On Monday 28 July 2014 15:20:06 Andre Przywara wrote: > On 28/07/14 11:46, Arnd Bergmann wrote: > > On Monday 28 July 2014 10:23:57 Graeme Gregory wrote: > >> The PL011 UART is the use-case I keep hitting, that IP block has a > >> variable input clock on pretty much everything I have seen in the wild. > > > > Ok, I see. What does ACPI-5.1 say about pl011? > > > > Interestingly, the subset of pl011 that is specified by SBSA does not > > contain the IBRD/FBRD registers, effectively making it a fixed-rated > > UART (I guess that would be a ART, without the U then), and you > > consequently don't even need to know the clock rate. > > The idea of this was probably to let the baudrate set by some firmware > code to the "right" value and the spec just didn't want to expose the > details for the generic UART: > "This specification does not cover registers needed to configure the > UART as these are considered hardware-specific and will be set up by > hardware-specific software." > To me that reads like the SBSA UART is just for debugging, and you are > expected just to access the data register. Right, makes sense. It also avoids the case where Linux for some reason ends up using a different line rate than the firmware, which can cause a lot of unnecessary pain. > > However, my guess is that most hardware in the real world contains > > an actual pl011 and it does make a lot of sense to allow setting > > the baud rate on it, which then requires knowing the input clock. > > > > If there is any hardware that implements just the SBSA-mandated subset > > rather than the full pl011, we should probably implement both > > in the kernel: a dumb driver that can only send and receive, and the > > more complex one that can set the bit rates and flow-control but that > > requires a standardized ACPI table with the input clock rate. > > The fast model I use can be switched to use the SBSA restricted PL011, > and as expected the Linux kernel crashes at the device doesn't support > DMA (and a lot more stuff) - but the current code requires it. It does? We have a lot of platforms that don't have DMA support for pl011. > So I am about to implement a new driver for that SBSA subset. So far > this will be a separate driver, starting from a copy of amba-pl011.c, > but removing most of the code ;-) Ok. You might want to consider starting from a different base though. IIRC, pl011 uses uart_port as the basic abstraction, while the new driver should probably use the raw tty_port instead. drivers/tty/goldfish.c is probably a good example to look at for that. You could also make it a hvc_driver like drivers/tty/hvc/hvc_vio.c, but I'm not sure if that model seen favorable by the tty maintainers. It would probably be the shortest driver though. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
