linux-kernel - Re: [PATCH] PCI: dwc: Move allocate and map page for msi out of dw_pcie_msi

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Date:   Thu, 24 Sep 2020 15:45:48 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Jisheng Zhang <Jisheng.Zhang@...aptics.com>,
        Kishon Vijay Abraham I <kishon@...com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Thierry Reding <treding@...dia.com>,
        Vidya Sagar <vidyas@...dia.com>,
        PCI <linux-pci@...r.kernel.org>,
        linux-omap <linux-omap@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] PCI: dwc: Move allocate and map page for msi out of dw_pcie_msi_init()

On Thu, 24 Sep 2020 at 15:28, Rob Herring <robh@...nel.org> wrote:
>
> On Thu, Sep 24, 2020 at 5:00 AM Ard Biesheuvel <ardb@...nel.org> wrote:
> >
> > On Wed, 23 Sep 2020 at 08:28, Jisheng Zhang <Jisheng.Zhang@...aptics.com> wrote:
> > >
> > > Currently, dw_pcie_msi_init() allocates and maps page for msi, then
> > > program the PCIE_MSI_ADDR_LO and PCIE_MSI_ADDR_HI. The Root Complex
> > > may lose power during suspend-to-RAM, so when we resume, we want to
> > > redo the latter but not the former. If designware based driver (for
> > > example, pcie-tegra194.c) calls dw_pcie_msi_init() in resume path, the
> > > previous msi page will be leaked.
> > >
> > > Move the allocate and map msi page from dw_pcie_msi_init() to
> > > dw_pcie_host_init() to fix this problem.
> > >
> > > Fixes: 56e15a238d92 ("PCI: tegra: Add Tegra194 PCIe support")
> > > Signed-off-by: Jisheng Zhang <Jisheng.Zhang@...aptics.com>
> >
> > Why do you allocate a page for this in the first place? Isn't
> > PCIE_MSI_ADDR_HI:PCIE_MSI_ADDR_LO simply a magic DMA address that
> > never gets forwarded across to the CPU side of the host bridge, and
> > triggers a SPI instead, which gets handled by reading
> > PCIE_MSI_INTR0_STATUS ?
>
> My question too after digging into this some more. I've asked the
> question on the thread that further complicated all this changing from
> virt_to_phys() to dma_map_page()[1].
>
> > Couldn't you just map the zero page instead?
>
> Why a page even? You could use PCIE_MSI_ADDR_LO address itself even.
> Or just an address in the driver data which is what some other drivers
> do.
>

PCIE_MSI_ADDR_LO itself could collide with an actual DRAM address if
any translation is applied on inbound transactions. Using an actual
DRAM address avoids that.