Re: Patch: ATI Xilleon port 2/11 net/e100 Memory barriers and write flushing

[David Daney <ddaney@avtrex.com>]

Ralf Baechle wrote:
> On Thu, Oct 20, 2005 at 12:01:01PM -0700, Jesse Brandeburg wrote:
> 
> 
>>>@@ -584,6 +584,7 @@ static inline void e100_write_flush(stru
>>> {
>>>        /* Flush previous PCI writes through intermediate bridges
>>>         * by doing a benign read */
>>>+       wmb();
>>>        (void)readb(&nic->csr->scb.status);
>>> }
>>
>>I find it odd that this is needed, the readb is meant to flush all
>>posted writes on the pci bus, if your bus is conforming to pci
>>specifications, this must succeed.  wmb is for host side (processor
>>memory) writes to complete, and since we're usually only try to force
>>a writeX command to execute immediately with the readb (otherwise lazy
>>writes work okay) we shouldn't need a wmb *here*.  not to say it might
>>not be missing somewhere else.
> 
> 
> wmb is defined as a sync instruction which will only complete once the
> write has actually left the CPU, that is citing the spec "has become
> globally visible".  Uncached stores such as writeX() may be held in a
> writeback buffers potencially infinitely, until this buffer is needed
> by another write operation.  The real surprise is to see such behaviour
> in a modern piece of silicon; the only that I knew of were the R3000-class
> processors and that era has ended over a decade ago, so ATI seems to have
> done something funny here.

In light of all the comments, and:

1) the fact that the drivers for the e100 in the 2.4.30 kernel 
distribution work well.

2) other pci drivers work well with this port (usb/ohci, net/8139too).

3) the properties of the write back buffer are not well documented.

I am going to take a more detailed look at trying to fix this problem in 
a less intrusive manner.

David Daney.