Re: RFC: being more anal about iospace accesses..

[Jesse Barnes <jbarnes@engr.sgi.com>]

On Thursday, September 16, 2004 10:17 pm, Benjamin Herrenschmidt wrote:
> Hrm... that's weird... on sane DMA hardward, most PIO/MMIO reads are
> actually here just to guarantee that completion, interrupts isn't enough
> especially if we do things like polling (NAPI) etc... besides,
> interrupts are usually totally asynchronous to anything else, so I
> wouldn't say it's sane.

Well, it can definitely be problematic.  Devices that want to complete more 
than one transaction per interrupt have always been troublesome on sn 
platforms.

>
> > Sure, that's the way I'd like to go, but that's a dangerous route. 
> > Devices that want to do a PIO read to see if their DMA is complete will
> > work most of the time, but you'll get occasional data corruption.  IOW,
> > specifying reads that you know *aren't* dependent is much easier to do,
> > and safer.  It also allows SGI hardware to run all current drivers w/o
> > modification, which is nice.
>
> Agreed. Reads must NOT be relaxed by default

At least not for the current interface, but maybe for the new one.  The 
proposed dma_sync interface could take both an iomem address *and* a device 
if that would make it easier (it would for us).

Jesse