Re: [PATCH] net: ftgmac100: Fix missing TX-poll issue

[Andrew Jeffery]

On Wed, 21 Oct 2020, at 08:40, Benjamin Herrenschmidt wrote:
> On Tue, 2020-10-20 at 21:49 +0200, Arnd Bergmann wrote:
> > On Tue, Oct 20, 2020 at 11:37 AM Dylan Hung <dylan_hung@xxxxxxxxxxxxxx> wrote:
> > > > +1 @first is system memory from dma_alloc_coherent(), right?
> > > > 
> > > > You shouldn't have to do this. Is coherent DMA memory broken on your
> > > > platform?
> > > 
> > > It is about the arbitration on the DRAM controller.  There are two queues in the dram controller, one is for the CPU access and the other is for the HW engines.
> > > When CPU issues a store command, the dram controller just acknowledges cpu's request and pushes the request into the queue.  Then CPU triggers the HW MAC engine, the HW engine starts to fetch the DMA memory.
> > > But since the cpu's request may still stay in the queue, the HW engine may fetch the wrong data.
> 
> Actually, I take back what I said earlier, the above seems to imply
> this is more generic.
> 
> Dylan, please confirm, does this affect *all* DMA capable devices ? If
> yes, then it's a really really bad design bug in your chips
> unfortunately and the proper fix is indeed to make dma_wmb() do a dummy
> read of some sort (what address though ? would any dummy non-cachable
> page do ?) to force the data out as *all* drivers will potentially be
> affected.
> 
> I was under the impression that it was a specific timing issue in the
> vhub and ethernet parts, but if it's more generic then it needs to be
> fixed globally.
> 

We see a similar issue in the XDMA engine where it can transfer stale data to 
the host. I think the driver ended up using memcpy_toio() to work around that 
despite using a DMA reserved memory region.