path: root/Documentation/bus-devices
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2012-11-09ARM: OMAP2+: gpmc: generic timing calculationAfzal Mohammed
Presently there are three peripherals that gets it timing by runtime calculation. Those peripherals can work with frequency scaling that affects gpmc clock. But timing calculation for them are in different ways. Here a generic runtime calculation method is proposed. Input to this function were selected so that they represent timing variables that are present in peripheral datasheets. Motive behind this was to achieve DT bindings for the inputs as is. Even though a few of the tusb6010 timings could not be made directly related to timings normally found on peripherals, expressions used were translated to those that could be justified. There are possibilities of improving the calculations, like calculating timing for read & write operations in a more similar way. Expressions derived here were tested for async onenand on omap3evm (as vanilla Kernel does not have omap3evm onenand support, local patch was used). Other peripherals, tusb6010, smc91x calculations were validated by simulating on omap3evm. Regarding "we_on" for onenand async, it was found that even for muxed address/data, it need not be greater than "adv_wr_off", but rather could be derived from write setup time for peripheral from start of access time, hence would more be in line with peripheral timings. With this method it was working fine. If it is required in some cases to have "we_on" same as "wr_data_mux_bus" (i.e. greater than "adv_wr_off"), another variable could be added to indicate it. But such a requirement is not expected though. It has been observed that "adv_rd_off" & "adv_wr_off" are currently calculated by adding an offset over "oe_on" and "we_on" respectively in the case of smc91x. But peripheral datasheet does not specify so and so "adv_rd(wr)_off" has been derived (to be specific, made ignorant of "oe_on" and "we_on") observing datasheet rather than adding an offset. Hence this generic routine is expected to work for smc91x (91C96 RX51 board). This was verified on smsc911x (9220 on OMAP3EVM) - a similar ethernet controller. Timings are calculated in ps to prevent rounding errors and converted to ns at final stage so that these values can be directly fed to gpmc_cs_set_timings(). gpmc_cs_set_timings() would be modified to take ps once all custom timing routines are replaced by the generic routine, at the same time generic timing routine would be modified to provide timings in ps. struct gpmc_timings field types are upgraded from u16 => u32 so that it can hold ps values. Whole of this exercise is being done to achieve driver and DT conversion. If timings could not be calculated in a peripheral agnostic way, either gpmc driver would have to be peripheral gnostic or a wrapper arrangement over gpmc driver would be required. Signed-off-by: Afzal Mohammed <>