linux-kernel - Re: [PATCH] PCI: mediatek: Fix sparse warning caused to virt_to

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Message-ID: <9a774508cc54914bd50b649218a3a17b632f31d2.camel@mediatek.com>
Date:   Wed, 22 Nov 2023 04:15:59 +0000
From:   Jianjun Wang (王建军) 
        <Jianjun.Wang@...iatek.com>
To:     "skinsburskii@...ux.microsoft.com" <skinsburskii@...ux.microsoft.com>,
        "helgaas@...nel.org" <helgaas@...nel.org>
CC:     "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "skinsburskii@...il.com" <skinsburskii@...il.com>,
        "robh@...nel.org" <robh@...nel.org>, "kw@...ux.com" <kw@...ux.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "lpieralisi@...nel.org" <lpieralisi@...nel.org>,
        Ryder Lee <Ryder.Lee@...iatek.com>,
        "angelogioacchino.delregno@...labora.com" 
        <angelogioacchino.delregno@...labora.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH] PCI: mediatek: Fix sparse warning caused to
 virt_to_phys() prototype change

Hi,

On Tue, 2023-11-21 at 14:34 -0800, Stanislav Kinsburskii wrote:
>  	 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
>  On Tue, Nov 21, 2023 at 04:23:25PM -0600, Bjorn Helgaas wrote:
> > On Tue, Nov 21, 2023 at 02:05:56PM -0800, Stanislav Kinsburskii
> wrote:
> > > On Tue, Nov 21, 2023 at 03:37:55PM -0600, Bjorn Helgaas wrote:
> > > > On Mon, Nov 20, 2023 at 03:40:33PM -0800, Stanislav Kinsburskii
> wrote:
> > > > > Explicitly cast __iomem pointer to const void* with __force
> to fix the
> > > > > following warning:
> > > > > 
> > > > >   warning: incorrect type in argument 1 (different address
> spaces)
> > > > >      expected void const volatile *address
> > > > >      got void [noderef] __iomem *
> > > > 
> > > > I have two questions about this:
> > > > 
> > > >   1) There's no other use of __force in drivers/pci, so I don't
> know
> > > >   what's special about pcie-mediatek.c.  There should be a way
> to fix
> > > >   the types so it's not needed.
> > > 
> > > __force suppreses the following sparse warning:
> > > 
> > >     warning: cast removes address space '__iomem' of expression
> > 
> > I'm suggesting that the cast is a band-aid that covers up a type
> > mismatch, and there shouldn't be a mismatch in the first place.
> > 
> > > >   2) virt_to_phys() is not quite right to begin with because
> what we
> > > >   want is a *bus* address, not the CPU physical address we get
> from
> > > >   virt_to_phys().  Obviously the current platforms that use
> this must
> > > >   not apply any offset between bus and CPU physical addresses,
> but
> > > >   it's not something we should rely on.
> > > > 
> > > >   There are only three drivers (pci-aardvark.c, pcie-xilinx.c,
> and
> > > >   this one) that use virt_to_phys(), and they're all slightly
> wrong
> > > >   here.
> > > > 
> > > > The *_compose_msi_msg() methods could use a little more
> consistency
> > > > across the board.
> > > 
> > > Could you elaborate on what do you suggest?
> > > Should virt_to_phys() be simply removed?
> > 
> > The DMA API (Documentation/core-api/dma-api.rst) is the usual way
> to
> > get bus addresses, since an MSI is basically a DMA on the PCI bus.
> > 
> > https://lore.kernel.org/linux-pci/20230914203146.GA77870@bhelgaas/
> > 
> > Nobody is very motivated to fix these, I guess ;)  I sort of hate
> to
> > just throw in a cast to shut up the warning because it doesn't
> really
> > solve the problem.
> > 
> 
> This is fair.
> However I have neither expertize to assess, no hardware to test such
> intrusive
> changes.
> I guess it's a no-op then, is it?

I think I can try and test 'dmam_alloc_coherent()' or other proper APIs
on the MediaTek platform, and if it works as expected, I can send the
patch to fix this.

Thanks.

> 
> > > > > Signed-off-by: Stanislav Kinsburskii <
> skinsburskii@...ux.microsoft.com>
> > > > > ---
> > > > >  drivers/pci/controller/pcie-mediatek.c |    4 ++--
> > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/pci/controller/pcie-mediatek.c
> b/drivers/pci/controller/pcie-mediatek.c
> > > > > index 66a8f73296fc..27f0f79810a1 100644
> > > > > --- a/drivers/pci/controller/pcie-mediatek.c
> > > > > +++ b/drivers/pci/controller/pcie-mediatek.c
> > > > > @@ -397,7 +397,7 @@ static void mtk_compose_msi_msg(struct
> irq_data *data, struct msi_msg *msg)
> > > > >  phys_addr_t addr;
> > > > >  
> > > > >  /* MT2712/MT7622 only support 32-bit MSI addresses */
> > > > > -addr = virt_to_phys(port->base + PCIE_MSI_VECTOR);
> > > > > +addr = virt_to_phys((__force const void *)port->base +
> PCIE_MSI_VECTOR);
> > > > >  msg->address_hi = 0;
> > > > >  msg->address_lo = lower_32_bits(addr);
> > > > >  
> > > > > @@ -520,7 +520,7 @@ static void mtk_pcie_enable_msi(struct
> mtk_pcie_port *port)
> > > > >  u32 val;
> > > > >  phys_addr_t msg_addr;
> > > > >  
> > > > > -msg_addr = virt_to_phys(port->base + PCIE_MSI_VECTOR);
> > > > > +msg_addr = virt_to_phys((__force const void *)port->base +
> PCIE_MSI_VECTOR);
> > > > >  val = lower_32_bits(msg_addr);
> > > > >  writel(val, port->base + PCIE_IMSI_ADDR);