Re: [tpmdd-devel] [PATCH v2] tpm_tis: fix stall after iowrite*()s

[Jason Gunthorpe]

On Thu, Aug 17, 2017 at 12:38:07PM +0200, Sebastian Andrzej Siewior wrote:

> > I worry a bit about "appears to fix".  It seems odd that the TPM device
> > driver would be the first code to uncover this.  Can anyone confirm that the
> > chipset does indeed have this bug?
> 
> What Haris says makes sense. It is just not all architectures
> accumulate/ batch writes to HW.

It doesn't seem that odd to me.. In modern Intel chipsets the physical
LPC bus is used for very little. Maybe some flash and possibly a
winbond super IO at worst?  Plus the TPM.

I can't confirm what Intel has done, but if writes are posted, then it
is not a 'bug', but expected operation for a PCI/LPC bridge device to
have an ordered queue of posted writes, and thus higher latency when
processing reads due to ordering requirments.

Other drivers may not see it because most LPC usages would not be
write heavy, or might use IO instructions which are not posted..

I can confirm that my ARM systems with a custom PCI-LPC bridge will
have exactly the same problem, and that the readl is the only
solution.

This is becuase writes to LPC are posted over PCI and will be buffered
in the root complex, device end port and internally in the LPC
bridge. Since they are posted there is no way for the CPU to know when
the complete and when it would be 'low latency' to issue a read.

> So powerpc (for instance) has a sync operation after each write to HW. I
> am wondering if we could need something like that on x86.

Even on something like PPC 'sync' is not defined to globally flush
posted writes, and wil not help. WMB is probably similar.

Jason