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

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Message-ID: <55F1A651.5090102@tronnes.org>
Date:	Thu, 10 Sep 2015 17:48:33 +0200
From:	Noralf Trønnes <noralf@...nnes.org>
To:	Eric Anholt <eric@...olt.net>, kernel@...tin.sperl.org,
	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,
	Phil <phil@...pberrypi.org>
Subject: Re: [PATCH v5 0/6] bcm2835: auxiliar device support for spi


Den 09.09.2015 03:48, skrev Eric Anholt:
> kernel@...tin.sperl.org writes:
>
>> From: Martin Sperl <kernel@...tin.sperl.org>
>>
>> The BCM2835 contains 3 auxiliar devices:
>> * spi1
>> * spi2
>> * uart1
>>
>> All of those 3 devices are enabled/disabled via a shared register,
>> which is set by default to be disabled.
>>
>> Access to this register needs to get serialized.
>>
>> So after several iterations of discussions with the following ideas:
>> * syscon - device tree should describe HW not drivers to use -
>>             'compatiblity = "brcm,bcm2835-aux-enable", "syscon";'
>>             is not acceptable
>> * regulator - it is not necessarily a regulator or a power gate
>>                that is implemented in HW, so it is not valid to use
>>                this framework
>>
>> The recommendation was made to create a new minimal API in soc
>> just for access to this shared enable/disable register.
>>
>> This patch-series implements:
>> * the bcm2835-auxiliar device enable/disable api in soc.
>> * the bcm2835-auxiliar spi device driver
>>
>> The uart1 device driver (ns16550 based) is not implemented so far
>> but would be using the same API.
>>
>> Both spi and uart drivers can run with shared interrupts,
>> so there is no need for an interrupt-controller to get implemented.
> I finally had a chance to sit down and look at what the hardware's doing
> with the enable bit (also, I've read a whole lot more of the hardware
> now, so I'm a lot faster at answering questions like this).  The enable
> bits are a clock gate off of the VPU clock.
>
> I knocked together the enable bits as a clock gate driver, since I'd
> just written very similar code for the audio domain clock driver (and I
> assume you are grumpy about how much time you've spent on this one
> stupid register).  It's up at
> https://github.com/anholt/linux/tree/bcm2835-clock-aux and I can submit
> it if you like the result.  I've compile tested it only, but I'm hoping
> you could just drop your aux SPI driver on top of it and have things
> work.
>

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.


Noralf.



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