Re: [Xen-devel] [PATCH v2 1/2] ns16550: Add compatible string for Raspberry Pi 4

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 29/07/2019 17:06, Andre Przywara wrote:
> On Mon, 29 Jul 2019 09:19:19 -0400
> Stewart Hildebrand <stewart.hildebrand@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> > Per the BCM2835 peripherals datasheet [1] page 10:
> > "The UART core is build to emulate 16550 behaviour ... The implemented
> > UART is not a 16650 compatible UART However as far as possible the
> > first 8 control and status registers are laid out like a 16550 UART. Al
> > 16550 register bits which are not supported can be written but will be
> > ignored and read back as 0. All control bits for simple UART receive/
> > transmit operations are available."
> >
> > Additionally, Linux uses the 8250/16550 driver for the aux UART [2].
> >
> > Unfortunately the brcm,bcm2835-aux-uart device tree binding doesn't
> > have the reg-shift and reg-io-width properties [3]. Thus, the reg-shift
> > and reg-io-width properties are inherent properties of this UART.
> >
> > Thanks to Andre Przywara for contributing the reg-shift and
> > reg-io-width setting snippet.
> >
> > In my testing, I have relied on enable_uart=1 being set in config.txt,
> > a configuration file read by the Raspberry Pi's firmware. With
> > enable_uart=1, the firmware performs UART initialization.
> >
> > [1] 
> > https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2835/BCM2835-ARM-Peripherals.pdf
> > [2] 
> > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/tty/serial/8250/8250_bcm2835aux.c
> > [3] 
> > https://www.kernel.org/doc/Documentation/devicetree/bindings/serial/brcm,bcm2835-aux-uart.txt
> >
> > Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxxxxxxxxxx>
> Reviewed-by: Andre Przywara <andre.przywara@xxxxxxx>
> Tested-by: Andre Przywara <andre.przywara@xxxxxxx>
Acked-by: Julien Grall <julien.grall@xxxxxxx>

Cheers,

--
Julien Grall

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