Re: [PATCH] PCI: dwc: ep: Cache MSI outbound iATU mapping

[Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>]

On 12/22/2025 2:06 PM, Niklas Cassel wrote:
> On Mon, Dec 22, 2025 at 01:49:21PM +0530, Krishna Chaitanya Chundru wrote:
>> On 12/10/2025 12:43 PM, Niklas Cassel wrote:
>>> @@ -786,14 +819,36 @@ int dw_pcie_ep_raise_msix_irq(struct dw_pcie_ep *ep, u8 func_no,
>>>    	}
>>>    	msg_addr = dw_pcie_ep_align_addr(epc, msg_addr, &map_size, &offset);
>>> -	ret = dw_pcie_ep_map_addr(epc, func_no, 0, ep->msi_mem_phys, msg_addr,
>>> -				  map_size);
>>> -	if (ret)
>>> -		return ret;
>>> -	writel(msg_data, ep->msi_mem + offset);
>>> +	/*
>>> +	 * Program the outbound iATU once and keep it enabled.
>>> +	 *
>>> +	 * The spec warns that updating iATU registers while there are
>>> +	 * operations in flight on the AXI bridge interface is not
>>> +	 * supported, so we avoid reprogramming the region on every MSI-X,
>>> +	 * specifically unmapping immediately after writel().
>>> +	 */
>>> +	if (!ep->msi_iatu_mapped) {
>> This is wrong, in MSIX each vector can give you different address, you can't
>> expect same address for
>> all the vectors in MSIX table. In ARM based system you might see only single
>> address for X86 this will
>> change.
> Ok, thank you. I did not know.
>
>
>> And also we see in MSIX the address are getting updated at runtime with x86
>> windows host machines.
> My idea was to add a pci_epc_set_currently_enabled_irq_type() API, and then
> let the EPF driver call that if it wants to change the IRQ type.
>
> But... if the msg_addr can change at runtime, even when the IRQ type does
> not change, then a pci_epc_set_currently_enabled_irq_type() API will not
> help.
>
> I guess we will need to come up with something else for the MSI-X case.
>
>
>> Use the MSIX doorbell method which will not use iATU at all,
>> dw_pcie_ep_raise_msix_irq_doorbell().
> AFAICT, right now, the only driver ever calling this function is:
> drivers/pci/controller/dwc/pci-layerscape-ep.c
>
> Are you suggesting that we somehow change all the other DWC based EPC
> drivers' .raise_irq() callback to call dw_pcie_ep_raise_msix_irq_doorbell()
> instead of dw_pcie_ep_raise_msix_irq() for case PCI_IRQ_MSIX ?
Yes.
> That sounds like a big change that would need to be tested and verified
> for each DWC based EPC driver.
I agree, but this will be clean solution to avoid iATU for MSIX.

- Krishna Chaitanya.
> Kind regards,
> Niklas