linux-kernel - Re: [PATCH v5 0/6] bcm2835: auxiliar device support for spi

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Date:	Thu, 10 Sep 2015 18:02:47 +0100
From:	Phil Elwell <phil@...pberrypi.org>
To:	Martin Sperl <kernel@...tin.sperl.org>,
	Noralf Trønnes <noralf@...nnes.org>
CC:	Eric Anholt <eric@...olt.net>,
	Stephen Warren <swarren@...dotorg.org>,
	Lee Jones <lee@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	Mark Brown <broonie@...nel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	<devicetree@...r.kernel.org>,
	<linux-rpi-kernel@...ts.infradead.org>,
	<linux-spi@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 0/6] bcm2835: auxiliar device support for spi

[ Sending again in plain text ]
Noralf pointed me at fixed-factor-clock, and that works in our
(downstream) environment:

    soc: soc {
        ...
        uart1: uart@...15040 {
            compatible = "brcm,bcm2835-aux-uart", "ns16550";
            reg = <0x7e215040 0x40>;
            interrupts = <1 29>;
            clocks = <&clk_uart1>;
            reg-shift = <2>;
            no-loopback-test;
            status = "disabled";
            };
    };

    clocks: clocks {
        ...
        clk_core: clock@2 {
            compatible = "fixed-clock";
            reg = <2>;
            #clock-cells = <0>;
            clock-output-names = "core";
            clock-frequency = <250000000>;
        };
        ...
        clk_uart1: clock@6 {
            compatible = "fixed-factor-clock";
            clocks = <&clk_core>;
            #clock-cells = <0>;
            clock-div = <1>;
            clock-mult = <2>;
        };
    };

Phil

On 10/09/2015 16:57, Martin Sperl wrote:
>> On 10.09.2015, at 17:48, Noralf Trønnes <noralf@...nnes.org> wrote:
>>
>> This looks interesting.
>> But there's a challenge with the uart1 and the 8250 driver.
>>
>> Phil Elwell has this to say:
>> This means that that UART1 isn't an exact clone of a 8250 UART.
>> In a particular, the clock divisor is calculated differently.
>> A standard 8250 derives the baud rate as clock/(divisor16),
>> whereas the BCM2835 mini UART uses clock/(divisor8). This means
>> that if you want to use the standard driver then you need to lie
>> about the clock frequency, providing a value is twice the real
>> value, in order for a suitable divisor to be calculated.
>>
>> Ref: https://github.com/raspberrypi/linux/pull/1008#issuecomment-139234607
>>
>> So either we need a new uart1 driver or a doubled clock freq. somehow.
> Found out the same thing and communicated it to Eric - not 
> knowing about the different divider…
>
> Martin



On 10/09/2015 16:57, Martin Sperl wrote:
>> On 10.09.2015, at 17:48, Noralf Trønnes <noralf@...nnes.org> wrote:
>>
>> This looks interesting.
>> But there's a challenge with the uart1 and the 8250 driver.
>>
>> Phil Elwell has this to say:
>> This means that that UART1 isn't an exact clone of a 8250 UART.
>> In a particular, the clock divisor is calculated differently.
>> A standard 8250 derives the baud rate as clock/(divisor16),
>> whereas the BCM2835 mini UART uses clock/(divisor8). This means
>> that if you want to use the standard driver then you need to lie
>> about the clock frequency, providing a value is twice the real
>> value, in order for a suitable divisor to be calculated.
>>
>> Ref: https://github.com/raspberrypi/linux/pull/1008#issuecomment-139234607
>>
>> So either we need a new uart1 driver or a doubled clock freq. somehow.
> Found out the same thing and communicated it to Eric - not 
> knowing about the different divider…
>
> Martin

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