On Wed, Jun 19, 2013 at 11:45:40AM +0200, Wolfram Sang wrote: > 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! Actually, in a protocol like I2C, every device on the bus must respect timing constraints like hold time etc. These parameters apply at the same time to the master and to all slaves. > - 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. This might be a solution but given that many I2C drivers are written as an afterthought by device manufacturers and are released under more or less open terms of licensing into the wild I doubt this would work very well in practise. > - 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 reality, the I2C specification is more subtle than that: The "data hold time" is specified as 0ns with a footnote [3] stating that devices "must internally provide a hold time of at least 300ns for the SDA signal...". Revision 5 contains a relatively understandable explanation about how to interpret this but earlier versions are less helpful. I think this confusion is at the root of many timing issues encountered with I2C (and the reason why Synopsys made this configurable). In fact, especially earlier specs are _all but_ clear in this point and we cannot assume that all peripherals were designed after Revision 5 was released in October 2012. > > 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. If you have access to DesignWare documentation, check out the "DesignWare DW_apb_i2c Databook" Version 1.17a from March 2012. Unluckily, I clearly don't have the right to share this document with you. Do you know the version of the blocks in the spear SoC which do not support this register? > > 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? Not according to our PCB guys. The highest value I have found in a quick check of our device trees is 650ns with others being just slightly above 300ns. > > 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 > -- Christian Ruppert , <christian.ruppert@xxxxxxxxxx> /| Tel: +41/(0)22 816 19-42 //| 3, Chemin du Pré-Fleuri _// | bilis Systems CH-1228 Plan-les-Ouates -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
