Re: [PATCH] [RFC] tpm_tis: tpm_tcg_flush() after iowrite*()s

[Haris Okanovic]

On 08/08/2017 04:58 PM, Jarkko Sakkinen wrote:
> On Mon, Aug 07, 2017 at 09:59:35AM -0500, Julia Cartwright wrote:
> > On Fri, Aug 04, 2017 at 04:56:51PM -0500, Haris Okanovic wrote:
> > > I have a latency issue using a SPI-based TPM chip with tpm_tis driver
> > > from non-rt usermode application, which induces ~400 us latency spikes
> > > in cyclictest (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 writes are cached (in cpu or somewhere along the bus), which
> > > gets flushed on the first LOAD instruction (ioread*()) that follows.
> >
> > To use the ARM parlance, these accesses aren't "cached" (which would
> > imply that a result could be returned to the load from any intermediate
> > node in the interconnect), but instead are "bufferable".
> >
> > It is really unfortunate that we continue to run into this class of
> > problem across various CPU vendors and various underlying bus
> > technologies; it's the continuing curse of running an PREEMPT_RT on
> > commodity hardware.  RT is not easy :)
> >
> > > The enclosed change appears to fix this issue: read the TPM chip's
> > > access register (status code) after every iowrite*() operation.
> >
> > Are we engaged in a game of wack-a-mole with all of the drivers which
> > use this same access pattern (of which I imagine there are quite a
> > few!)?
> >
> > I'm wondering if we should explore the idea of adding a load in the
> > iowriteN()/writeX() macros (marking those accesses in which reads cause
> > side effects explicitly, redirecting to a _raw() variant or something).
> >
> > Obviously that would be expensive for non-RT use cases, but for helping
> > constrain latency, it may be worth it for RT.
> >
> >     Julia
>
> What if we as quick resort we add tpm_tis_iowrite8() to the TPM driver.
> Would be easy to move to iowrite8() if the problem is sorted out there
> later on.

Sounds OK to me. I will submit a PATCH shortly.

> /Jarkko