[EXT] Re: The problem about arm64: io: Relax implicit barriers in default I/O accessors

[Frank Li]

> -----Original Message-----
> From: Will Deacon <will at kernel.org>
> Sent: Monday, June 21, 2021 12:00 PM
> To: Frank Li <frank.li at nxp.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>; Zhi Li <lznuaa at gmail.com>;
> Shenwei Wang <shenwei.wang at nxp.com>; Han Xu <han.xu at nxp.com>; Nitin Garg
> <nitin.garg at nxp.com>; Jason Liu <jason.hui.liu at nxp.com>; linux-arm-
> kernel at lists.infradead.org
> Subject: Re: [EXT] Re: The problem about arm64: io: Relax implicit barriers
> in default I/O accessors
> 
> Caution: EXT Email
> 
> On Mon, Jun 21, 2021 at 05:26:41PM +0100, Will Deacon wrote:
> > On Mon, Jun 21, 2021 at 04:11:57PM +0000, Frank Li wrote:
> > > > Oh, interesting. Maybe this is a case where OSH vs SY actually makes
> a
> > > > difference. I'm not quite sure what it means for the coherency of
> normal,
> > > > non-cacheable accesses (which are outer-shareable) so that probably
> needs a
> > > > bit more thought.
> > > >
> > > > Can you confirm that the issue *does* still occur if you use dmb(osh)
> > > > instead of dmb(oshst), please?
> > >
> > > After get ARM support
> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fservices.
> arm.com%2Fsupport%2Fs%2Fcase%2F5003t00001RuJHw&data=04%7C01%7Cfrank.li%
> 40nxp.com%7Ca319ac5213a14aa6bb2508d934d5facc%7C686ea1d3bc2b4c6fa92cd99c5c30
> 1635%7C0%7C0%7C637598915908588560%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwM
> DAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=6%2F%2FK
> ScsCmnUgNPnzcvyjRrOLjLVPrHtbVgI3J959U%2BQ%3D&reserved=0,
> > > This issue have some progress.
> > >
> > > Our system configure SYSBARDISABLE = 0x0, So ARM core barrier propagate
> to CCI-400
> > >
> > > Our DMA and USB is located below downstream of CCI-400. So USB or DMA
> is located
> > > in system shared domain. Only use dmb(st), CCI-400 wait for previous
> transaction
> > > Complete. When dma(osh), the response is sent when snoop responses are
> received for
> > > all earlier transactions. CCI-400 don't wait for previous write finish.
> >
> > Thanks for following up. I'll cook a patch to fix this...
> 
> ... and in doing so, I realised I still have a question about this.
> 
> If a CPU is writing to a zero-initialised non-cacheable buffer in memory
> and does something like:
> 
>         buffer[0] = 1;
>         dma_wmb();      // DMB OSHST
>         buffer[64] = 1;
> 
> would a non-coherent device reading this be able to see buffer[64] == 1
> but buffer[0] = 0? In other words, do we need to upgrade the dmb_* barriers
> as well as the I/O accessors, or are they still ordered by the bus fabric
> because all of the accesses are going to the DDR?

I think re-order is possible. According to my understanding, 
If cci ack dmb(oshst), the follow order is not guaranteed if no address overlap
for normal memory. 

A6.6.1 of AXI protocol spec. 

A write W1 must be ordered before a write W2 with the same ID, to the *same Memory location*, where W2
is received after W1 is received.    

> 
> Will