On 12/09/2018 11:54:07+0100, Lee Jones wrote: > On Wed, 12 Sep 2018, Geert Uytterhoeven wrote: > > On Wed, Sep 12, 2018 at 10:41 AM Lee Jones <lee.jones@xxxxxxxxxx> wrote: > > > On Wed, 12 Sep 2018, Alexandre Belloni wrote: > > > > On 11/09/2018 23:54:40+0100, Lee Jones wrote: > > > > > > > http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-6438-32-bit-ARM926-Embedded-Microprocessor-SAM9G45_Datasheet.pdf > > > > > > > > > > > > > > USART doc starting p572, registers p621. > > > > > > > > > > After looking at the datasheet, I don't see any reason why one of the > > > > > two drivers can't be selected using different compatible strings. > > > > > > > > Because there is only one IP and we don't use the device tree to selecet > > > > linux specific drivers. > > > > > > We do it all the time. There are loads of MFDs (def: same IP, with > > > different functions) which have separate compatibles for their various > > > functions. If you wish this IP to operate as an SPI controller, it > > > should have an SPI compatible, if you wish it to operate as a U(S)ART, > > > then it should have a UART compatible. It's what we do for most of > > > the other MFDs in the kernel. > > > > There is a big difference: MFD functions are(more or less) independent > > functions, which can be used at the same time. It makes perfect sense for a > > single IP block that has both SPI and UART interfaces, that can be used at > > the same time. > > > > In this case, there is a single piece of hardware that can perform > > different functions, but not at the same time. Performing a different > > function means configuring the hardware for that function, hence using a > > different driver (from a different subsystem). > > Yes, I can see that PoV. > > But ... we can't have it both ways. *Either* it's a true MFD, in > which case it can/should have 2 separate compatible strings which can > be specified directly from the DT. *Or* it's not an MFD. In the > latter case, which I think we're all agreeing on (else we'd have 2 > compatible strings), MFD is not the place to handle this (my original > point). > If that is what bothers you, then let's move it out of mfd. > So ... this is a USART device which can do SPI, right? > > My current thinking is that; as this is a USART device first & > foremost, the USART should be probed in the first instance regardless, > then if SPI mode is specified it (the USART driver) registers the SPI > platform driver (as MFD does currently) and exits gracefully, allowing > the SPI driver to take over. > > Spanner in the works: is it physically possible to change the mode at > run-time? :s Yes it is possible but on Linux that will not happen without probing the drivers again. I think DT overlays will be the only possible use case because on SPI, you'd still have to provide nodes for the connected SPI devices. -- Alexandre Belloni, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
