Hi, On Wed, Mar 19, 2014 at 09:15:03AM +0000, Mark Jackson wrote: > On 18/03/14 17:11, Felipe Balbi wrote: > > On Tue, Mar 18, 2014 at 05:04:47PM +0000, Mark Jackson wrote: > >> On 18/03/14 16:55, Felipe Balbi wrote: > >>> Hi Mark, > >>> > >>> I'm looking at the omap-serial driver and saw that you added rts-gpio > >>> binding in commit 4a0ac0f55b18dc297a87a85417fcf068658bf103 (OMAP: add > >>> RS485 support) but, as it turns out, gpio0_13 and gpio2_15 are both > >>> actual RTS signals. > >>> > >>> Instead of adding that extra GPIO handling, why didn't you just mux > >>> those signals as RTS and enable auto-RTS/auto-CTS feature ? > >>> > >>> It looks to me like that's highly unnecessary binding. > >> > >> I agree !! > >> > >> I think it was to allow delays pre- and post- sending the comms data. > >> > >> Several RS485 drivers require a "warm up" time before they will > >> transmit data correctly, and also need a "cool down" time to prevent > >> clipping of the last few bits of data. > >> > >> IIRC the built-in RTS handling did not allow for this. > > > > you might be right here. I can't find anywhere to write rts delays. > > Weird... digging TRM. > > > >> It also allows the RTS signal to be "inverted", again required for > >> some RS485 driver chips. > >> > >> However, I'll dig into why I did this and report back. > > > > cool, thanks > > Okay ... it comes back to me now. > > When using RS485 drivers, we're not actually using RTS as a "Ready > To Send", we're really using it as an "enable RS485 driver". > > I just used the "RTS" mnemonic as "we're now wanting to send some > data so please now enable the RS485 driver", rather than the normal > "I'm ready for your to send me some data". > > So it's the opposite function. > > Maybe it was a poor choice of abbreviation ? > > As I said before, RS485 drivers might have active or active low > enables, so we might need to invert the "RTS" polarity. This is > not handled by the hardware RTS signal. > > Is that a good enough explanation ? fair, but considering you can toggle RTS by hand with writes to MCR register, I still don't see the need for remuxing the lines as GPIOs. Just don't use auto-RTS and toggle the line by hand with MCR, no ? -- balbi
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