[U-Boot] i.MX51: FEC: Cache coherency problem?

[David Jander]

Hi Aneesh,

On Wed, 20 Jul 2011 15:59:42 +0530
Aneesh V <aneesh at ti.com> wrote:
> On Wednesday 20 July 2011 02:51 PM, David Jander wrote:
> > On Wed, 20 Jul 2011 10:56:07 +0200
> 
> [snip ..]
> 
> >>> Any ideas?
> >>
> >> Yes, one: I had issues with the Marvell Ethernet adapter, which has DMA
> >> as well, not because of cache (it was not active at the time) but
> >> because of instruction reordering done by the compiler when optimizing,
> >> which resulted in out-of-order accesses to the descritpors and DMA
> >> registered, so that the DMA start was written before the descriptors
> >> were set up. The (proper and clean) solution was to introduce memory
> >> barriers at strategic points of the driver to ensure that specific DMA
> >> starts were done only after all writes to the descriptors (and possibly
> >> buffers).
> >
> > Hmmm. I know that issue, but AFAICS, the driver already uses readX() and
> > writeX() macros when accessing register and DMA memory. Those macros have
> > compiler barriers included.... and armv7 is still in-order execution ;-)
> 
> 1. armv7 is not necessarily in-order. Cortex-A8 is in-order while
> Cortex-A9 is out-of-order.
> 2. I am not sure if in-order execution guarantees memory ordering. My
> understanding is that it doesn't.
> 3. In u-boot's implementation of MMU for ARM, the register
> space(everything other than SDRAM) is 'strongly-ordered' memory.
> Strongly ordered accesses will be correctly ordered w.r.to all other
> accesses in program order.

Wow, I hadn't noticed dcache_enable() also enables the MMU. Cool. One step
closer to splitting the mapping between a cached and uncached, strongly-ordered
region of RAM.

> 4. Compiler barriers prevent the compiler from doing certain
> optimizations, but doesn't help in these situations.

Thanks for this excellent summary!

> In short, I think you don't need to worry about ordering, but not
> because of compiler barriers or armv7 being in-order.

Well, I do need to worry about the buffer-descriptors and buffers then, since
they are in SDRAM, right?

Best regards,

-- 
David Jander
Protonic Holland.