[PATCH v3 3/6] iommu: add ARM short descriptor page table allocator.

Yong Wu yong.wu at mediatek.com
Mon Jul 27 22:08:14 PDT 2015

On Mon, 2015-07-27 at 15:11 +0100, Will Deacon wrote:
> On Mon, Jul 27, 2015 at 03:05:38PM +0100, Robin Murphy wrote:
> > On 27/07/15 05:21, Yong Wu wrote:
> > >>>>> +       } else {        /* page or largepage */
> > >>>>> +               if (quirk & IO_PGTABLE_QUIRK_SHORT_MTK) {
> > >>>>> +                       if (large) { /* special Bit */
> > >>>>
> > >>>> This definitely needs a better comment! What exactly are you doing here
> > >>>> and what is that quirk all about?
> > >>>
> > >>> I use this quirk is for MTK Special Bit as we don't have the XN bit in
> > >>> pagetable.
> > >>
> > >> I'm still not really clear about what this is.
> > >
> > > There is some difference between the standard spec and MTK HW,
> > > Our hw don't implement some bits, like XN and AP.
> > > So I add a quirk for MTK special.
> > 
> > When you say it doesn't implement these bits, do you mean that having 
> > them set will lead to Bad Things happening in the hardware, or that it 
> > will simply ignore them and not enforce any of the protections they 
> > imply? The former case would definitely want clearly documenting 
> > somewhere, whereas for the latter case I'm not sure it's even worth the 
> > complication of having a quirk - if the value doesn't matter there seems 
> > little point in doing a special dance just for the sake of semantic 
> > correctness of the in-memory PTEs, in my opinion.
> Agreed. We should only use quirks if the current (architecturally
> compliant) code causes real issues with the hardware. Then the quirk can
> be used to either avoid the problematic routines or to take extra steps
> to make things work as the architecture intended.
> I've asked how this IOMMU differs from the architecture on a number of
> occasions, but I'm still yet to receive a response other than "it's special".

After check further with DE, Our pagetable is refer to ARM-v7's
short-descriptor which is a little different from ARM-v8. like bit0(PXN)
in section and supersection, I didn't read ARM-v7 spec before, so I add
a MTK quirk to disable PXN bit in section and supersection.(if the PXN
bit is wrote in ARM-v7 spec, HW will page fault.)

Then I write this code according to ARM-v8 spec defaultly, and add a
ARM-v7 quirk?

And there is a little different between ARM-v7 spec and MTK pagetable.
It's the XN(bit0) in small page. MTK don't implement XN bit. 
The bit[1:0] in MTK's small page should be 2'b10, if it's 2'b11, HW will
page fault.
(MTK don't implement AP bits too, but HW don't use them, it is ok even
though AP bits is wrote)

In the end, I will add two quirk like this, is it OK?

#define ARM_PGTABLE_QUIRK_SHORT_ARM_V7   BIT(2)  /* for ARM-v7 while
default is the ARM-v8 spec */
#define ARM_PGTABLE_QUIRK_SHORT_MTK  BIT(3)      /* MTK special */

In the ARM_V7 quirk, I only disable PXN bit in section and supersection,
In the MTK quirk, I only disbable XN in small page.

The other bits seems the same. I'm not sure I write clearly and It seems
we could not copy a image of mtk pagetable here..If any question, please
help tell me.
Thanks very much.

> Will

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