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

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Message-ID: <20170320160636.GB24572@bhelgaas-glaptop.roam.corp.google.com>
Date:   Mon, 20 Mar 2017 11:06:36 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Liviu Dudau <Liviu.Dudau@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....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 Fri, Mar 17, 2017 at 01:33:21AM +0100, Luis R. Rodriguez wrote:
> On Thu, Mar 16, 2017 at 04:48:44PM -0500, Bjorn Helgaas wrote:
> > [+cc Luis]
> > 
> > On Mon, Feb 27, 2017 at 03:14:13PM +0000, Lorenzo Pieralisi wrote:
> > > According to the PCI local bus specifications (Revision 3.0, 3.2.5),
> > > I/O Address space transactions are non-posted. On architectures where
> > > I/O space is implemented through a chunk of memory mapped space mapped
> > > to PCI address space (ie IA64/ARM/ARM64) the memory mapping for the
> > > region backing I/O Address Space transactions determines the I/O
> > > transactions attributes (before the transactions actually reaches the
> > > PCI bus where it is handled according to the PCI specifications).
> > > 
> > > Current pci_remap_iospace() interface, that is used to map the PCI I/O
> > > Address Space into virtual address space, use pgprot_device() as memory
> > > attribute for the virtual address mapping, that in some architectures
> > > (ie ARM64) provides non-cacheable but write bufferable mappings (ie
> > > posted writes), 
> 
> <sarcasm>
> Gee wiz, I am glad this is so well documented.
> </sarcasm>

I'm not sure this is actionable feedback.  The two paragraphs above
seem clear and useful to me.  How would you like to see them improved?

> > > ...
> > > @@ -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.

Bjorn