On 19:59 Wed 31 Oct , Grant Likely wrote: > Hi Roland > > On Wed, Oct 31, 2012 at 6:19 PM, Roland Stigge <sti...@antcom.de> wrote: > > On 10/31/2012 04:00 PM, Grant Likely wrote: > >> For the API, I don't think it is a good idea at all to try and > >> abstract away gpios on multiple controllers. I understand that it > >> makes life a lot easier for userspace to abstract those details away, > >> but the problem is that it hides very important information about how > >> the system is actually constructed that is important to actually get > >> things to work. For example, say you have a gpio-connected device with > >> the constraint that GPIOA must change either before or at the same > >> time as GPIOB, but never after. If those GPIOs are on separate > >> controllers, then the order is completely undefined > > > > It is correct that it's not (yet) well documented and the API is also > > not very explicit about it, but the actual approach of the manipulation > > order is to let drivers handle gpios "as simultaneous as possible" and > > when not possible, do it in the _order of bits specified_ (either > > defined at the device tree level, or when created via > > block_gpio_create() directly). > > The documentation is actually fine. I do understand that the intent is > "as simultaneous as possible", but I accept the point that the order > of specification affects the behaviour*. However, it still remains > that the method used by the ABI abstracts at the wrong level and that > blocking arbitrary GPIO pins into a single virtual GPIO register is a > bad idea. > > *note that the current code doesn't implement that intended behaviour > either since the gpios are processed in the order of the controllers, > not the order of the bits. > > > I'm not sure how far you tested the API in depth: You can already define > > a block that maps onto a subset of gpios on a controller and internally > > of course maps onto those set and clear operations. Whenever you need to > > manipulate a different subset (whether disjoint or overlapping), you can > > easily define _additional_ blocks. From my experience, this solves most > > of the real world problems when n-bit busses are bit banged over GPIOs. > > Doesn't this already solve this (in a different way, though)? > > Blech! Requiring a new block for each possible combination of > write-at-once bits is a horrible ABI. That just strengthens my opinion > that the abstraction isn't right yet. > > > Pin direction currently needs to be set up separately, analogous to > > requesting gpios. Need to document this better, right. The assumption is > > that I/O needs to be efficient primarily, before bloating the API with > > direction functions. Or should I add functions for this? > > Since this is a userspace facing ABI, once it is merged it cannot be > changed in an incompatible way. I cannot merge it until there is at > least a plan for how to handle all of the reasonable use cases. That > means it must support set/clear or mask operations. Also, if it sticks > with the design of grouping pins from multiple controllers, then it > needs to handle explicitly constraining what order operations are > performed in at the time of the operation. At the time of setup > doesn't work since constraints between pins may not always be in the > same order. > > I really think you should consider implementing a command stream type > of interface. I agreed with Grant and I'm not also a fan of the sysfs ABI
as I already point out in the previous version and Linus too Best Regards, J. > > g. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
