Re: [PATCH REBASE] i2c-designware: make SDA hold time configurable

[Wolfram Sang <wsa@the-dreams.de>]

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Hi,

On Wed, Jun 12, 2013 at 04:47:45PM +0200, Christian Ruppert wrote:
> On Mon, Jun 10, 2013 at 05:29:55PM +0200, Wolfram Sang wrote:
> > On Tue, May 14, 2013 at 03:04:02PM +0200, Christian Ruppert wrote:
> > > This patch makes the SDA hold time configurable through device tree.
> > > 
> > > [rebased to i2c-current/i2c-next/mainline-3.10-rc1]
> > > 
> > > Signed-off-by: Christian Ruppert <christian.ruppert@abilis.com>
> > > Signed-off-by: Pierrick Hascoet <pierrick.hascoet@abilis.com>
> > 
> > Hmm, I really have problems adding a generic property. I need to better
> > understand why this is needed? What is the usecase? Can't a safe value
> > be calculated depending on the bus-speed? Is there a public datasheet
> > available somewhere?
> 
> I checked with our PCB/Applications team and the data sheets for the
> peripherals in question (DVB demodulator front ends) are under NDA.
> Mika, you seem to be interested in this patch as well. Do you know of
> any publicly available data sheets for hardware requiring this
> adjustment?

So, I looked around and found:
http://www.maximintegrated.com/app-notes/index.mvp/id/3268

which after thinking further about it gives me the following
conclusions:

- sda-hold-time is a property/requirement of a device not following
  the I2C spec. It is not a property of the master!

- It should not be encoded in the devicetree, since the flaw is implicit
  to the device, so only the driver needs to know about it. I wonder
  about something like this in the i2c slave driver:

	ret = i2c_request_sda_hold_time(client);

  The core then can collect the requests and forward them to the host
  driver. This driver then can set up the hardware or return -EOPNOTSUPP
  and we can even warn the user that there might be problems ahead.

- I wonder if we really need to have a parameter time-in-ns? The
  specs cleary say 300ns, so I'd think this is the value we should
  always use. This is from a theorhetical pov though, maybe your
  practical experience is different. What values do you need?


> In the case of the Designware block, the parameter both changes SDA and
> START hold times, however, and you'll find lots of data sheets for
> hardware with START hold time requirements on the net, e.g.
> http://ww1.microchip.com/downloads/en/DeviceDoc/21805B.pdf

What I couldn't find is a reference manual for a designware IP that
supports sda hold time? I found some spear SoC which do not have that
register, so that should surely be reflected in the patchset, too.

> The empirical solution in the function i2c_dw_scl_hcnt does not seem to
> work in all cases: Our lab guys confirmed that we have several PCB
> designs which do not work without adjusting the sda-hold-time parameter
> to an appropriate value. The value seems to be different for different
> PCBs.

I'd hope that 300ns is a safe value for all PCBs?

> I suspect that this kind of configurability is not the same for all i2c
> bus master hardware.

Yeah, maybe some do sda-holding by default? Dunno, never checked for
that detail.

Regards,

   Wolfram


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