Re: [PATCH] PCI: dwc: Move allocate and map page for msi out of dw_pcie_msi_init()

From: Ard Biesheuvel <ardb@kernel.org>
To: Rob Herring <robh@kernel.org>
Cc: Jisheng Zhang <Jisheng.Zhang@synaptics.com>,
	Kishon Vijay Abraham I <kishon@ti.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Jingoo Han <jingoohan1@gmail.com>,
	Gustavo Pimentel <gustavo.pimentel@synopsys.com>,
	Thierry Reding <treding@nvidia.com>,
	Vidya Sagar <vidyas@nvidia.com>, PCI <linux-pci@vger.kernel.org>,
	linux-omap <linux-omap@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] PCI: dwc: Move allocate and map page for msi out of dw_pcie_msi_init()
Date: Thu, 24 Sep 2020 15:45:48 +0200	[thread overview]
Message-ID: <CAMj1kXH4aNZkST2QrKUDTPRPVq6Ybm0O0GLTwZiBrt-7hfYfVA@mail.gmail.com> (raw)
In-Reply-To: <CAL_JsqLkQ_NqrDDJZkm5ef-mf4_Vh0sW1DqQjitz-GzGBNbWhA@mail.gmail.com>

On Thu, 24 Sep 2020 at 15:28, Rob Herring <robh@kernel.org> wrote:
>
> On Thu, Sep 24, 2020 at 5:00 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > On Wed, 23 Sep 2020 at 08:28, Jisheng Zhang <Jisheng.Zhang@synaptics.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@synaptics.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.