Re: [PATCH v3 1/7] AMSO1100 Low Level Driver.

[Steve Wise <swise@opengridcomputing.com>]

> ok pci posting...
> 
> basically, if you use writel() and co, the PCI bridges in the middle are
> allowed (and the more fancy ones do) cache the write, to see if more
> writes follow, so that the bridge can do the writes as a single burst to
> the device, rather than as individual writes. This is of course great...
> ... except when you really want the write to hit the device before the
> driver continues with other actions. 
> 
> Now the PCI spec is set up such that any traffic in the other direction
> (basically readl() and co) will first flush the write through the system
> before the read is actually sent to the device, so doing a dummy readl()
> is a good way to flush any pending posted writes.
> 
> Where does this matter? 
> it matters most at places such as irq enabling/disabling, IO submission
> and possibly IRQ acking, but also often in eeprom-like read/write logic
> (where you do manual clocking and need to do delays between the
> write()'s). But in general... any place where you do writel() without
> doing any readl() before doing nothing to the card for a long time, or
> where you are waiting for the card to do something (or want it done NOW,
> such as IRQ disabling) you need to issue a (dummy) readl() to flush
> pending writes out to the hardware.
> 
> 
> does this explanation make any sense? if not please feel free to ask any
> questions, I know I'm not always very good at explaining things.

Yep.  I get it.  I believe we're ok in this respect, but I'll review the
code again with an eye for this issue...

Steve.



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