linux-kernel - Re: [RFC PATCH v2 19/27] PCI: dwc: ep: Cache MSI outbound iATU mapping

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Message-ID: <a7b94f8f-8773-43b0-b481-29828aba9abd@oss.qualcomm.com>
Date: Mon, 22 Dec 2025 13:44:02 +0530
From: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
To: Niklas Cassel <cassel@...nel.org>
Cc: Koichiro Den <den@...inux.co.jp>, ntb@...ts.linux.dev,
        linux-pci@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org, Frank.Li@....com, mani@...nel.org,
        kwilczynski@...nel.org, kishon@...nel.org, bhelgaas@...gle.com,
        corbet@....net, vkoul@...nel.org, jdmason@...zu.us,
        dave.jiang@...el.com, allenbh@...il.com, Basavaraj.Natikar@....com,
        Shyam-sundar.S-k@....com, kurt.schwemmer@...rosemi.com,
        logang@...tatee.com, jingoohan1@...il.com, lpieralisi@...nel.org,
        robh@...nel.org, jbrunet@...libre.com, fancer.lancer@...il.com,
        arnd@...db.de, pstanner@...hat.com, elfring@...rs.sourceforge.net
Subject: Re: [RFC PATCH v2 19/27] PCI: dwc: ep: Cache MSI outbound iATU
 mapping



On 12/22/2025 1:20 PM, Niklas Cassel wrote:
> On Mon, Dec 22, 2025 at 10:40:12AM +0530, Krishna Chaitanya Chundru wrote:
>> On 12/8/2025 1:27 PM, Niklas Cassel wrote:
>>> On Sun, Nov 30, 2025 at 01:03:57AM +0900, Koichiro Den wrote:
>>>
>>> I guess the problem is that some EPF drivers, even if only
>>> one capability can be enabled (MSI/MSI-X), call both
>>> pci_epc_set_msi() and pci_epc_set_msix(), e.g.:
>>> https://github.com/torvalds/linux/blob/v6.18/drivers/pci/endpoint/functions/pci-epf-test.c#L969-L987
>>>
>>> To fill in the number of MSI/MSI-X irqs.
>>>
>>> While other EPF drivers only call either pci_epc_set_msi() or
>>> pci_epc_set_msix(), depending on the IRQ type that will actually
>>> be used:
>>> https://github.com/torvalds/linux/blob/v6.18/drivers/nvme/target/pci-epf.c#L2247-L2262
>>>
>>> I think both versions is okay, just because the number of IRQs
>>> is filled in for both MSI/MSI-X, AFAICT, only one of them will
>>> get enabled.
>>>
>>>
>>> I guess it might be hard for an EPC driver to know which capability
>>> that is currently enabled, as to enable a capability is only a config
>>> space write by the host side.
>> As the host is the one which enables MSI/MSIX, it should be better the
>> controller
>> driver takes this decision and the EPF driver just sends only raise_irq.
>> Because technically, host can disable MSI and enable MSIX at runtime also.
>>
>> In the controller driver,  it can check which is enabled and chose b/w
>> MSIX/MSI/Legacy.
> I'm not sure if I'm following, but if by "the controller driver", you
> mean the EPC driver, and not the host side driver, how can the EPC
> driver know how many interrupts a specific EPF driver wants to use?
I meant the dwc drivers here.
Set msi & set msix still need to called from the EPF driver only to tell 
how many
interrupts they want to configure etc.
>
>  From the kdoc to pci_epc_set_msi(), the nr_irqs parameter is defined as:
> @nr_irqs: number of MSI interrupts required by the EPF
> https://github.com/torvalds/linux/blob/v6.19-rc2/drivers/pci/endpoint/pci-epc-core.c#L305
>
>
> Anyway, I posted Koichiro's patch here:
> https://lore.kernel.org/linux-pci/20251210071358.2267494-2-cassel@kernel.org/
I will comment on that patch.
>
> See my comment:
>    pci-epf-test does change between MSI and MSI-X without calling
>    dw_pcie_ep_stop(), however, the msg_addr address written by the host
>    will be the same address, at least when using a Linux host using a DWC
>    based controller. If another host ends up using different msg_addr for
>    MSI and MSI-X, then I think that we will need to modify pci-epf-test to
>    call a function when changing IRQ type, such that pcie-designware-ep.c
>    can tear down the MSI/MSI-X mapping.
Maybe for arm based systems we are getting same address but for x86 
based systems
it is not guarantee that you will get same address.
> So if we want to improve things, I think we need to modify the EPF drivers
> to call a function when changing the IRQ type. The EPF driver should know
> which IRQ type that is currently in use (see e.g. nvme_epf->irq_type in
> drivers/nvme/target/pci-epf.c).
My suggestion is let EPF driver call raise_irq with the vector number 
then the dwc driver
can raise IRQ based on which IRQ host enables it.
> Additionally, I don't think that the host side should be allowed to change
> the IRQ type (using e.g. setpci) when the EPF driver is in a "running state".
In the host driver itelf they can choose to change it by using 
pci_alloc_irq_vectors 
<https://elixir.bootlin.com/linux/v6.18.2/C/ident/pci_alloc_irq_vectors>, 
Currently it is not present but in future someone can change it, as spec 
didn't say you
can't update it.
> I think things will break badly if you e.g. try to do this on an PCIe
> connected network card while the network card is in use.
I agree on this.

I just want to highlight there is possibility of this in future, if 
someone comes up with a
clean logic.

- Krishna Chaitanya.
>
>
> Kind regards,
> Niklas