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

[Sebastian Andrzej Siewior]

On 2017-08-16 17:15:55 [-0400], Ken Goldman wrote:
> On 8/15/2017 4:13 PM, Haris Okanovic wrote:
> > ioread8() operations to TPM MMIO addresses can stall the cpu when
> > immediately following a sequence of iowrite*()'s to the same region.
> > 
> > For example, cyclitest measures ~400us latency spikes when a non-RT
> > usermode application communicates with an SPI-based TPM chip (Intel Atom
> > E3940 system, PREEMPT_RT_FULL kernel). The spikes are caused by a
> > stalling ioread8() operation following a sequence of 30+ iowrite8()s to
> > the same address. I believe this happens because the write sequence is
> > buffered (in cpu or somewhere along the bus), and gets flushed on the
> > first LOAD instruction (ioread*()) that follows.
> > 
> > The enclosed change appears to fix this issue: read the TPM chip's
> > access register (status code) after every iowrite*() operation to
> > amortize the cost of flushing data to chip across multiple instructions.

Haris, could you try a wmb() instead the read?

> 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.

> I'd also like an indication of the performance penalty.  We're doing a lot
> of work to improve the performance and I worry that "do a read after every
> write" will have a performance impact.
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.

Sebastian