Subject: Re: [PATCH net-next 2/4] net: stmmac: use correct barrier between coherent memory and MMIO

From: Niklas Cassel <[email-masked]>
Date: Sat, 3 Mar 2018 00:28:53 +0100

On Fri, Mar 02, 2018 at 09:54:11AM -0500, David Miller wrote:
> From: Pavel Machek <[email-masked]>
> Date: Fri, 2 Mar 2018 10:20:00 +0100
>

Hello Pavel, David

> >> This barrier cannot be a simple dma_wmb(), since a dma_wmb() is only
> >> used to guarantee the ordering, with respect to other writes,
> >> to cache coherent DMA memory.
> > 
> > Could you explain this a bit more (and perhaps in code comment)?

Have a look at:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/memory-barriers.txt?h=v4.16-rc1#n1913

AFAICT, a dma_wmb() can only be used to guarantee that the
writes to cache coherent memory (e.g. memory allocated with
dma_alloc_coherent()) before the dma_wmb() will be performed
before the writes to cache coherent memory after the dma_wmb().

Since most of our writes are simply writing new buffer addresses
and sizes to TDES0/TDES1/TDES2/TDES3, and since these TX DMA
descriptors have been allocated with dma_alloc_coherent(),
a dma_wmb() should be enough to e.g. make sure that TDES3
(which contains the OWN bit), is written after the writes to
TDES0/TDES1/TDES2.

However, the last write we do is "DMA start transmission",
this is a register in the IP, i.e. it is a write to the cache
incoherent MMIO region (rather than a write to cache coherent memory).
To ensure that all writes to cache coherent memory have
completed before we start the DMA, we have to use the barrier
wmb() (which performs a more extensive flush compared to
dma_wmb()).

So the only place where we have to use a wmb() instead
of a dma_wmb() is where we have a write to coherent memory,
followed by a write to cache incoherent MMIO.
The only obvious place where we have this situtation is
where we write the OWN bit immediately followed by a write
to the "DMA start transmission" register.

Note that this also matches how it's done in other other drivers:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/ethernet/amd/xgbe/xgbe-dev.c?h=v4.16-rc1#n1638

There is already a comment describing the barrier in
stmmac_xmit() and stmmac_tso_xmit() that says:
/* The own bit must be the latest setting done when prepare the
 * descriptor and then barrier is needed to make sure that
 * all is coherent before granting the DMA engine.
 */
However, if you want, we could mention wmb() explicitly in this comment.

> > 
> > Ensuring other writes are done before writing the "GO!" bit should be
> > enough, no?
> 
> Indeed, the chip should never look at the descriptor contents unless
> the GO bit is set.
> 
> If there are ways that it can, this must be explained and documented
> since it is quite unusual compared to other hardware.
> 
> > (If it is not, do we need heavier barriers in other places, too?)
> 
> Right.

I hope that my explaination above has cleared any potential confusion.

Best regards,
Niklas