Re: [PATCH] bus: mhi: Disable IRQs instead of freeing them during power down
From: Jeffrey Hugo
Date: Mon Jun 20 2022 - 15:59:23 EST
On 6/16/2022 2:59 PM, Manivannan Sadhasivam wrote:
On Thu, Jun 16, 2022 at 09:53:34AM -0600, Jeffrey Hugo wrote:
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.
No. When the PCI device gets removed during runtime, the remove() callback will
get called with relevant "struct pci_dev" and that should take care of all
resource cleanup for that particular device (including free_irq).
That is what I expected, so I was confused. Seems like we are on the
same page now.
You do not need to manually rmmod the driver as that will be done by the
hotplug driver when there are no devices making use of it. And yes, the commit
message needs to be changed. >
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?
Yep!
Reviewed, with additional comments. I guess I remove my NACK, but there
is a lot to address with v2.