On Thu, Jan 02, 2020 at 11:27:57PM +0100, Luca Ceresoli wrote: > Hi Wolfram, > > On 02/01/20 22:13, Wolfram Sang wrote: > > Hi Luca, > > > >>> This looks quite inefficient, especially if the beginning of the range > >>> is populated with devices. Furthermore, I think there's a high risk of > >>> false negatives, as acquiring a free address and reprogramming the > >>> client to make use of it are separate operations. > >> > >> Right. Applying the alias could raise other errors, thus one would need > >> i2c_new_alias_device() to keep the alias locked until programming it has > >> either failed or has been successfully programmed. > > > > Please see my reply to Laurent, I don't think it is racy. But please > > elaborate if you think I am wrong. > > Uhm, you are right here, it's not racy. Sorry, I had read the code > quickly and didn't notice the i2c_new_dummy_device() call. > > So this means if i2c_new_alias_device() succeeds but the caller later > fails while applying the alias, then it has to call > i2c_unregister_device() to free the alias. Correct? I was wrong as well, sorry about that. > >>> What happened to the idea of reporting busy address ranges in the > >>> firmware (DT, ACPI, ...) ? > >> > >> Indeed that's how I remember it as well, and I'm a bit suspicious about > >> sending out probe messages that might have side effects (even if the > >> false negative issue mentioned by Laurent were solved). You know, I've > >> been taught to "expect the worse" :) so I'd like to better understand > >> what are the strong reasons in favor of probing, as well as the > >> potential side effects. > > > > As I said to Laurent, too, I think the risk that a bus is not fully > > described is higher than a device which does not respond to a read_byte. > > In both cases, we would wrongly use an address in use. I don't fully agree with this, I think we shouldn't impose a penalty on every user because some device trees don't fully describe the hardware. I think we should, at the very least, skip the probe and rely on DT if DT explicitly states that all used addresses are listed. We discussed a property to report addresses used by devices not described in DT, if that property is listed I would prefer trusting DT. > OK, I'm still uncomfortable with sending unexpected transactions to the > dark outer space, but this is more a feeling than based on facts, and > you know more than me, so I guess I can live with that. > > > Also, all the best for you in 2020! > > Thanks. Best wishes to you too for the new year! Likewise. Let's start with a simple wish of getting this issue resolved :-) -- Regards, Laurent Pinchart
