linux-kernel - Re: [RFC PATCH] coresight: etm4x: Match all ETM4 instances based on DEVARCH and DEVTYPE

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Message-ID: <50399db6-f7f0-8e9d-cc13-fe4fbc0287e1@arm.com>
Date:   Fri, 2 Jun 2023 17:11:59 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Mike Leach <mike.leach@...aro.org>
Cc:     coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
        james.clark@....com, anshuman.khandual@....com,
        linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        frowand.list@...il.com, linux@...linux.org.uk
Subject: Re: [RFC PATCH] coresight: etm4x: Match all ETM4 instances based on
 DEVARCH and DEVTYPE

On 01/06/2023 16:59, Mike Leach wrote:
> HI Suzuki,
> 
> On Thu, 25 May 2023 at 10:58, Suzuki K Poulose <suzuki.poulose@....com> wrote:
>>
>> All,
>>
>> This is an RFC patch to allow all ETM4 instances to be detected via AMBA driver
>> without having to add the PIDs to the list. The AMBA driver already supports
>> checking the DEVTYPE and DEVARCH registers for CoreSight components. This patch
>> adds a pid,mask value that is bound to match all PIDs (with PIDR2.JEDEC field
>> mandated to be RA0).
>>
>> With this patch, we wouldn't need to add the PIDs for newer CPUs to be able to
>> use them. An entry in the device tree is all we need. The only side effect of
>> this patch is :
>>      If a DT description exists for an ETM and the CPU ETM has an erratum, the
>>      driver may still probe it and use it. But then the DT shouldn't have
>>      described it in the first place.
>>
> 
> Don't think this is an issue.
> 
> In the previous mechanism, with an ETM with an erratum - or indeed
> need of some arch specific extension as we allow now - someone would
> have added the PID - tested it, hit the erratum, and would have to
> investigate and fix according to what is required. This changes
> nothing in terms of handling errata on ETM hardware - it just removes
> the add PID step.
> 
> For new ETM that work out of the box, this saves time re-spinning the
> driver every time - which is kind of what we want from device tree!
> 
> I'd go for it.
> 
> Moreover, the same principle could be added to the CTI drivers -
> though these are generally pretty standard anyway (i.e. based on
> Coresight Soc 600/400), so may be no pressing need for this right now.
> 

Thank you Mike, I will send it as a proper patch.

Cheers
Suzuki