[alsa-devel] [PATCH 4/4] ASoC: fsl: make fsl_ssi driver compilable on ARM/IMX

[Trent Piepho]

On Thu, Feb 23, 2012 at 12:14 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Thu, Feb 23, 2012 at 11:04:59AM -0600, Timur Tabi wrote:
>> Russell King - ARM Linux wrote:
>> We almost never use readl.  I see that it is used in some places, but
>> honestly I can't see how it can be valid.  For instance, the mpic driver
>> uses it in mpic_startup_ht_interrupt().  I don't understand how that's
>> valid, since there's nothing preventing the readl() from happening
>> *before* the writeb.
>
> That just sounds broken.  If readl() can happen before a writeb() issued
> in program order, how do most of the PCI drivers work?

I was under the impression that on powerpc io memory was mapped
cache-inhibited and guarded, which provides most ordered, but not
store to load ordering without an additional synchronization
instruction.  See
http://cache.freescale.com/files/32bit/doc/app_note/AN3441.pdf on page
8.

PCI adds some additional complexity, since the PCI device is not
connected directly to the CPU.  One must consider the actions of the
PCI host controller.