[virtio-dev] [PATCH] virtio-mmio: Specify wait needed in driver during reset

[virtio-dev] [PATCH] virtio-mmio: Specify wait needed in driver during reset

[Srivatsa Vaddagiri]

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This is following up on the discussion we had earlier on MMIO reset behavior:

https://lists.oasis-open.org/archives/virtio-dev/202107/msg00012.html

Suggesting below change to spec to make it explicit what's expected in driver. If this is accepted, I will follow up with a patch to Linux driver.

===

Device reset is accomplished by writing 0 to Status register. Explicitly note
that a driver needs to wait on Status register read returning 0 before assuming that
device reset operation is complete.

Signed-off-by: Srivatsa Vaddagiri <svaddagi@qti.qualcomm.com>

diff --git a/content.tex b/content.tex
index 5c70a3c..7631be4 100644
--- a/content.tex
+++ b/content.tex
@@ -1985,6 +1985,10 @@ \subsubsection{Device Initialization}\label{sec:Virtio Transport Options / Virti
 and if its value is zero (0x0) MUST abort initialization and
 MUST NOT access any other register.

+After writing 0 to \field{Status}, the driver MUST wait for a read of
+\field{Status} to return 0 before proceeding with the remaining steps of
+initializing the device.
+
 Drivers not expecting shared memory MUST NOT use the shared
 memory registers.


---

Qualcomm Innovation Center, Inc. is submitting the attached "feedback" as a non-member to the virtio-dev mailing list for consideration and inclusion.

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Re: [virtio-dev] [PATCH] virtio-mmio: Specify wait needed in driver during reset

[Jason Wang]

在 2021/7/22 下午5:19, Srivatsa Vaddagiri 写道:
> This is following up on the discussion we had earlier on MMIO reset 
> behavior:
>
> https://lists.oasis-open.org/archives/virtio-dev/202107/msg00012.html 
> <https://lists.oasis-open.org/archives/virtio-dev/202107/msg00012.html>
>
> Suggesting below change to spec to make it explicit what's expected in 
> driver. If this is accepted, I will follow up with a patch to Linux 
> driver.
>
> ===
>
> Device reset is accomplished by writing 0 to Status register. 
> Explicitly note
> that a driver needs to wait on Status register read returning 0 before 
> assuming that
> device reset operation is complete.
> Signed-off-by: Srivatsa Vaddagiri <svaddagi@qti.qualcomm.com>



I wonder if it deserves a feature bit.


Thanks


>
> diff --git a/content.tex b/content.tex
> index 5c70a3c..7631be4 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -1985,6 +1985,10 @@ \subsubsection{Device 
> Initialization}\label{sec:Virtio Transport Options / Virti
>  and if its value is zero (0x0) MUST abort initialization and
>  MUST NOT access any other register.
> +After writing 0 to \field{Status}, the driver MUST wait for a read of
> +\field{Status} to return 0 before proceeding with the remaining steps of
> +initializing the device.
> +
>  Drivers not expecting shared memory MUST NOT use the shared
>  memory registers.
>
>
> ---
>
> Qualcomm Innovation Center, Inc. is submitting the attached "feedback" 
> as a non-member to the virtio-dev mailing list for consideration and 
> inclusion.


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Re: [virtio-dev] [PATCH] virtio-mmio: Specify wait needed in driver during reset

[Cornelia Huck]

On Thu, Jul 22 2021, Jason Wang <jasowang@redhat.com> wrote:

> 在 2021/7/22 下午5:19, Srivatsa Vaddagiri 写道:
>> This is following up on the discussion we had earlier on MMIO reset 
>> behavior:
>>
>> https://lists.oasis-open.org/archives/virtio-dev/202107/msg00012.html 
>> <https://lists.oasis-open.org/archives/virtio-dev/202107/msg00012.html>
>>
>> Suggesting below change to spec to make it explicit what's expected in 
>> driver. If this is accepted, I will follow up with a patch to Linux 
>> driver.
>>
>> ===
>>
>> Device reset is accomplished by writing 0 to Status register. 
>> Explicitly note
>> that a driver needs to wait on Status register read returning 0 before 
>> assuming that
>> device reset operation is complete.
>> Signed-off-by: Srivatsa Vaddagiri <svaddagi@qti.qualcomm.com>
>
>
>
> I wonder if it deserves a feature bit.

Not sure what that feature bit should govern; if it is not negotiated,
the device must assume that the driver considers the reset to be
complete right after writing? I'm not sure what the device is supposed
to do then.

>
>
> Thanks
>
>
>>
>> diff --git a/content.tex b/content.tex
>> index 5c70a3c..7631be4 100644
>> --- a/content.tex
>> +++ b/content.tex
>> @@ -1985,6 +1985,10 @@ \subsubsection{Device 
>> Initialization}\label{sec:Virtio Transport Options / Virti
>>  and if its value is zero (0x0) MUST abort initialization and
>>  MUST NOT access any other register.
>> +After writing 0 to \field{Status}, the driver MUST wait for a read of
>> +\field{Status} to return 0 before proceeding with the remaining steps of
>> +initializing the device.

Maybe move this to the "MMIO Device Register Layout" driver normative
section.

Should that have a companion device normative statement (just as for
PCI)?

>> +
>>  Drivers not expecting shared memory MUST NOT use the shared
>>  memory registers.


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Re: [virtio-dev] [PATCH] virtio-mmio: Specify wait needed in driver during reset

[Jason Wang]

在 2021/7/22 下午6:57, Cornelia Huck 写道:
> On Thu, Jul 22 2021, Jason Wang <jasowang@redhat.com> wrote:
>
>> 在 2021/7/22 下午5:19, Srivatsa Vaddagiri 写道:
>>> This is following up on the discussion we had earlier on MMIO reset
>>> behavior:
>>>
>>> https://lists.oasis-open.org/archives/virtio-dev/202107/msg00012.html
>>> <https://lists.oasis-open.org/archives/virtio-dev/202107/msg00012.html>
>>>
>>> Suggesting below change to spec to make it explicit what's expected in
>>> driver. If this is accepted, I will follow up with a patch to Linux
>>> driver.
>>>
>>> ===
>>>
>>> Device reset is accomplished by writing 0 to Status register.
>>> Explicitly note
>>> that a driver needs to wait on Status register read returning 0 before
>>> assuming that
>>> device reset operation is complete.
>>> Signed-off-by: Srivatsa Vaddagiri <svaddagi@qti.qualcomm.com>
>>
>>
>> I wonder if it deserves a feature bit.
> Not sure what that feature bit should govern; if it is not negotiated,
> the device must assume that the driver considers the reset to be
> complete right after writing? I'm not sure what the device is supposed
> to do then.


The (hardware) device can refuse the feature negotiation if driver 
doesn't support the feature.

This makes sure the driver is not broken by the device.

Thanks


>
>>
>> Thanks
>>
>>
>>> diff --git a/content.tex b/content.tex
>>> index 5c70a3c..7631be4 100644
>>> --- a/content.tex
>>> +++ b/content.tex
>>> @@ -1985,6 +1985,10 @@ \subsubsection{Device
>>> Initialization}\label{sec:Virtio Transport Options / Virti
>>>   and if its value is zero (0x0) MUST abort initialization and
>>>   MUST NOT access any other register.
>>> +After writing 0 to \field{Status}, the driver MUST wait for a read of
>>> +\field{Status} to return 0 before proceeding with the remaining steps of
>>> +initializing the device.
> Maybe move this to the "MMIO Device Register Layout" driver normative
> section.
>
> Should that have a companion device normative statement (just as for
> PCI)?
>
>>> +
>>>   Drivers not expecting shared memory MUST NOT use the shared
>>>   memory registers.

Re: [virtio-dev] [PATCH] virtio-mmio: Specify wait needed in driver during reset

[Srivatsa Vaddagiri]

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> >> I wonder if it deserves a feature bit.
> > Not sure what that feature bit should govern; if it is not negotiated,
> > the device must assume that the driver considers the reset to be
> > complete right after writing? I'm not sure what the device is supposed
> > to do then.
>
>
> The (hardware) device can refuse the feature negotiation if driver
> doesn't support the feature.
>
> This makes sure the driver is not broken by the device.

Let me send the next patch with a new feature bit added. I guess I can use bit
40 for it (VIRTIO_F_MMIO_RESET_WAIT)?

- vatsa

---

Qualcomm Innovation Center, Inc. is submitting the attached "feedback" as a
non-member to the virtio-dev mailing list for consideration and inclusion.

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Re: [virtio-dev] [PATCH] virtio-mmio: Specify wait needed in driver during reset

[Srivatsa Vaddagiri]

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> > This is following up on the discussion we had earlier on MMIO reset
> > behavior:
> >
> > https://lists.oasis-open.org/archives/virtio-dev/202107/msg00012.html
> > <https://lists.oasis-open.org/archives/virtio-dev/202107/msg00012.html>
> >
> > Suggesting below change to spec to make it explicit what's expected in
> > driver. If this is accepted, I will follow up with a patch to Linux
> > driver.
> >
> > ===
> >
> > Device reset is accomplished by writing 0 to Status register.
> > Explicitly note
> > that a driver needs to wait on Status register read returning 0 before
> > assuming that
> > device reset operation is complete.
> > Signed-off-by: Srivatsa Vaddagiri <svaddagi@qti.qualcomm.com>
>
>
>
> I wonder if it deserves a feature bit.

I see this as fixing a bug in spec/code rather than adding any new feature, so
perhaps no.

- vatsa

---

Qualcomm Innovation Center, Inc. is submitting the attached "feedback" as a non-member to the virtio-dev mailing list for consideration and inclusion.


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