linux-kernel - RE: [PATCH V3 2/4] ARM64 LPC: LPC driver implementation on Hip06

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Date:   Thu, 22 Sep 2016 15:20:31 +0000
From:   Gabriele Paoloni <gabriele.paoloni@...wei.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     zhichang <zhichang.yuan02@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "minyard@....org" <minyard@....org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        John Garry <john.garry@...wei.com>,
        "will.deacon@....com" <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Yuanzhichang <yuanzhichang@...ilicon.com>,
        Linuxarm <linuxarm@...wei.com>,
        "xuwei (O)" <xuwei5@...ilicon.com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "zourongrong@...il.com" <zourongrong@...il.com>,
        "liviu.dudau@....com" <liviu.dudau@....com>,
        "kantyzc@....com" <kantyzc@....com>
Subject: RE: [PATCH V3 2/4] ARM64 LPC: LPC driver implementation on Hip06



> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd@...db.de]
> Sent: 22 September 2016 15:59
> To: Gabriele Paoloni
> Cc: zhichang; linux-arm-kernel@...ts.infradead.org;
> devicetree@...r.kernel.org; lorenzo.pieralisi@....com; minyard@....org;
> linux-pci@...r.kernel.org; gregkh@...uxfoundation.org; John Garry;
> will.deacon@....com; linux-kernel@...r.kernel.org; Yuanzhichang;
> Linuxarm; xuwei (O); linux-serial@...r.kernel.org;
> benh@...nel.crashing.org; zourongrong@...il.com; liviu.dudau@....com;
> kantyzc@....com
> Subject: Re: [PATCH V3 2/4] ARM64 LPC: LPC driver implementation on
> Hip06
> 
> On Thursday, September 22, 2016 2:47:14 PM CEST Gabriele Paoloni wrote:
> > > >  static int of_empty_ranges_quirk(struct device_node *np)
> > > >  {
> > > >         if (IS_ENABLED(CONFIG_PPC)) {
> > > > @@ -503,7 +512,7 @@ static int of_translate_one(struct
> device_node
> > > *parent, struct of_bus *bus,
> > > >          * This code is only enabled on powerpc. --gcl
> > > >          */
> > > >         ranges = of_get_property(parent, rprop, &rlen);
> > > > -       if (ranges == NULL && !of_empty_ranges_quirk(parent)) {
> > > > +       if (ranges == NULL && !of_empty_ranges_quirk(parent) &&
> > > !of_isa_indirect_io(parent)) {
> > > >                 pr_debug("OF: no ranges; cannot translate\n");
> > > >                 return 1;
> > > >         }
> > >
> > > I don't see what effect that would have. What do you want to
> > > achieve with this?
> >
> > If I read the code correctly adding the function above would end
> > up in a 1:1 mapping:
> > http://lxr.free-electrons.com/source/drivers/of/address.c#L513
> >
> > so taddr will be assigned with the cpu address space specified
> > in the children nodes of LPC and we are not using a quirk function
> > (we are just checking that we have the indirect io assigned and
> > that we are on a ISA bus). Now probably there is a nit in my
> > code sketch where of_isa_indirect_io should be probably an
> architecture
> > specific function...
> 
> But the point is that it would then return an incorrect address,
> which in the worst case could be the same as another I/O space
> if that happens to be at CPU address zero.

If we do not touch __of_address_to_resource after taddr is returned
by of_translate_address we will check for (flags & IORESOURCE_IO),
then we call pci_address_to_pio to retrieve the unique token (remember
that LPC driver will register the LPC io range to pci io_range_list).

I do not think that we can have any conflict with any other I/O space
as pci_register_io_range will guarantee that the LPC range does not
overlap with any other I/O range... 

> 
> > > I think all we need from this function is to return '1' if
> > > we hit an ISA I/O window, and that should happen for the two
> > > interesting cases, either no 'ranges' at all, or no translation
> > > for the range in question, so that __of_translate_address can
> > > return OF_BAD_ADDR, and we can enter the special case
> > > handling in the caller, that handles it like
> > >
> >
> > I don't think this is very right as you may fail for different
> > reasons other than a missing range property, e.g:
> > http://lxr.free-electrons.com/source/drivers/of/address.c#L575
> >
> > And even if the only failure case was a missing range if in the
> > future __of_translate_address had to be reworked we would again
> > make a wrong assumption...you get my point?
> 
> The newly introduced function would clearly have to make
> some sanity checks. The idea is that treat the case of
> not being able to translate a bus specific I/O address
> into a CPU address literally and fall back to another method
> of translating that address.
> 
> This matches my mental model of how we find the resource:
> 
> - start with the bus address
> - try to translate that into a CPU address
> - if we arrive at a CPU physical address for IORESOURCE_MEM, use that
> - if we arrive at a CPU physical address for IORESOURCE_IO, translate
>   that into a Linux IORESOURCE_IO token
> - if there is no valid CPU physical address, try to translate
>   the address into an IORESOURCE_IO using the ISA accessor
> - if that fails too, give up.
> 
> If you try to fake a CPU physical address inbetween, it just
> gets more confusing.
> 
> 	Arnd