On Tue, Jul 28, 2015 at 06:08:14AM +0100, Yong Wu wrote:
> 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.)
I've been reviewing this using the ARMv7 ARM (Rev.C of DDI0406C) the whole
time. PXN is there as an optional field in non-LPAE implementations. That's
fine and doesn't require any quirks.
> Then I write this code according to ARM-v8 spec defaultly, and add a
> ARM-v7 quirk?
No, I don't think you need this, as the v8 and v7 short-descriptor formats
look compatible to me. You should only need a quirk if architecturally
compliant code cannot work on your hardware.
> 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.
Aha, thanks! *That* is worthy of a quirk. Something like:
IO_PGTABLE_QUIRK_ARM_NO_XN
> (MTK don't implement AP bits too, but HW don't use them, it is ok even
> though AP bits is wrote)
Yeah, I think that's fine. The pgtable code will honour the request but
the h/w will ignore it.
> In the end, I will add two quirk like this, is it OK?
I think you only need the one I mentioned above. I don't see the need
for PXN at all (as I said in the last review).
Will
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