On 12/05/2013 01:16 PM, Peter Chen wrote: >> Personally I think you should include the text of the errata >> in one of the files. > Yes, some may be interested in detail, not only the solution, > will do it next time. > >> I'm not suggesting you change the guts of the patch, but... >> >> Having read the errata it isn't 100% clear to me that using SWP >> is necessary for all register accesses. > > Only write USB controller register is needed. > >> Since SWP is a locked bus cycle it will have performance penalties, >> depending on the number of write operations this might be measurable. >> >> AFAICT the reason that SWP works is that does a read cycle prior >> to the write that ensures that logic is all in the correct state >> (I think the read returns garbage). >> There is no requirement for a locked bus cycle, or for the cycles >> to be adjacent (from the cpu's point of view). Just that there >> can be no other cycles on what I assume is a peripheral bus. >> >> So if the code can guarantee no other cycles on the same bus (the >> ones that are documented in the errata to cause problems) then >> there is no need to use the SWP instruction. >> Typically this would be single cpu systems doing a series of >> accesses (the first could be a read) with interrupts hard disabled. >> > > Thanks for your analysis. > > As far as I know, the workaround for this problem is it needs a read > usb controller register operation before every write, the swp instruction > may be the easiest way to implement it, so the design team suggests it, > this patch just implements that workaround. > > We can't guarantee there is no other access on the AHB without disable > interrupt, would you think below operation will be more efficient than swp, > or any better solutions? > > local_irq_save(flags); > readl(addr); > writel(val, addr); > local_irq_restore(flags); Does this work without modifications on PREEMPT_RT? Marc -- Pengutronix e.K. | Marc Kleine-Budde | Industrial Linux Solutions | Phone: +49-231-2826-924 | Vertretung West/Dortmund | Fax: +49-5121-206917-5555 | Amtsgericht Hildesheim, HRA 2686 | http://www.pengutronix.de |
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