[PATCH v5 0/6] bcm2835: auxiliar device support for spi

Noralf Trønnes noralf at tronnes.org
Thu Sep 10 08:48:33 PDT 2015

Den 09.09.2015 03:48, skrev Eric Anholt:
> kernel at martin.sperl.org writes:
>> From: Martin Sperl <kernel at martin.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.


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