Re: [PATCH V3 06/26] csky: Cache and TLB routines

[Guo Ren]

On Fri, Sep 07, 2018 at 10:14:38AM +0200, Arnd Bergmann wrote:
> On Fri, Sep 7, 2018 at 5:04 AM Guo Ren <ren_guo@xxxxxxxxx> wrote:
> >
> > On Thu, Sep 06, 2018 at 04:31:16PM +0200, Arnd Bergmann wrote:
> > > On Wed, Sep 5, 2018 at 2:08 PM Guo Ren <ren_guo@xxxxxxxxx> wrote:
> > >
> > > Can you describe how C-Sky hardware implements MMIO?
> > Our mmio is uncachable and strong-order address, so there is no need
> > barriers for access these io addr.
> >
> >  #define ioremap_wc ioremap_nocache
> >  #define ioremap_wt ioremap_nocache
> >
> > Current ioremap_wc and ioremap_wt implementation are too simple and
> > we'll improve it in future.
> >
> > > In particular:
> > >
> > > - Is a read from uncached memory always serialized with DMA, and with
> > >   other CPUs doing MMIO access to a different address?
> > CPU use ld.w to get data from uncached strong order memory.
> > Other CPUs use the same mmio vaddr to access the uncachable strong order
> > memory paddr.
> 
> Ok, but what about the DMA? The most common requirement for
> serialization here is with a DMA transfer, where you first write
> into a buffer in memory, then write to an MMIO register to trigger
> a DMA-load, and then the device reads the data from memory.
> Without a barrier before the MMIO, the data may still be in a
> store queue of the CPU, and the DMA gets stale data.

> 
> Similarly, an MMIO read may be used to see if a DMA has completed
> and the device register tells you that the DMA has left the device,
> but without a barrier, the CPU may have prefetched the DMA
> data while waiting for the MMIO-read to complete. The __io_ar()
> barrier() in asm-generic/io.h prevents the compiler from reordering
> the two reads, but if an weakly ordered read (in coherent DMA buffer)
> can bypass a strongly ordered read (MMIO), then it's still still
> broken.
__io_ar() barrier()? not rmb() ?! I've defined the rmb in asm/barrier, So
I got rmb() here not barrier().

Only __io_br() is barrier().

> > > - How does endianess work? Are there any buses that flip bytes around
> > >   when running big-endian, or do you always do that in software?
> > Currently we only support little-endian and soc will follow it.
> 
> Ok, that makes it easier. If you think that you won't even need big-endian
> support in the long run, you could also remove your asm/byteorder.h
> header. If you're not sure, it doesn't hurt to keep it of course.
Em... I'm not sure, so let me keep it for a while.

Best Regards
 Guo Ren