Hi James,
A couple of points - relating mainly to docs:
1. Activating branch broadcast overrides any setting of return stack. As a minimum there needs to be a documentation update to reflect this -Setting both options is not prohibited in hardware - and in the case where we can use branch broadcast over a range both are then relevant.
2. A documented note to reflect that choosing this option will result in a significant increase in the amount of trace generated - possible danger of overflows, or less actual instructions covered. In addition perhaps documents could reflect the intended use-case for this option, given the disadvantages.
3. Has this been tested in a situation where it will be of use? Testing against static code images will show the same decoded trace output as not using branch broadcast. (although the packet dumps will show additional output)
Given a primary use is for situations where code is patched or dynamically altered at runtime - then this can affect the full decode output. If the code is being patched to only alter the branch addresses then decode should work against static images. If, however, we are tracing code that adds in new branches, on top of NOPs for example, then the decoding against the original static image will be wrong, as the image will have the NOPs, rather than the branch instructions so the apparent location of E atoms will be in a different position to the actual code. Is there anything in perf that will ensure that the patched code is presented to the decoder?
If there are potential decode issues - these too need documenting.
Other than the documents and testing, I cannot see any issues with this patch set in terms of setting and enabling the option.
Regards
Mike
On Fri, 10 Dec 2021 at 17:22, Mathieu Poirier mathieu.poirier@linaro.org wrote:
Hi James,
On Thu, Dec 09, 2021 at 11:13:55AM +0000, James Clark wrote:
On 09/12/2021 11:00, Suzuki K Poulose wrote:
On 08/12/2021 16:09, James Clark wrote:
Maintain consistency with the other options by failing to open when they aren't supported. For example ETM_OPT_TS, ETM_OPT_CTXTID2 and the newly added ETM_OPT_BRANCH_BROADCAST all return with -EINVAL if they are requested but not supported by hardware.
The consequence of not doing this is that the user may not be aware that they are not enabling the feature as it is silently disabled.
Signed-off-by: James Clark james.clark@arm.com
drivers/hwtracing/coresight/coresight-etm4x-core.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c index d2bafb50c66a..0a9bb943a5e5 100644 --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c @@ -674,10 +674,15 @@ static int etm4_parse_event_config(struct coresight_device *csdev, } /* return stack - enable if selected and supported */
- if ((attr->config & BIT(ETM_OPT_RETSTK)) && drvdata->retstack)
/* bit[12], Return stack enable bit */config->cfg |= BIT(12);
- if (attr->config & BIT(ETM_OPT_RETSTK)) {
if (!drvdata->retstack) {ret = -EINVAL;goto out;} else {/* bit[12], Return stack enable bit */config->cfg |= BIT(12);}nit: While at this, please could you change the hard coded value to ETM4_CFG_BIT_RETSTK ?
I started changing them all because I had trouble searching for bits by name but then I thought it would snowball into a bigger change so I undid it.
I think I'll just go and do it now if it's an issue here.
I can apply this set right away and you send another patch to fix all hard coded bitfields or you can send another revision with all 4 patches included in it (bitfields fix plus these 3). Just let me know what you want to do. And next time please add a cover letter.
Thanks, Mathieu
Otherwise, looks good to me
Suzuki