[RFC PATCH] riscv: Kconfig: do not select PCI_MSI if CONIFG_PCI is enabled

[Vincent Chen]

Just a gentle ping.
Does anyone have any comments on this RFC patch? If not, I will remove
the "note" part from the git description and resend it as a formal
patch. Thank you.

On Fri, Jul 9, 2021 at 11:12 AM Vincent Chen <vincent.chen at sifive.com> wrote:
>
> The CONFIG_PCI_MSI is used to allow device drivers to enable MSI. The MSI
> enables a device to generate an interrupt using an inbound Memory Write
> on its PCI bus instead of asserting a device IRQ pin. The whole mechanism
> needs support from the PCI controller or generic interrupt controller and
> the corresponding software driver.
>
> The RISC-V Kconfig file actively selects the PCI_MSI  if users enable
> CONFIG_PCI. However, the RISC-V specification does not require every RISC-V
> platform shall have MSI support. In other words, Kconfig enables CONFIG_PCI
> to allow PCI devices to use MSI, but due to lack of MSI support, the kernel
> may not have any function to deal with the MSI from PCI devices. When this
> case happens, it leads to the following warning message displayed in
> booting a v5.10~v5.12 kernel.
>
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 1 at include/linux/msi.h:219 __pci_enable_msix_range+0x4b6/0x50e
> Modules linked in:
> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.10.34 #76
> epc: ffffffe0004fa1e0 ra : ffffffe0004fa02e sp : ffffffe07fea7920
>  gp : ffffffe0010e25c0 tp : ffffffe07fea8000 t0 : ffffffe0808c3180
>  t1 : ffffffd000239000 t2 : 0000000000000400 s0 : ffffffe07fea79e0
>  s1 : ffffffe080803000 a0 : 0000000000000000 a1 : ffffffe080803210
>  a2 : ffffffe0808c30c0 a3 : 0000000000000101 a4 : ffffffe0808c30c0
>  a5 : 0000000000000010 a6 : ffffffe0004dcf98 a7 : ffffffe07fe0f010
>  s2 : 0000000000000002 s3 : 0000000000000014 s4 : 0000000000000000
>  s5 : ffffffe080803210 s6 : 0000000000000000 s7 : ffffffe0808c3120
>  s8 : ffffffe0808c30c0 s9 : 0000000000000000 s10: 0000000000000002
>  s11: ffffffe0808030b0 t3 : 0000000000001000 t4 : 0000000000001000
>  t5 : ffffffd040239000 t6 : ffffffe07ff8b2c0
> status: 0000000000000120 badaddr: 0000000000000000 cause: 0000000000000003
>
> A simple reproduce way is to use defconfig to configure the 64-bit riscv
> v5.10 ~ v5.12 kernel but disable CONFIG_PCIE_XILINX, and then booting this
> kernel on the QEMU virt platform with virtio-mouse-pci.
>
> ARM's implementation may be a good sample for this case. Its Kconfig file
> does not select CONFIG_PCI_MSI. Instead, It makes the selection of
> CONFIG_PCI_MSI depends on the capability of the interrupt controller. This
> way seems to be more straightforward. Therefore, this patch follows the
> same way to remove the dependency between CONFIG_PCI and CONFIG_PCI_MSI
> from Kconfig, which allows users to enable or disable CONFIG_PCI_MSI
> according to the capabilities of the platform.
>
> (note) Why does this warning message only happen in the v5.10 to v5.12 kernel?
> When the kernel wants to register the MSI ID of the first PCI device, if
> the PCI device does not belong to any MSI domain and there is no valid
> arch_setup_msi_irqs() defined by architecture or PCI controller, the
> generic arch_setup_msi_irqs() will be called to display this warning
> message. However, before v5.10, this flow is different. A valid and weak
> arch_setup_msi_irqs() is defined in driver/pci/msi.c. The kernel can use
> this arch_setup_msi_irqs() to set up the MSI. Therefore, it will not
> encounter any problems even if the architecture and PCI controller does
> not define it. After v5.13, the kernel uses the MSI domain to deal with
> all the setup of MSI issues. Only some old PCI controller still uses
> arch_setup_msi_irqs(). Through the hierarchy of the MSI domain, the kernel
> can know the bus used by the PCI device has MSI support or not before
> doing the MSI setup. In this condition, the kernel will return early before
> calling the generic arch_setup_msi_irqs(), which will trigger the warning
> message.
>
> Signed-off-by: Vincent Chen <vincent.chen at sifive.com>
> ---
>  arch/riscv/Kconfig | 1 -
>  1 file changed, 1 deletion(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 469a70bd8da6..563d550cb682 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -97,7 +97,6 @@ config RISCV
>         select OF_EARLY_FLATTREE
>         select OF_IRQ
>         select PCI_DOMAINS_GENERIC if PCI
> -       select PCI_MSI if PCI
>         select RISCV_INTC
>         select RISCV_TIMER if RISCV_SBI
>         select SPARSE_IRQ
> --
> 2.7.4
>