Re: [RFC] MMIO accessors & barriers documentation

[Jeff Garzik <jgarzik@pobox.com>]

Benjamin Herrenschmidt wrote:
> Well, the argument currently is to make writel and readl imply the above
> barriers by making them fully ordered (and slow on some platforms) and
> so also provide more weakly ordered routines along with barriers for
> people who know what they do. The above 2 barriers are what I've called
> io_to_memory_rb() and memory_to_io_wb() (actually,
> prepare_to_read_dma_memory() by itself doesn't really make much sense.
> It does in conjunction with an MMIO read to flush DMA buffers, in which
> case the barrier provides an ordering guarantee that the memory reads
> will only be performed after the MMIO read has fully completed).

<jgarzik throws a monkey wrench into the works>

I think focusing on MMIO just confuses the issue.

wmb() is often used to make sure a memory store is visible to a 
busmastering PCI device... before the code proceeds with some more 
transactions in the memory space shared by the host and PCI device.

prepare_to_read_dma_memory() is the operation that an ethernet driver's 
RX code wants.  And this is _completely_ unrelated to MMIO.  It just 
wants to make sure that the device and host are looking at the same 
data.  Often this involves polling a DMA descriptor (or index, stored 
inside DMA-able memory) looking for changes.

flush_my_writes_to_dma_memory() is the operation that an ethernet 
driver's TX code wants, to precede either an MMIO "poke" or any other 
non-MMIO operation where the driver needs to be certain that the write 
is visible to the PCI device, should the PCI device desire to read that 
area of memory.

	Jeff