[PATCH 18/19] coresight: etm4x: Add support for sysreg only devices

[Suzuki K Poulose]

Hi Mike

On 09/18/2020 04:35 PM, Mike Leach wrote:
> Hi Suzuki,
> 
> On Fri, 11 Sep 2020 at 09:41, Suzuki K Poulose <suzuki.poulose at arm.com> wrote:
>>
>> Add support for devices with system instruction access only.
>> They don't have a memory mapped interface and thus are not
>> AMBA devices.
>>
>> Cc: Mathieu Poirier <mathieu.poirier at linaro.org>
>> Cc: Mike Leach <mike.leach at linaro.org>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose at arm.com>
>> ---
>>   drivers/hwtracing/coresight/coresight-etm4x.c | 42 ++++++++++++++++++-
>>   1 file changed, 40 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/hwtracing/coresight/coresight-etm4x.c b/drivers/hwtracing/coresight/coresight-etm4x.c
>> index 7d5f942c2108..212713ffa37e 100644
>> --- a/drivers/hwtracing/coresight/coresight-etm4x.c
>> +++ b/drivers/hwtracing/coresight/coresight-etm4x.c
>> @@ -26,6 +26,7 @@
>>   #include <linux/seq_file.h>
>>   #include <linux/uaccess.h>
>>   #include <linux/perf_event.h>
>> +#include <linux/platform_device.h>
>>   #include <linux/pm_runtime.h>
>>   #include <linux/property.h>
>>   #include <asm/sections.h>
>> @@ -1712,6 +1713,20 @@ static int etm4_probe_amba(struct amba_device *adev, const struct amba_id *id)
>>          return ret;
>>   }
>>
>> +static int etm4_probe_platform_dev(struct platform_device *pdev)
>> +{
>> +       int ret;
>> +
>> +       pm_runtime_get_noresume(&pdev->dev);
>> +       pm_runtime_set_active(&pdev->dev);
>> +       pm_runtime_enable(&pdev->dev);
>> +
> 
> Right about here is where I would expect the sysreg access to
> TRCDEVARCH etc, to determine if this is an ETM device that can be
> supported by the driver.
> This matches approximately the similar ID table checks that the AMBA
> driver did to ensure a valid device match.

The problem is, we have to do this on the target CPU and later do another
one for feature check.

> This logically separates "is this a device we support" from "what
> features does this supported device have"

While I understand the logical argument, it doesn't buy us much. Even now we do an
additional check on the supported architecture in the etm4x_probe() anyway and reject
the unsupported CPUs there. The only change here is we move the supported architecture
check in to the etm4_init_arch_data() and stop the hard work if it is not supported.

I would prefer to keep the current method if possible, while cleaning up the detection
of the supported (old) devices as agreed in the other patch.


Cheers
Suzuki