linux-kernel - Re: [PATCH v4 19/23] drivers/fsi: Add GPIO based FSI master

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Date:   Mon, 10 Apr 2017 07:53:55 +1000
From:   Benjamin Herrenschmidt <benh@....ibm.com>
To:     Christopher Bostic <cbostic@...ux.vnet.ibm.com>,
        Joel Stanley <joel@....id.au>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>, rostedt@...dmis.org,
        mingo@...hat.com, Greg KH <gregkh@...uxfoundation.org>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Jeffery <andrew@...id.au>,
        Alistair Popple <alistair@...ple.id.au>,
        "Edward A . James" <eajames@...ibm.com>,
        Jeremy Kerr <jk@...abs.org>
Subject: Re: [PATCH v4 19/23] drivers/fsi: Add GPIO based FSI master

On Mon, 2017-04-10 at 07:41 +1000, Benjamin Herrenschmidt wrote:
> 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 ?

FYI. pdbg in userspace operates without any  delays in practice, the
overhead between the various load/store instructions seems sufficient.

The only delay that's needed is when going through the FSI2PIB (to do
SCOMs) where it seems like back-2-back accesses can be problematic.

Cheers,
Ben