linux-kernel - Re: [PATCH 02/20] PCI: fix pci_remap

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Message-ID: <20170320163843.GD24572@bhelgaas-glaptop.roam.corp.google.com>
Date:   Mon, 20 Mar 2017 11:38:43 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Liviu Dudau <Liviu.Dudau@....com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-pci@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Wenrui Li <wenrui.li@...k-chips.com>,
        Gabriele Paoloni <gabriele.paoloni@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Will Deacon <will.deacon@....com>,
        Michal Simek <michal.simek@...inx.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Tanmay Inamdar <tinamdar@....com>, linux-arch@...r.kernel.org,
        Pratyush Anand <pratyush.anand@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Jon Mason <jonmason@...adcom.com>,
        Murali Karicheri <m-karicheri2@...com>,
        Arnd Bergmann <arnd@...db.de>,
        Bharat Kumar Gogada <bharat.kumar.gogada@...inx.com>,
        Ray Jui <rjui@...adcom.com>,
        John Garry <john.garry@...wei.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Mingkai Hu <mingkai.hu@...escale.com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        Jingoo Han <jingoohan1@...il.com>,
        linux-kernel@...r.kernel.org,
        Stanimir Varbanov <svarbanov@...sol.com>,
        Minghuan Lian <minghuan.Lian@...escale.com>,
        Zhou Wang <wangzhou1@...ilicon.com>,
        Roy Zang <tie-fei.zang@...escale.com>
Subject: Re: [PATCH 02/20] PCI: fix pci_remap_iospace() remap attribute

On Mon, Mar 20, 2017 at 04:26:27PM +0000, Lorenzo Pieralisi wrote:
> On Mon, Mar 20, 2017 at 11:06:36AM -0500, Bjorn Helgaas wrote:
> 
> [...]
> 
> > > > > @@ -3375,7 +3375,7 @@ int pci_remap_iospace(const struct resource *res, phys_addr_t phys_addr)
> > > > >  		return -EINVAL;
> > > > >  
> > > > >  	return ioremap_page_range(vaddr, vaddr + resource_size(res), phys_addr,
> > > > > -				  pgprot_device(PAGE_KERNEL));
> > > > > +				  pgprot_noncached(PAGE_KERNEL));
> > > > 
> > > > ...
> > > > I do find this puzzling because I naively expected pgprot_noncached()
> > > > to match up with ioremap_nocache(), and apparently it doesn't.
> > > > 
> > > > For example, ARM64 ioremap_nocache() uses PROT_DEVICE_nGnRE, which
> > > > doesn't match the MT_DEVICE_nGnRnE in pgprot_noncached().
> > > > 
> > > > The point of these patches is to use non-posted mappings.  Apparently
> > > > you can do that with pgprot_noncached() here, but ioremap_nocache()
> > > > isn't enough for the config space mappings?
> > > 
> > > This is for iospace it seems, so the other patch I think was for
> > > config space.
> > 
> > I understand that 02/20 is for PCI I/O port space and 03/20 is for PCI
> > config space.  I'm confused because I thought we wanted the same
> > non-posted mapping for both, but looks like they're different.
> > 
> > Patch 02/20 uses ioremap_page_range(..., pgprot_noncached(PAGE_KERNEL))
> > to map PCI I/O port space:
> > 
> >   #define pgprot_noncached(prot) \
> >           __pgprot_modify(prot, PTE_ATTRINDX_MASK, PTE_ATTRINDX(MT_DEVICE_nGnRnE) | PTE_PXN | PTE_UXN)
> > 
> > Patch 03/20 uses ioremap_nocache() to map PCI config space:
> > 
> >   #define ioremap_nocache(addr, size)     __ioremap((addr), (size), __pgprot(PROT_DEVICE_nGnRE))
> > 
> > So the I/O port mapping uses MT_DEVICE_nGnRnE, while the config space
> > mapping uses PROT_DEVICE_nGnRE, which looks different.
> 
> On ARM64 (PATCH 4) and ARM (PATCH 5) we override pci_remap_cfgspace()
> with implementations that provide non-posted writes bus attributes,
> PATCH 3 is just there to provide a "safe" (well, I need input on that)
> fall-back.

Ah, thanks, that makes sense to me finally!  After patch 4,
pci_remap_cfgspace() uses PROT_DEVICE_nGnRnE, which does look like the
MT_DEVICE_nGnRnE used in the ioremap_page_range() path.

I wonder if we can get rid of pgprot_device() altogether?  Patch 2
removed the use in pci_remap_iospace(), and patch 18 removes the use
in tegra_pcie_bus_alloc().

Bjorn