Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section

[Arnd Bergmann]

On Tue, Feb 19, 2019 at 12:36 PM Will Deacon <will.deacon@xxxxxxx> wrote:
> On Tue, Feb 19, 2019 at 12:31:50PM +0100, Arnd Bergmann wrote:
> > On Tue, Feb 19, 2019 at 11:27 AM Thomas Petazzoni
> > <thomas.petazzoni@xxxxxxxxxxx> wrote:
> >
> > I think an example of this would be a driver using outb() to disable
> > an interrupt, and then relying on the the interrupt no longer happening
> > after the outb().
>
> Isn't that racy already? i.e. the interrupt could fire just before you
> disabled it, but not get delivered by the irq controller until after you'd
> disabled it at the device?

Probably, I had a hard enough time trying to come up with any example ;-)

One reference to non-posted transaction in a comment is in
drivers/net/ethernet/dec/tulip/de4x5.c:

/*
** The DE4X5 interrupt handler.
**
** I/O Read/Writes through intermediate PCI bridges are never 'posted',
** so that the asserted interrupt always has some real data to work with -
** if these I/O accesses are ever changed to memory accesses, ensure the
** STS write is read immediately to complete the transaction if the adapter
** is not on bus 0. Lost interrupts can still occur when the PCI bus load
** is high and descriptor status bits cannot be set before the associated
** interrupt is asserted and this routine entered.
*/

I found another comment in the via-rhine driver:

/* Beware of PCI posted writes */
#define IOSYNC  do { ioread8(ioaddr + StationAddr); } while (0)

this one is used in the chip reset function, and in the transmit
path.

       Arnd


     Arnd