + * > + * For the memory-mapped interface, the registers are mapped as Device > + * type (Device-nGnRE). Reading back the value of any register in the > + * trace unit ensures that all writes have completed. Therefore, polling > + * on TRCSTATR guarantees that the writing TRCPRGCTLR is complete, and > + * no explicit dsb() is required at here. > */ > isb(); > } > > -- > 2.34.1 > -- Sincerely, Yeoreum Yun