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Subject: Re: [PATCH v4 19/23] drivers/fsi: Add GPIO based FSI master
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Christopher Bostic <cbostic@linux.vnet.ibm.com>,
        Joel Stanley <joel@jms.id.au>
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Date: Mon, 10 Apr 2017 07:41:32 +1000
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On Sun, 2017-04-09 at 16:22 -0500, Christopher Bostic wrote:
> A 3 microsecond delay is required, however, to prevent occasional issues 
> during heavy FSI bus load stress testing.
> A 1 nanosecond delay using ndelay(1) had been specified prior to this 
> but after looking more closely at real time performance it turned out to 
> actually be roughly 1-2 microseconds.   This appears to be the minimum 
> resolution using the delay() linux libraries on the AST2400/2500.   
> Given this, increasing delay to 3 microseconds doesn't impact 
> performance much considering I can now remove the sample input delay 
> based on your recommendations to re-order the two clock delays.

This is huge delays. We should consider a AST2xxx specific variant of
the backend that uses nops or similar lab-calibrated constructs
instead. Otherwise we are stuck in the kHz range, this is a >200Mhz bus
:)

I don't understand why 3us delay would thus be necessary.

Where about did you observe issues ? Could it be that you don't wait
long enough in the transitions from write to read ?

Cheers,
Ben.