Re: [PATCH 3/3] net: hisilicon: new hip04 ethernet driver

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, Apr 03, 2014 at 03:42:00PM +0000, David Laight wrote:
> From: Russell King - ARM Linux
> > DMA coherent memory is write combining, so multiple writes will be
> > coalesced.  This also means that barriers may be required to ensure the
> > descriptors are pushed out in a timely manner if something like writel()
> > is not used in the transmit-triggering path.
> 
> You also have to ensure that the write that changes the 'owner'
> bit is the one that happens last.
> 
> If (for example) a descriptor has two words, one containing the
> buffer address and the other containing the length and flags,
> then you have to absolutely ensure that the hardware will not
> read the new 'flags' with the old 'buffer address'.
> Any write to tell the hardware to look at the tx ring won't
> help you - the hardware might be reading the descriptor anyway.
> 
> Even if the accesses are uncached, you need the appropriate
> barrier (or volatiles) to stop gcc reordering the writes.
> (I think accesses to volatiles can't be reordered - check.)

Exactly... I wish Freescale were as thoughtful as you are on this point. :)

-- 
FTTC broadband for 0.8mile line: now at 9.7Mbps down 460kbps up... slowly
improving, and getting towards what was expected from it.