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Re: [PATCH 02/17] irqchip: Add PLX Technology RPS IRQ Controller

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On Thu, Mar 03, 2016 at 02:08:19PM +0100, Arnd Bergmann wrote:
> On Thursday 03 March 2016 13:01:13 Marc Zyngier wrote:
> > > +/* Routines to acknowledge, disable and enable interrupts */
> > > +static void rps_mask_irq(struct irq_data *d)
> > > +{
> > > +     u32 mask = BIT(d->hwirq);
> > > +
> > > +     iowrite32(mask, rps_data.base + RPS_MASK);
> > 
> > I do question the use of iowrite32 here (and its ioread32 pendent
> > anywhere else), as it actually translates in a writel, which contains a
> > memory barrier. Do you have any case that requires the use of such a
> > barrier? if not, consider switching to relaxed accessors (which are the
> > 
> 
> I really ask everyone to do the opposite: we have seen several drivers
> blindlessly using the relaxed accessors and actually introducing bugs
> that way, so I'd rather see the readl/writel ones used by default.

I actually agree with Marc - we have far too many drivers using the
barriered IO accessors, which are really very expensive on 32-bit
ARM.

For most ARM systems, the rules are quite simple: a write which causes
DMA memory to be accessed by the device must be using the barriered
IO accessor, and a read from a DMA status register must be too.
Everything else need not be.  Barriered IO accessors are only about
access ordering.

That's independent of whether you need a read-back to ensure that the
write has hit the hardware: that's a completely different problem, and
one which is harder for people to understand and get right.  (Eg, for
interrupt registers.)

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
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[PATCH 00/17] Add Initial support for PLX Technology OX810SE Neil Armstrong <narmstrong@baylibre.com> - 2016-03-03 13:00 +0100
  [PATCH 06/17] reset: Add PLX Technology Reset Controller driver Neil Armstrong <narmstrong@baylibre.com> - 2016-03-03 13:00 +0100
    Re: [PATCH 06/17] reset: Add PLX Technology Reset Controller driver Philipp Zabel <p.zabel@pengutronix.de> - 2016-03-03 15:20 +0100
      Re: [PATCH 06/17] reset: Add PLX Technology Reset Controller driver Neil Armstrong <narmstrong@baylibre.com> - 2016-03-03 15:40 +0100
        Re: [PATCH 06/17] reset: Add PLX Technology Reset Controller driver Philipp Zabel <p.zabel@pengutronix.de> - 2016-03-03 16:10 +0100
  [PATCH 07/17] dt-bindings: Add PLX Technology Reset Controller bindings Neil Armstrong <narmstrong@baylibre.com> - 2016-03-03 13:00 +0100
    Re: [PATCH 07/17] dt-bindings: Add PLX Technology Reset Controller  bindings Philipp Zabel <p.zabel@pengutronix.de> - 2016-03-03 15:30 +0100
      Re: [PATCH 07/17] dt-bindings: Add PLX Technology Reset Controller  bindings Neil Armstrong <narmstrong@baylibre.com> - 2016-03-03 15:30 +0100
        Re: [PATCH 07/17] dt-bindings: Add PLX Technology Reset Controller  bindings Philipp Zabel <p.zabel@pengutronix.de> - 2016-03-03 15:40 +0100
  [PATCH 05/17] dt-bindings: Add PLX Technology RPS Timer bindings Neil Armstrong <narmstrong@baylibre.com> - 2016-03-03 13:00 +0100
  [PATCH 02/17] irqchip: Add PLX Technology RPS IRQ Controller Neil Armstrong <narmstrong@baylibre.com> - 2016-03-03 13:00 +0100
    Re: [PATCH 02/17] irqchip: Add PLX Technology RPS IRQ Controller Arnd Bergmann <arnd@arndb.de> - 2016-03-03 14:10 +0100
      Re: [PATCH 02/17] irqchip: Add PLX Technology RPS IRQ Controller Russell King - ARM Linux <linux@arm.linux.org.uk> - 2016-03-03 14:40 +0100
        Re: [PATCH 02/17] irqchip: Add PLX Technology RPS IRQ Controller Arnd Bergmann <arnd@arndb.de> - 2016-03-03 18:40 +0100
    Re: [PATCH 02/17] irqchip: Add PLX Technology RPS IRQ Controller Marc Zyngier <marc.zyngier@arm.com> - 2016-03-03 14:10 +0100
    RE: [PATCH 02/17] irqchip: Add PLX Technology RPS IRQ Controller "Ma Haijun" <mahaijuns@gmail.com> - 2016-03-03 16:40 +0100
      Re: [PATCH 02/17] irqchip: Add PLX Technology RPS IRQ Controller Neil Armstrong <narmstrong@baylibre.com> - 2016-03-03 18:00 +0100
        Re: [PATCH 02/17] irqchip: Add PLX Technology RPS IRQ Controller Marc Zyngier <marc.zyngier@arm.com> - 2016-03-03 18:20 +0100
  [PATCH 09/17] dt-bindings: Add PLX Technology OXNAS Standard Clocks bindings Neil Armstrong <narmstrong@baylibre.com> - 2016-03-03 13:00 +0100
  Re: [PATCH 00/17] Add Initial support for PLX Technology OX810SE Arnd Bergmann <arnd@arndb.de> - 2016-03-03 13:30 +0100
    Re: [PATCH 00/17] Add Initial support for PLX Technology OX810SE Neil Armstrong <narmstrong@baylibre.com> - 2016-03-03 13:40 +0100

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