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Date:   Thu, 16 Jun 2022 09:53:34 -0600
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Subject: Re: [PATCH] bus: mhi: Disable IRQs instead of freeing them during
 power down
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To:     Manivannan Sadhasivam <mani@kernel.org>
CC:     Qiang Yu <quic_qianyu@quicinc.com>, <quic_hemantk@quicinc.com>,
        <loic.poulain@linaro.org>, <mhi@lists.linux.dev>,
        <linux-arm-msm@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <quic_cang@quicinc.com>
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From:   Jeffrey Hugo <quic_jhugo@quicinc.com>
In-Reply-To: <20220615211621.GD3606@thinkpad>
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On 6/15/2022 3:16 PM, Manivannan Sadhasivam wrote:
> On Mon, Jun 13, 2022 at 07:07:02AM -0600, Jeffrey Hugo wrote:
>> On 6/12/2022 7:48 PM, Qiang Yu wrote:
>>>
>>> On 6/10/2022 10:00 PM, Jeffrey Hugo wrote:
>>>> On 6/9/2022 9:21 PM, Qiang Yu wrote:
>>>>> On 6/9/2022 9:54 PM, Jeffrey Hugo wrote:
>>>>>
>>>>>> On 6/9/2022 7:43 AM, Qiang Yu wrote:
>>>>>>> EP tends to read MSI address/data once and cache them
>>>>>>> after BME is set.
>>>>>>> So host should avoid changing MSI address/data after BME is set.
>>>>>>>
>>>>>>> In pci reset function, host invokes free_irq(), which also clears MSI
>>>>>>> address/data in EP's PCIe config space. If the invalid address/data
>>>>>>> are cached and used by EP, MSI triggered by EP wouldn't be received by
>>>>>>> host, because an invalid MSI data is sent to an invalid MSI address.
>>>>>>>
>>>>>>> To fix this issue, after host runs request_irq() successfully during
>>>>>>> mhi driver probe, let's invoke enable_irq()/disable_irq() instead of
>>>>>>> request_irq()/free_irq() when we want to power on and power down MHI.
>>>>>>> Meanwhile, Host should invoke free_irq() when mhi host driver is
>>>>>>> removed.
>>>>>>
>>>>>> I don't think this works for hotplug, nor cases where there
>>>>>> are multiple MHI devices on the system.
>>>>>>
>>>>>> The EP shouldn't be caching this information for multiple
>>>>>> reasons. Masking the MSIs, disabling the MSIs, changing the
>>>>>> address when the affinity changes, etc.
>>>>>>
>>>>>> It really feels like we are solving the problem in the wrong place.
>>>>>>
>>>>>> Right now, this gets a NACK from me.
>>>>>>
>>>>> After free_irq(), MSI is still enabled but MSI address and data
>>>>> are cleared. So there is a chance that device initiates MSI
>>>>> using zero address. How to fix this race conditions.
>>>>
>>>> On what system is MSI still enabled?  I just removed the AIC100
>>>> controller on an random x86 system, and lspci is indicating MSIs are
>>>> disabled -
>>>>
>>>> Capabilities: [50] MSI: Enable- Count=32/32 Maskable+ 64bit+
>>>
>>> system: Ubuntu18.04, 5.4.0-89-generic,  Intel(R) Core(TM) i7-6700 CPU @
>>> 3.40GHz
>>>
>>> After removing MHI driver, I also see MSI enable is cleared.  But I
>>> don't think free_irq clears it. I add log before free_irq and after
>>> free_irq as following show:
>>>
>>> [62777.625111] msi cap before free irq
>>> [62777.625125] msi control=0x1bb, address=0xfee00318, data=0x0
>>> [62777.625301] msi cap after free irq
>>> [62777.625313] msi control=0x1bb, address=0x0, data=0x0
>>> [62777.625496] mhi-pci-generic 0000:01:00.0: mhi_pci_remove end of line,
>>> block 90 secs.
>>> # lspci -vvs 01:00.0
>>>           Capabilities: [50] MSI: Enable+ Count=8/32 Maskable+ 64bit+
>>>                   Address: 0000000000000000  Data: 0000
>>>                   Masking: ffffffff  Pending: 00000000
>>
>> At this point, the MSI functionality is still enabled, but every MSI is
>> masked out (Masking), so per the PCIe spec, the endpoint may not trigger a
>> MSI to the host.  The device advertises that it supports maskable MSIs
>> (Maskable+), so this is appropiate.
>>
>> If your device can still send a MSI at this point, then it violates the PCIe
>> spec.
>>
>> disable_irq() will not help you with this as it will do the same thing.
>>
>> I still think you are trying to fix an issue in the wrong location (host vs
>> EP), and causing additional issues by doing so.
>>
> 
> Irrespective of caching the MSI data in endpoint, I'd like to get rid of
> request_irq/free_irq during the mhi_{power_down/power_up} time. As like the MHI
> endpoint stack, we should just do disable/enable irq. Because, the MHI device
> may go down several times while running and we do not want to deallocate the
> IRQs all the time. And if the device gets removed, ultimately the MHI driver
> will get removed and we are fine while loading it back (even if MSI count
> changes).
> 
> I didn't had time to look into the patch in detail but I'm in favour of
> accepting the proposal.
> 
> @Jeff: Any specific issue you are seeing with hotplug etc...?

Perhaps I'm getting confused by the commit text of this change.

The issue described is that we free the irq, and then the EP sends a 
MSI, and the host doesn't receive it.  To me, that is expected.  The 
host doesn't care about the irq anymore because it freed it, therefore 
it would be expected that the host doesn't receive the irq.  So, the 
described issue is not an issue since it is expected behavior from what 
I can tell.

The proposed fix, is to disable the interrupts, and not free them until 
the driver is removed.  I interpret removing the driver as "rmmod mhi". 
  Based on this, the problem I see is a scenario where we have N devices 
in a system, and one device is hotplugged.  On hotplug, we would want to 
clean up all resources (free irq), but according to the description, we 
need to rmmod mhi, which is both not automatic and also affects the 
other N-1 devices which are presumed to be operational.

Now, if we throw all of that out the window, and say that the goal is to 
register the irqs when the controller is registered, free them when the 
controller is unregistered, and enable/disable based on power up/down as 
a optimization, that could be sane.  If that is what this change is 
attempting to do, it is not what the commit text describes.

Under the assumption that you want the optimization I just described, I 
will re-review the code next week when I get back from my travel. 
Assuming the implementation is good (other than what I've already 
pointed out), I think the commit text needs to be rewritten.

Does that clarify things for you?