On Fri, 12 Jun 2009, Hans Verkuil wrote: > On Friday 12 June 2009 14:59:03 Guennadi Liakhovetski wrote: > > On Fri, 12 Jun 2009, Hans Verkuil wrote: > > > > > > 1. it is very unusual that the board designer has to mandate what signal > > > > polarity has to be used - only when there's additional logic between the > > > > capture device and the host. So, we shouldn't overload all boards with > > > > this information. Board-code authors will be grateful to us! > > > > > > I talked to my colleague who actually designs boards like that about what > > > he would prefer. His opinion is that he wants to set this himself, rather > > > than leave it as the result of a software negotiation. It simplifies > > > verification and debugging the hardware, and in addition there may be > > > cases where subtle timing differences between e.g. sampling on a falling > > > edge vs rising edge can actually become an important factor, particularly > > > on high frequencies. > > > > I'd say this is different. You're talking about cases where you _want_ to > > be able to configure it explicitly, I am saying you do not have to _force_ > > all to do this. Now, this selection only makes sense if both are > > configurable, right? In this case, e.g., pxa270 driver does support > > platform-specified preference. So, if both the host and the client can > > configure either polarity in the software you _can_ still specify the > > preferred one in platform data and it will be used. > > > > I think, the ability to specify inverters and the preferred polarity > > should cover all possible cases. > > In my opinion you should always want to set this explicitly. This is not > something you want to leave to chance. Say you autoconfigure this. Now > someone either changes the autoconf algorithm, or a previously undocumented > register was discovered for the i2c device and it can suddenly configure the > polarity of some signal that was previously thought to be fixed, or something > else happens causing a different polarity to be negotiated. TBH, the argumentation like "someone changes the autoconf algorithm" or "previously undocumented register is discovered" doesn't convince me. In any case, I am adding authors, maintainers and major contributors to various soc-camera host drivers to CC and asking them to express their opinion on this matter. I will not add anything else here to avoid any "unfair competition":-) they will have to go a couple emails back in this thread to better understand what is being discussed here. > Suddenly the board > doesn't work because it was never verified or tested with that different > polarity. Or worse: it glitches only 0.001% of the time. That's going to be a > nasty bug to find. > > You generally verify your board with specific bus settings, and that's what > should also be configured explicitly. Sure, it is nice not to have to think > about this. The problem is that I believe that you *have* to think about it. > > The longer I think about this, the more convinced I am that relying on > autoconfiguration is a bad design. Thanks Guennadi --- Guennadi Liakhovetski, Ph.D. Freelance Open-Source Software Developer http://www.open-technology.de/ -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
