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linux-next/Documentation/bus-devices/ti-gpmc.txt
Afzal Mohammed 246da26d37 ARM: OMAP2+: gpmc: generic timing calculation
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 <afzal@ti.com>
2012-11-09 18:07:11 +05:30

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GPMC (General Purpose Memory Controller):
=========================================
GPMC is an unified memory controller dedicated to interfacing external
memory devices like
* Asynchronous SRAM like memories and application specific integrated
circuit devices.
* Asynchronous, synchronous, and page mode burst NOR flash devices
NAND flash
* Pseudo-SRAM devices
GPMC is found on Texas Instruments SoC's (OMAP based)
IP details: http://www.ti.com/lit/pdf/spruh73 section 7.1
GPMC generic timing calculation:
================================
GPMC has certain timings that has to be programmed for proper
functioning of the peripheral, while peripheral has another set of
timings. To have peripheral work with gpmc, peripheral timings has to
be translated to the form gpmc can understand. The way it has to be
translated depends on the connected peripheral. Also there is a
dependency for certain gpmc timings on gpmc clock frequency. Hence a
generic timing routine was developed to achieve above requirements.
Generic routine provides a generic method to calculate gpmc timings
from gpmc peripheral timings. struct gpmc_device_timings fields has to
be updated with timings from the datasheet of the peripheral that is
connected to gpmc. A few of the peripheral timings can be fed either
in time or in cycles, provision to handle this scenario has been
provided (refer struct gpmc_device_timings definition). It may so
happen that timing as specified by peripheral datasheet is not present
in timing structure, in this scenario, try to correlate peripheral
timing to the one available. If that doesn't work, try to add a new
field as required by peripheral, educate generic timing routine to
handle it, make sure that it does not break any of the existing.
Then there may be cases where peripheral datasheet doesn't mention
certain fields of struct gpmc_device_timings, zero those entries.
Generic timing routine has been verified to work properly on
multiple onenand's and tusb6010 peripherals.
A word of caution: generic timing routine has been developed based
on understanding of gpmc timings, peripheral timings, available
custom timing routines, a kind of reverse engineering without
most of the datasheets & hardware (to be exact none of those supported
in mainline having custom timing routine) and by simulation.
gpmc timing dependency on peripheral timings:
[<gpmc_timing>: <peripheral timing1>, <peripheral timing2> ...]
1. common
cs_on: t_ceasu
adv_on: t_avdasu, t_ceavd
2. sync common
sync_clk: clk
page_burst_access: t_bacc
clk_activation: t_ces, t_avds
3. read async muxed
adv_rd_off: t_avdp_r
oe_on: t_oeasu, t_aavdh
access: t_iaa, t_oe, t_ce, t_aa
rd_cycle: t_rd_cycle, t_cez_r, t_oez
4. read async non-muxed
adv_rd_off: t_avdp_r
oe_on: t_oeasu
access: t_iaa, t_oe, t_ce, t_aa
rd_cycle: t_rd_cycle, t_cez_r, t_oez
5. read sync muxed
adv_rd_off: t_avdp_r, t_avdh
oe_on: t_oeasu, t_ach, cyc_aavdh_oe
access: t_iaa, cyc_iaa, cyc_oe
rd_cycle: t_cez_r, t_oez, t_ce_rdyz
6. read sync non-muxed
adv_rd_off: t_avdp_r
oe_on: t_oeasu
access: t_iaa, cyc_iaa, cyc_oe
rd_cycle: t_cez_r, t_oez, t_ce_rdyz
7. write async muxed
adv_wr_off: t_avdp_w
we_on, wr_data_mux_bus: t_weasu, t_aavdh, cyc_aavhd_we
we_off: t_wpl
cs_wr_off: t_wph
wr_cycle: t_cez_w, t_wr_cycle
8. write async non-muxed
adv_wr_off: t_avdp_w
we_on, wr_data_mux_bus: t_weasu
we_off: t_wpl
cs_wr_off: t_wph
wr_cycle: t_cez_w, t_wr_cycle
9. write sync muxed
adv_wr_off: t_avdp_w, t_avdh
we_on, wr_data_mux_bus: t_weasu, t_rdyo, t_aavdh, cyc_aavhd_we
we_off: t_wpl, cyc_wpl
cs_wr_off: t_wph
wr_cycle: t_cez_w, t_ce_rdyz
10. write sync non-muxed
adv_wr_off: t_avdp_w
we_on, wr_data_mux_bus: t_weasu, t_rdyo
we_off: t_wpl, cyc_wpl
cs_wr_off: t_wph
wr_cycle: t_cez_w, t_ce_rdyz
Note: Many of gpmc timings are dependent on other gpmc timings (a few
gpmc timings purely dependent on other gpmc timings, a reason that
some of the gpmc timings are missing above), and it will result in
indirect dependency of peripheral timings to gpmc timings other than
mentioned above, refer timing routine for more details. To know what
these peripheral timings correspond to, please see explanations in
struct gpmc_device_timings definition. And for gpmc timings refer
IP details (link above).