> From: Peter Chen > > According to Freescale imx28 Errata, "ENGR119653 USB: ARM to USB > register error issue", All USB register write operations must > use the ARM SWP instruction. So, we implement special hw_write > and hw_test_and_clear for imx28. > > Discussion for it at below: > http://marc.info/?l=linux-usb&m=137996395529294&w=2 Personally I think you should include the text of the errata in one of the files. 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. 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. David -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
