linux-kernel - Re: [PATCH v2 1/2] tpm: devicetree: document properties for cr50

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Date:	Wed, 20 Jul 2016 12:49:12 -0700
From:	Andrey Pronin <apronin@...omium.org>
To:	Rob Herring <robh@...nel.org>
Cc:	Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
	Peter Huewe <peterhuewe@....de>,
	Marcel Selhorst <tpmdd@...horst.net>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	tpmdd-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	Christophe Ricard <christophe.ricard@...il.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 1/2] tpm: devicetree: document properties for cr50

On Wed, Jul 20, 2016 at 02:03:03PM -0500, Rob Herring wrote:
> On Tue, Jul 19, 2016 at 08:41:24PM -0700, Andrey Pronin wrote:

Hi Rob,

> As I mentioned, there may be common properties. It doesn't seem you 
> looked, so I did:
> 
> - spi-rx-delay-us  - (optional) Microsecond delay after a read transfer.
> - spi-tx-delay-us  - (optional) Microsecond delay after a write transfer.
> 
> Seems to me setting one or both of these should work for you.
>

Yes, good catch, my fault I didn't see those.
But they are not exactly what I mean and need. I don't need delay after
each read or write transfer. What is needed is a guaranteed time
between transfers.

So, if the next transaction doesn't come withing the next X ms (or us),
we don't waste time on inserting a delays after this transaction at all.
Following the description and always inserting a delay must work well
for short microseconds-long delays. For longer milliseconds-long delays
a different strategy of checking the time when the previous transaction
was and only delaying if it was not too long ago is better.

Thus, I won't be able to re-use these properties anyways based on their
current description in bindings/spi/spi-bus.txt.

> > +- sleep-delay-ms: Time after the last SPI activity, after which the chip
> > +  may go to sleep.
> > +- wake-start-delay-ms: Time after initiating wake up before the chip is
> > +  ready to accept commands over SPI.
> 
> I also asked why these 2 can't be hard-coded in the driver?
>

Sorry, I just updated this patch description in v2 to indicate why they are not
hard-coded, but didn't answer explicitly. As the firmware changes, a different
revision of it can have a different time before it sleeps in its configuration,
or the time it takes it to startup may be different. Thus, there's a way to
set it here w/o changing the driver.

Andrey