Re: [PATCH 5/7] coresight: tmc: Add support for reading tracedata from previous boot

[James Clark]

On 29/09/2023 14:37, Linu Cherian wrote:
> * Introduce a new mode CS_MODE_READ_PREVBOOT for reading tracedata
>   captured in previous boot.
> 
> * Add special handlers for preparing ETR/ETF for this special mode
> 
> * User can read the trace data as below
> 
>   For example, for reading trace data from tmc_etf sink
> 
>   1. cd /sys/bus/coresight/devices/tmc_etfXX/
> 
>   2. Change mode to READ_PREVBOOT
> 
>      #echo 1 > read_prevboot
> 
>   3. Dump trace buffer data to a file,
> 
>      #dd if=/dev/tmc_etrXX of=~/cstrace.bin
> 
>   4. Reset back to normal mode
> 
>      #echo 0 > read_prevboot
> 
> Signed-off-by: Anil Kumar Reddy <areddy3@xxxxxxxxxxx>
> Signed-off-by: Tanmay Jagdale <tanmay@xxxxxxxxxxx>
> Signed-off-by: Linu Cherian <lcherian@xxxxxxxxxxx>
> ---
>  .../coresight/coresight-etm4x-core.c          |   1 +
>  .../hwtracing/coresight/coresight-tmc-core.c  |  81 +++++++++-
>  .../hwtracing/coresight/coresight-tmc-etf.c   |  62 ++++++++
>  .../hwtracing/coresight/coresight-tmc-etr.c   | 145 +++++++++++++++++-
>  drivers/hwtracing/coresight/coresight-tmc.h   |   6 +
>  include/linux/coresight.h                     |  13 ++
>  6 files changed, 306 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> index 77b0271ce6eb..513baf681280 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> @@ -1010,6 +1010,7 @@ static void etm4_disable(struct coresight_device *csdev,
>  
>  	switch (mode) {
>  	case CS_MODE_DISABLED:
> +	case CS_MODE_READ_PREVBOOT:
>  		break;
>  	case CS_MODE_SYSFS:
>  		etm4_disable_sysfs(csdev);
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-core.c b/drivers/hwtracing/coresight/coresight-tmc-core.c
> index 6658ce76777b..65c15c9f821b 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-core.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-core.c
> @@ -103,6 +103,45 @@ u32 tmc_get_memwidth_mask(struct tmc_drvdata *drvdata)
>  	return mask;
>  }
>  
> +int tmc_read_prepare_prevboot(struct tmc_drvdata *drvdata)
> +{
> +	int ret = 0;
> +	struct tmc_register_snapshot *reg_ptr;
> +	struct coresight_device *csdev = drvdata->csdev;
> +
> +	if (!drvdata->metadata.vaddr) {
> +		ret = -ENOMEM;
> +		goto out;
> +	}
> +
> +	reg_ptr = drvdata->metadata.vaddr;
> +	if (!reg_ptr->valid) {
> +		dev_err(&drvdata->csdev->dev,
> +			"Invalid metadata captured from previous boot\n");
> +		ret = -EINVAL;
> +		goto out;
> +	}

I'm wondering if a more robust check is needed than the valid flag, like
a checksum or something. I didn't debug it yet but I ended up with an
invalid set of metadata after a panic reboot, see below. I'm not sure if
it's just a logic bug or something got lost during the reboot, I didn't
debug it yet. But I suppose unless you assume the panic didn't affect
writing the metadata, then it could be partially written and shouldn't
be trusted?

[...]
> +
> +static int tmc_etr_sync_prevboot_buf(struct tmc_drvdata *drvdata)
> +{
> +	u32 status;
> +	u64 rrp, rwp, dba;
> +	struct tmc_register_snapshot *reg_ptr;
> +	struct etr_buf *etr_buf = drvdata->prevboot_buf;
> +
> +	reg_ptr = drvdata->metadata.vaddr;
> +
> +	rrp = reg_ptr->rrp;
> +	rwp = reg_ptr->rwp;
> +	dba = reg_ptr->dba;
> +	status = reg_ptr->sts;
> +
> +	etr_buf->full = !!(status & TMC_STS_FULL);
> +
> +	/* Sync the buffer pointers */
> +	etr_buf->offset = rrp - dba;
> +	if (etr_buf->full)
> +		etr_buf->len = etr_buf->size;
> +	else
> +		etr_buf->len = rwp - rrp;
> +
> +	/* Sanity checks for validating metadata */
> +	if ((etr_buf->offset > etr_buf->size) ||
> +	    (etr_buf->len > etr_buf->size))
> +		return -EINVAL;

The values I got here are 0x781b67182aa346f9 0x8000000 0x8000000 for
offset, size and len respectively. This fails the first check. It would
also be nice to have a dev_dbg here as well, it's basically the same as
the valid check above which does have one.