linux-kernel - Write to memory-mapped register has actually reached the device

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Message-ID: <55AFACAF.7020004@free.fr>
Date:	Wed, 22 Jul 2015 16:46:07 +0200
From:	Mason <slash.tmp@...e.fr>
To:	Linux ARM <linux-arm-kernel@...ts.infradead.org>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Write to memory-mapped register has actually reached the device

Hello everyone,

I'm using an ARMv7 platform (Cortex A9).

The hardware designer said something that confused me: he said
that when a driver writes to a device memory-mapped register,
there is no way to "know" when the write has actually reached
the device, other than to read the value back.

I had been using this kind of code:

  static void __iomem *device_base;

  device_base = ioremap(DEVICE_ADDR, RANGE);
  writel_relaxed(val, device_base + N);

The situation where he said this would bite me is:

  write to a device register to clear an interrupt notification
  unmask interrupts

The interrupt might fire because the interrupt bit has
not been cleared yet. Does that make any sense?

Should I use writel instead of writel_relaxed in that
situation? Do I really have to read-after-write?

Regards.
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