Initial values of dtmcs.dmistat and dmi.op

[Peter Ashenden]

Folks,

We've just been bringing up a new debug module and debug JTAG transport design with OpenOCD and encountered an issue with initialization. We're implementing debug spec Version 0.13-DRAFT and using the OpenOCD driver for V0.13.

In the debug spec, dtmcs.dmistat is specified as having a reset value of 0, and dmi.op as having a reset value of 2 (error). We found that OpenOCD does a DMI read during its init, but sees the op=2 value in the first dmi capture and thinks this is a failure. When we worked around this by doing a dmireset before the first DMI access, it all went ok.

So questions arising:

• Why is OpenOCD surprised to see op=2 initially, when that's what the spec says it should be?
• Why are dmi.op and dtmcs.dmistat reset to different values? If they were the same, dtmcs.dmistat could just be a mirror of dmi.op capture value.

Cheers,

PA

-- 
Peter Ashenden, CTO IC Design, ASTC

[Tim Newsome]

On Tue, Sep 25, 2018 at 5:17 PM Peter Ashenden <peter.a...@astc-design.com> wrote:

We've just been bringing up a new debug module and debug JTAG transport design with OpenOCD and encountered an issue with initialization. We're implementing debug spec Version 0.13-DRAFT and using the OpenOCD driver for V0.13.

In the debug spec, dtmcs.dmistat is specified as having a reset value of 0, and dmi.op as having a reset value of 2 (error). We found that OpenOCD does a DMI read during its init, but sees the op=2 value in the first dmi capture and thinks this is a failure. When we worked around this by doing a dmireset before the first DMI access, it all went ok.

So questions arising:

• Why is OpenOCD surprised to see op=2 initially, when that's what the spec says it should be?

Sounds like a bug in OpenOCD. 

• Why are dmi.op and dtmcs.dmistat reset to different values? If they were the same, dtmcs.dmistat could just be a mirror of dmi.op capture value.

I don't even know why dtmcs.dmistat exists. Probably left over from an older interface.

I agree that they should be the same value. Do you prefer to reset to 0, or to 2? Probably existing implementations use 0, because OpenOCD works with that.

Tim 

Cheers,

PA

-- 
Peter Ashenden, CTO IC Design, ASTC

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[Peter Ashenden]

Hi Tim,

I would prefer 0. The spec is that 2 indicates a previous operation
failed. If there was no previous operation, there was no previous
failure. Thanks.

PA

On 26/09/2018 09:54, Tim Newsome wrote:
> I don't even know why dtmcs.dmistat exists. Probably left over from an
> older interface.
> I agree that they should be the same value. Do you prefer to reset to
> 0, or to 2? Probably existing implementations use 0, because OpenOCD
> works with that.

[Tim Newsome]

I've created https://github.com/riscv/riscv-debug-spec/pull/369 to make this change.

Tim

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