From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Ruppert Subject: Re: [PATCH REBASE] i2c-designware: make SDA hold time configurable Date: Wed, 19 Jun 2013 15:58:56 +0200 Message-ID: <20130619135855.GB16483@ab42.lan> References: <20130514110745.GA10906@intel.com> <1368536642-7158-1-git-send-email-christian.ruppert@abilis.com> <20130610152954.GE2987@katana> <20130612144743.GB8102@ab42.lan> <20130619094540.GA2950@katana> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: <20130619094540.GA2950@katana> Sender: linux-doc-owner@vger.kernel.org To: Wolfram Sang Cc: Mika Westerberg , linux-i2c@vger.kernel.org, "Ben Dooks (embedded platforms)" , Grant Likely , Rob Herring , Rob Landley , devicetree-discuss@lists.ozlabs.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Vineet Gupta , Pierrick Hascoet List-Id: linux-i2c@vger.kernel.org On Wed, Jun 19, 2013 at 11:45:40AM +0200, Wolfram Sang wrote: > Hi, >=20 > 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. > > > >=20 > > > > [rebased to i2c-current/i2c-next/mainline-3.10-rc1] > > > >=20 > > > > Signed-off-by: Christian Ruppert > > > > Signed-off-by: Pierrick Hascoet > > >=20 > > > 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 datas= heet > > > available somewhere? > >=20 > > I checked with our PCB/Applications team and the data sheets for th= e > > 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? >=20 > So, I looked around and found: > http://www.maximintegrated.com/app-notes/index.mvp/id/3268 >=20 > which after thinking further about it gives me the following > conclusions: >=20 > - 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 impli= cit > to the device, so only the driver needs to know about it. I wonder > about something like this in the i2c slave driver: >=20 > ret =3D i2c_request_sda_hold_time(client); >=20 > The core then can collect the requests and forward them to the host > driver. This driver then can set up the hardware or return -EOPNOTS= UPP > 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 t= o interpret this but earlier versions are less helpful. I think this confusion is at the root of many timing issues encountered with I2C (an= d 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 >=20 > 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 no= t support this register? > > The empirical solution in the function i2c_dw_scl_hcnt does not see= m 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 param= eter > > to an appropriate value. The value seems to be different for differ= ent > > PCBs. >=20 > 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 quic= k check of our device trees is 650ns with others being just slightly abov= e 300ns. > > I suspect that this kind of configurability is not the same for all= i2c > > bus master hardware. >=20 > Yeah, maybe some do sda-holding by default? Dunno, never checked for > that detail. >=20 > Regards, >=20 > Wolfram >=20 --=20 Christian Ruppert , /| Tel: +41/(0)22 816 19-42 //| 3, Chemin du Pr=E9-F= leuri _// | bilis Systems CH-1228 Plan-les-Oua= tes