Re: [PATCH] PCI: dwc: skip MSI init if MSIs have been explicitly disabled

[Gustavo Pimentel <gustavo.pimentel@synopsys.com>]

Hi,

On 04/03/2019 20:18, Marc Zyngier wrote:
> Hi both,
> 
> On Mon, 04 Mar 2019 19:39:45 +0000,
> Lucas Stach <l.stach@pengutronix.de> wrote:
>>

(snipped)

>>>>
>>>> As MSI and legacy IRQs are already mutually exclusive on the DWC core,
>>>> as the core won't forward any legacy IRQs once any MSI has been enabled,
>>>> users wishing to use legacy IRQs already need to explictly disable MSI
>>>> support (usually via the pci=nomsi kernel commandline option). To avoid
>>>> any issues with MSI conflicting with legacy IRQs, just skip all of the
>>>> DWC MSI initalization, including the IRQ line claim, when MSI is disabled.
>>>
>>> Does this mean that if we have a device that uses legacy IRQs, the
>>> user has to figure out to boot with "pci=nomsi"?
>>
>> As long as there is only a single device connected and there are no
>> port services things will work. If port services are active, those will
>> start to use MSIs, breaking legacy IRQs in the process.
>>
>> I've asked Synopsys if there is a workaround for this, but it seems
>> that the core is working "as designed" with no workaround for this icky
>> behavior.
> 
> Is this the general DWC controller behaviour? Or something that is
> specific to a given implementation? I can't believe someone actually
> thought this is an acceptable behaviour...
> 
> Gustavo, can you please check with your HW colleagues and let
> everybody know what's the official Synopsys position on this?
> 

Sure, I can ask the HW team to provide me more info about this, This can take a
while. Unfortunately on my setup I only have MSI and MSI-X, therefore I can't
really test what has been statemented.

Regards,
Gustavo