Re: [PATCH 7/7] iommu/ipmmu-vmsa: Add suspend/resume support

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Laurent,

On Wed, Feb 20, 2019 at 4:42 PM Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
> On Wed, Feb 20, 2019 at 04:05:31PM +0100, Geert Uytterhoeven wrote:
> > During PSCI system suspend, R-Car Gen3 SoCs are powered down, and all
> > IPMMU state is lost.  Hence after s2ram, devices wired behind an IPMMU,
> > and configured to use it, will see their DMA operations hang.
> >
> > To fix this, restore all IPMMU contexts, and re-enable all active
> > micro-TLBs during system resume.
> >
> > To avoid overhead on platforms not needing it, the resume code has a
> > build time dependency on sleep and PSCI support, and a runtime
> > dependency on PSCI.
> >
> > Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> > ---
> > This patch takes a different approach than the BSP, which implements a
> > bulk save/restore of all registers during system suspend/resume.
>
> I like this approach better too.

Thanks ;-)

> > --- a/drivers/iommu/ipmmu-vmsa.c
> > +++ b/drivers/iommu/ipmmu-vmsa.c

> > @@ -58,6 +62,7 @@ struct ipmmu_vmsa_device {
> >       spinlock_t lock;                        /* Protects ctx and domains[] */
> >       DECLARE_BITMAP(ctx, IPMMU_CTX_MAX);
> >       struct ipmmu_vmsa_domain *domains[IPMMU_CTX_MAX];
> > +     s8 utlb_ctx[IPMMU_UTLB_MAX];
>
> How about making this a bitmask instead to save memory ? I would also
> rename it as utlb_ctx doesn't really carry the meaning of the field,
> whose purpose is to store whether the µTLB is enabled or disabled.

This field isn't just a binary flag, but stores the context used for the
uTLB, so we can map from micro-TLB to context.
Given there can be 8 contexts, plus the need to indicate unused contexts,
that means 4 bits/micro-TLB.  So the overhead is just 24 bytes per IPMMU
instance.

I considered allocating the array dynamically (by having s8 utlb_ctx[]
at the end of the structure), but didn't go that route, as the domains[]
array already uses more memory.

> > @@ -1158,10 +1166,52 @@ static int ipmmu_remove(struct platform_device *pdev)
> >       return 0;
> >  }
> >
> > +#if defined(CONFIG_PM_SLEEP) && defined(CONFIG_ARM_PSCI_FW)
> > +static int ipmmu_resume_noirq(struct device *dev)
> > +{
> > +     struct ipmmu_vmsa_device *mmu = dev_get_drvdata(dev);
> > +     unsigned int i;
> > +
> > +     /* This is the best we can do to check for the presence of PSCI */
> > +     if (!psci_ops.cpu_suspend)
> > +             return 0;
>
> PSCI suspend disabling power to the SoC completely may be a common
> behaviour on our development boards, but isn't mandated by the PSCI
> specification if I'm not mistaken. Is there a way to instead detect that
> power has been lost, perhaps by checking whether a register has been
> reset to its default value ?

The approach here is the same as in the clk and pinctrl drivers.

I think we could check if the IMCTR registers for allocated domains in root
IPMMUs are non-zero.  But that's about as expensive as doing the full
restore, I think.  And it may have to be done for each and every IPMMU
instance, or do you trust caching for this?

> > +
> > +     /* Reset root MMU and restore contexts */
>
> I think the rest of the code adds a period at the end of sentences in
> comments.

The balance seems to be just under 50% ;-)

> > +     if (ipmmu_is_root(mmu)) {
> > +             ipmmu_device_reset(mmu);
> > +
> > +             for (i = 0; i < mmu->num_ctx; i++) {
> > +                     if (!mmu->domains[i])
> > +                             continue;
> > +
> > +                     ipmmu_context_init(mmu->domains[i]);
> > +             }
> > +     }
> > +
> > +     /* Re-enable active micro-TLBs */
> > +     for (i = 0; i < mmu->features->num_utlbs; i++) {
> > +             if (mmu->utlb_ctx[i] == IPMMU_CTX_INVALID)
> > +                     continue;
> > +
> > +             ipmmu_utlb_enable(mmu->root->domains[mmu->utlb_ctx[i]], i);
> > +     }
> > +
> > +     return 0;
> > +}
> > +
> > +static const struct dev_pm_ops ipmmu_pm  = {
> > +     SET_NOIRQ_SYSTEM_SLEEP_PM_OPS(NULL, ipmmu_resume_noirq)
> > +};
> > +#define DEV_PM_OPS   &ipmmu_pm
> > +#else
> > +#define DEV_PM_OPS   NULL
> > +#endif /* CONFIG_PM_SLEEP && CONFIG_ARM_PSCI_FW */
> > +
> >  static struct platform_driver ipmmu_driver = {
> >       .driver = {
> >               .name = "ipmmu-vmsa",
> >               .of_match_table = of_match_ptr(ipmmu_of_ids),
> > +             .pm = DEV_PM_OPS,
>
> I would have used conditional compilation here instead of using a
> DEV_PM_OPS macro, as I think the macro decreases readability (and also
> given how its generic name could later conflict with something else).

You mean

    #ifdef ...
    .pm = &ipmmu_pm,
    #endif

and marking ipmmu_pm __maybe_unused__?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds