AW: [PATCH] fpga: altera_cvp: restrict registration to CvP enabled devices

[Andreas Puhm]

Hi Moritz, Matthew,

>> Hi Anatolij, Andreas,
>>
>> On Tue, Oct 23, 2018 at 06:46:47PM +0000, Andreas Puhm wrote:
>>> Hi Anatolij,
>>>
>>>> The CvP docs says that on some FPGAs (e.g. Arria 10) the assertion of CVP
>>>> status can take up to 500ms. However it is not clear whether this delay
>>>> might be required after peripheral image configuration and after PCIe
>>>> link activation. The diagram describing configuration sequence suggests
>>>> that CVP_EN should be polled until it is asserted. I can imaging the
>>>> situation that this bit is still not asserted when the device is being
>>>> probed. Maybe we should better defer device probing if CVP_EN bit is
>>>> cleared? When deferred probing fails again and sufficient period for
>>>> CVP_EN bit assertion elapsed, then stop deferred probing and return
>>>> -ENODEV?
>>>>
>>>> Thanks,
>>>>
>>>> Anatolij
>>>
>>> Anatolij, thank you for your feedback.
>>>
>>> My rationale behind the patch is as follows:
>>>
>>> The CVP_EN is part of the Hard PCIe IP core configuration,
>>> and therefore, has a defined and static value right from "the start".
>>>
>>> Remark in [1, fig 12]
>>> " For high density devices such as Intel Cyclone 10 GX,
>>> it may be necessary to wait up to 500 ms for the CvP
>>> status register bit assertion."
>>> According to [2] the Cyclone 10 GX devices achieve proper operation
>>> within 100 ms (via the PCIe IP core and CvP).
>>>
>>> I think (and here the documentation is a bit lacking),
>>> that this remark is valid only for other bits of the status register,
>>> e.g., CVP_CONFIG_DONE or USERMODE.
>>> I also think, that the 500 ms delay is calculated from peripheral + core image programming
>>> and that the time for peripheral image programming is far lower than that
>>> (i.e., low enough to allow PCI enumeration).
>>>
>>> But if this actually means that it can take up to 500 ms to program the peripheral image,
>>> than such FPGAs would have different problems.
>>> I.e., missing the deadline for PCI enumeration.
>>> This would need a solution outside of the scope of the
>>> altera_cvp module (e.g., soft-reset to re-start enumeration with a stable system).
>>>
>>> Bottom line:
>>> The CVP_EN should be deemed stable when altera_cvp is called,
>>> if not,
>>> the programming of the Intel/Altera FPGA and PCIe IP core has not been completed in time
>>> for the enumeration of the PCI device. Hence it would be questionable or, more likely, would not
>>> have completed successfully in the first place, i.e., altera_cvp would not have been called.
>>
>> Yeah I think this makes sense. If your config space isn't up on boot you
>> would run into issues. I agree the docs are soemwhat vague here. Maybe Matthew or Alan can shoot
>> an email to their HW folks internally to clarify?
>
>My experience with cvp is with Arria10 and Stratix 10.  The PCIe Hard IP
>gets configured when the IOring gets configured at power on.  The idea is
>that the load of the IOring is very fast, much before the infamous 100ms
>PCIe timeout for link training.  When the Hard IP is configured, the
>CVP_EN is set or cleared according to how it was configured.  Yes, you

So is it correct that the value of CVP_EN can be evaluated by the altera_cvp right in the first call of its probe function
(as would be the case with my proposed patch).

If it is, I will fix the remaining issues with the patch and submit it.

>will run into issues if config space is not up on boot.  The exact nature
>of the issues is dependent on the platform being used.
>
>For the record, if cvp is not being used then the initial full
>configuration of the FPGA can take much longer than just configuring the
>IOring.  In the worst case, if the initial FPGA image in flash is
>corrupted, it can take a while before the failover image gets configured
>into the fpga.  This might be the explanation for the 500 ms for Cyclone
>10 GX devices.  For an Arria10, the flash failover can take much longer
>than even the 500ms, which has been shown to have issues for many platforms.
>
>Thanks,
>Matthew
>
>>
>> Thanks,
>> Moritz
>>

Thanks,
Andreas