On 31/05/2017 16:58, Stefan Wahren wrote: > Am 31.05.2017 um 17:27 schrieb Stephen Warren: >> On 05/30/2017 06:23 AM, Phil Elwell wrote: >>> Hi, >>> >>> I've run into a problem using the fixed-factor clock on Raspberry Pi >>> and I'd >>> like some advice before I submit a patch. >>> >>> Some context: the aim is to use a standard UART and some external >>> circuitry >>> as a MIDI interface. This would be straightforward except that Linux >>> doesn't >>> recognise the required 31.25KHz as a valid UART baud rate. Rhe >>> workaround is >>> to declare the UART clock such that the reported rate differs from >>> the actual >>> rate. If one sets the reported rate to be (actual*38400)/31250 then >>> requesting a 38400 baud rate will result in an actual 31250 baud signal. >> >> This sounds like the wrong approach. Forcing the port to use a >> different clock rate than the user requests would prevent anyone from >> using that port for its standard purpose; it'd turn what should be a >> runtime decision into a compile-time decision. >> >> Are you sure there's no way to simply select the correct baud rate on >> the port? I see plenty of references to configuring custom baud rates >> under Linux when I search Google, e.g.: >> >>> https://stackoverflow.com/questions/12646324/how-to-set-a-custom-baud-rate-on-linux >>> >> "How to set a custom baud rate on Linux?" >> >>> https://sourceware.org/ml/libc-help/2009-06/msg00016.html >> "Re: Terminal interface and non-standard baudrates" >> > > I remember this gist from Peter Hurley: > > https://gist.github.com/peterhurley/fbace59b55d87306a5b8
Thank you, Stephen and Stephan. Stephen - the clock scaling is applied by a DT overlay so it effectively a runtime setting, but I take your point about the elegance of the solution. Stefan - anybaud looks promising, although I would have preferred for users to continue to use the existing user-space tools - kernel changes can be deployed more easily. For my edification, can you pretend for a moment that the application was a valid one and answer any of my original questions?: 1. Should all system clock drivers use OF_CLK_DECLARE? Doing so would probably avoid this problem, but further initialisation order dependencies may require more drivers to be initialised early. 2. Why does the clock initialisation hook registered by OF_CLK_DECLARE not return any indication of success? If it did, and if the OF_POPULATED flag was only set after successful initialisation then the normal retrying of a deferred probe would also avoid this problem. 3. Would adding the OF_CLK_DECLARE hook prevent the use of the devm_ managed functions like devm_kzalloc? If not, why doesn't fixed-factor-clock use it? Thanks, Phil