Re: [RFC PATCH 00/11] Refactor MSI to support Non-PCI device

[Yijing Wang <wangyijing@huawei.com>]

>>> The method you describe here makes sense for PCI devices that are required to support
>>> legacy interrupts and may or may not support MSI on a given system, but not so much
>>> for platform devices for which we know exactly whether we want to use MSI
>>> or legacy interrupts.
>>>
>>> In particular if you have a device that can only do MSI, the entire pci_enable_msi
>>> step is pointless: all we need to do is program the correct MSI target address/message
>>> pair into the device and register the handler.
>>
>> Yes, I almost agree if we won't change the existing hundreds of drivers, what
>> I worried about is some drivers may want to know the IRQ numbers, and use the IRQ
>> number to process different things, as I mentioned in another reply.
>> But we can also provide the interface which integrate MSI configuration and request_irq(),
>> if most drivers don't care the IRQ number.
> 
> The driver would still have the option of getting the IRQ number for now: With
> the interface I imagine, you would get a 'struct msi_desc' pointer, from which
> you can look up the 'struct irq_desc' pointer (either embedded in msi_desc,
> or using a pointer from a member of msi_desc), and you can already get the
> interrupt number from the irq_desc.
> 
> My point was that a well-written driver already does not care about the interrupt
> number: the only information a driver needs in the interrupt handler is a pointer
> to its own context, which we already derive from the irq_desc.

Agree, I will try to introduce this similar interface in next version, thanks!

> 
> The main interface that currently requires the irq number is free_irq(), but
> I would argue that we can just add a wrapper that takes the msi_desc pointer
> as its first argument so the driver does not have to worry about it.
> 
> We can add additional wrappers like that as needed.

OK

>>>> This is a huge change for device drivers, and some device drivers don't know which msi_chip
>>>> their MSI irq deliver to. I'm reworking the msi_chip, and try to use msi_chip to eliminate
>>>> all arch_msi_xxx() under every arch in kernel. And the important point is how to create the
>>>> binding for the MSI device to the target msi_chip.
>>>
>>> Which drivers are you thinking of? Again, I wouldn't expect to change any PCI drivers,
>>> but only platform drivers that do native MSI, so we only have to change drivers that
>>> do not support any MSI at all yet and that need to be changed anyway in order to add
>>> support.
>>
>> I mean platform device drivers, because we can find the target msi_chip by some platform
>> interfaces(like the existing of_pci_find_msi_chip_by_node()). So we no need to explicitly
>> provide the msi_chip as the function argument.
> 
> Right, that works too. I was thinking we might need an interface that allows us to
> pick a particular msi_chip if there are several alternatives (e.g. one in the GIC
> and one in the PCI host), but you are right: we should normally be able to hardwire
> that information in DT or elsewhere, and just need the 'struct device pointer' which
> should probably be the first argument here.
> 
> As you pointed out, it's common to have multiple MSIs for a single device, so we
> also need a context to pass around, so my suggestion would become something like:
> 
> struct msi_desc *msi_request(struct device *dev, irq_handler_t handler,
>  			unsigned long flags, const char *name, void *data);
> 
> It's possible that we have to add one or two more arguments here.

Good suggestion, thanks!

> 
>>> A degenerate case of this would be a system where a PCI device sends its MSI into
>>> the host controller, that generates a legacy interrupt and that in turn gets 
>>> sent to an irqchip which turns it back into an MSI for the GICv3. This would of
>>> course be very inefficient, but I think we should be able to express this with
>>> both the binding and the in-kernel framework just to be on the safe side.
>>
>> Yes, the best way to tell the kernel which msi_chip should deliver to is describe
>> the binding in DTS file. If a real degenerate case found, we can update the platform
>> interface which is responsible for getting the match msi_chip in future.
> 
> Ok.
> 
> 	Arnd
> 
> .
> 


-- 
Thanks!
Yijing