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@xxxxxxxxxx> > > > Signed-off-by: Pierrick Hascoet <pierrick.hascoet@xxxxxxxxxx> > > > > 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
Attachment:
signature.asc
Description: Digital signature
