RE: [PATCH v2] pinctrl: at91-pio4: implement .get_multiple and .set_multiple

[David Laight <David.Laight@ACULAB.COM>]

From: Alexandre Belloni
> Sent: 06 September 2019 10:12
> On 06/09/2019 09:05:36+0000, David Laight wrote:
> > From: Alexandre Belloni
> > > Implement .get_multiple and .set_multiple to allow reading or setting
> > > multiple pins simultaneously. Pins in the same bank will all be switched at
> > > the same time, improving synchronization and performances.
> >
> > Actually it won't 'improve synchronisation', instead it will lead to
> > random synchronisation errors and potential metastability if one
> > pin is used as a clock and another as data, or if the code is reading
> > a free-flowing counter.
> >
> 
> It does improve gpio switching synchronisation when they are in the same
> bank as it will remove the 250ns delay. Of course, if you need this
> delay between clk and data, then the consumer driver should ensure the
> delay is present.

With multiple requests the output pin changes will always be in the
same order and will be separated by (say) 250ns.
This is a guaranteed synchronisation.

If you change multiple pins with the same 'iowrite()' then the pins
will change at approximately the same time.
But the actual order will depend on internal device delays (which
may depend on the actual silicon and temperature).
You then have to take account of varying track lengths and the
target devices input stage properties before knowing which change
arrives first.
The delays might be sub-nanosecond, but they matter if you are
talking about synchronisation.

IIRC both SMBus and I2C now quote 0ns setup time.
Changing both clock and data with the same IOW isn't enough to
guarantee this.
(In practise the I2C setup time required by a device is probably
slightly negative (In order to support 0ns inputs) so a very small
-ve setup will (mostly) work.)

	David

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