Re: [PATCH v2] riscv: Kconfig: do not select PCI_MSI if CONIFG_PCI is enabled

From: Vincent Chen <vincent.chen@sifive.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-riscv <linux-riscv@lists.infradead.org>,
	Palmer Dabbelt <palmer@dabbelt.com>
Subject: Re: [PATCH v2] riscv: Kconfig: do not select PCI_MSI if CONIFG_PCI is enabled
Date: Mon, 2 Aug 2021 13:23:27 +0800	[thread overview]
Message-ID: <CABvJ_xhm=ZVfEpzQxTp7=OUrxxjdcJJmHzdwVcRmLuDqfdEYzg@mail.gmail.com> (raw)
In-Reply-To: <CABvJ_xiGZ29LQ+YFnraJbTSBihKi7t7VSDF0+AXcFQzA34VvtA@mail.gmail.com>

Just a gentle ping. If there are still problems with this patch,
please let me know. Thank you.

On Thu, Jul 22, 2021 at 10:21 AM Vincent Chen <vincent.chen@sifive.com> wrote:
>
> On Wed, Jul 21, 2021 at 1:57 PM Christoph Hellwig <hch@infradead.org> wrote:
> >
> > On Wed, Jul 21, 2021 at 11:39:12AM +0800, Vincent Chen 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.
> >
> > So make sure the runtime detection works fine.  An x86 kernel with
> > CONFIG_PCI_MSI also works on non-MSI systems.
> >
> > NAK to this patch.
>
> Thanks for your information. I think this will be a good reference to
> solve this warning message for the v5.10~v5.12 riscv kernel. However,
> the current RISC-V does not specify that each platform must have a
> controller to support PCI_MSI. Even there is no a ratified
> specification to provide an interrupt controller to support MSI. In
> this circumstance, removing the CONFIG_PCI_MSI selection from the
> Kconfig file seems to more close to the RISC-V platform setting. I
> guess that is why most architectures, such as ARM and x86, do not add
> "select PCI_MSI if PCI" to their Kconfig file. Therefore, I prefer to
> remove the "select PCI_MSI if PCI" from the Kconfig file. Please
> correct me if I have misunderstands.
>
> Thank you.

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