Messages in this thread | | | Subject | Re: [PATCH] [RFC] tpm_tis: tpm_tcg_flush() after iowrite*()s | From | Haris Okanovic <> | Date | Mon, 14 Aug 2017 17:53:15 -0500 |
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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 >
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