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From: Alexander Stein <>
To: Ken Goldman <>
Subject: Re: [tpmdd-devel] [PATCH v2] tpm_tis: fix stall after iowrite*()s
Date: Thu, 17 Aug 2017 07:57:08 +0200	[thread overview]
Message-ID: <2684601.PnLOm3uNmr@ws-stein> (raw)
In-Reply-To: <>

On Wednesday 16 August 2017 17:15:55, 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.
> 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?

No, there was already a similar problem in e1000e where a PCIe read stalled 
the CPU, hence no interrupts are serviced. See
AFAIK there was no outcome though.

> 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.

Realtime will always affect performance, but IMHO the latter is much more 

Best regards,

  reply	other threads:[~2017-08-17  5:57 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
2017-08-14 22:53 ` [PATCH] tpm_tis: fix stall after iowrite*()s Haris Okanovic
2017-08-15  6:11   ` Alexander Stein
2017-08-15 20:10     ` Haris Okanovic
     [not found] ` <>
2017-08-15 20:13   ` [PATCH v2] " Haris Okanovic
2017-08-16 21:15     ` [tpmdd-devel] " Ken Goldman
2017-08-17  5:57       ` Alexander Stein [this message]
2017-08-17 10:38       ` Sebastian Andrzej Siewior
     [not found]         ` <>
2017-08-17 17:17           ` Jason Gunthorpe
     [not found]             ` <20170817171732.GA22792-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/>
2017-08-17 20:12               ` Haris Okanovic
2017-08-19 17:03             ` [tpmdd-devel] " Jarkko Sakkinen

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