[GIT PULL 00/21] Renesas ARM based SoC Board Updates for v3.15

[Magnus Damm]

On Wed, Feb 26, 2014 at 1:04 AM, Arnd Bergmann <arnd at arndb.de> wrote:
> On Tuesday 25 February 2014, Magnus Damm wrote:
>> On Thu, Feb 20, 2014 at 5:23 PM, Olof Johansson <olof at lixom.net> wrote:
>> > On Thu, Feb 06, 2014 at 03:17:08PM +0900, Simon Horman wrote:
>> Thanks for your email. I agree that the number of board file commits
>> is definitely larger than zero so some clarification is in order.
>>
>> As you probably recall, we earlier agreed on not adding any new board
>> files. That part is clear I believe so I will skip that.
>>
>> Regarding the legacy board code, we have quite ok hardware support
>> coverage as it is now, but some devices drivers are of course still
>> under development. This means that in some cases integration is on
>> going or has not happened yet. You may see those kind of changes as
>> significant commits in the pull requests for board support.
>
> [adding Ben Dooks, since he was complaining about this as well]
>
> My feeling is that we should adjust the strategy for shmobile. We've
> had good success with the dual strategy of keeping board support
> separate for DT-enabled and ATAGS-only boards in the sense that
> we did not have to coordinate updates for bindings between subsystem
> and architecture git trees, which has always been source for
> problems on other platforms.

Hi Arnd,

You're right that many of our boot loaders only provide us with ATAG
information. In the kernel we have long since made use of appended DTB
to handle those cases. The last bit of code that depended on ATAGS and
mach-type being passed from the boot loader was removed in the
following patch:

[PATCH] ARM: shmobile: Update romImage to relocate appended DTB
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/arch/arm/boot/compressed/head-shmobile.S?id=2c408d149299e99c89fc4be80fb4fe00a7016f02

> However, the price for this seems to be that it's still not possible
> to get a properly working system without a board file, and my feeling
> is that it's taking too long to get there. In particular, we now see
> new drivers getting added (I noticed VIN, which Ben mentioned before)
> that start out with just platform_device support but no DT support.
> This is bad, because it means DT users are always behind.

>From my side it is possible to boot working systems with DT only
(without a board file) for a wide range of SoCs and boards. Actually,
for KZM9D we have no board code - instead there is only Multiplatform
DT. Any left over C board code for KZM9D and the legacy EMEV2SoC code
is removed in:

[PATCH 00/03] ARM: shmobile: Clean up the EMEV2 and KZM9D code base
http://www.spinics.net/lists/arm-kernel/msg307379.html

To be clear, I believe all our boards can boot without a board file.
The only exception is sh7372 and Mackerel which will be phased out.

Please note that device support may however be limited with DT. This
since several device drivers are under constant development to be able
to support the latest hardware features. And the hardware is being
updated quite regularly, so it seems to me that our only choice is to
keep on updating software support.

>> In the Legacy Lager/Koelsch board code the following devices are
>> supported as platform devices:
>>
>> ETHER, SCIF, DU, I2C, SATA*, USB*, MSIOF*, SDHI*, QSPI*, MMCIF,
>> Audio*, VIN*, Thermal, IRQC, PFC, CMT
>>
>> * Platform device support under development in v3.15-pre
>>
>> In the Multiplatform DT for Lager/Koelsch the following devices have DT support:
>>
>> ETHER, SCIF**, I2C, SATA, MSIOF, SDHI, QSPI, MMCIF, Thermal, IRQC, PFC, CMT**
>>
>> ** Driver DT binding development on-going but integration not finalized
>>
>> In Multiplatform DT for Lager/Koelsch the following devices lack DT bindings:
>>
>> DU (drm/kms)
>> USB (host/function/phy)
>> Audio (alsa-soc + dmac)
>> VIN (v4l2 + camera sensor)
>
> Ok, thanks for the list.
>
>> Our plan is to migrate over to the DT Multiplatform code base as soon
>> as ever possible, but at the same time we do not want to commit long
>> term support of potentially premature DT bindings. Our short term
>> solution to that is to use platform devices for a limited number of
>> devices together with DT Multiplatform.
>>
>> If you would like use to adjust our way forwards please let me know!
>
> I think you should try to close the gap between ATAGS and DT now and
> stop the dual strategy. You seem to have come far enough with the
> basic infrastructure (clock, pinctrl, irq, timer, ...) that all the
> devices missing DT support now are on-chip peripherals. If this is the
> case, it should be possible to use auxdata registration in the
> per-soc files to connect the platform data to the remaining devices
> that are lacking DT bindings. This means we have to be more careful
> with the dependencies when a driver gains a DT binding, but at least
> we can enforce that any driver in the future only gets merged with
> a proper DT binding as we do for other subarchitectures, and you don't
> have to implement probing twice for each new driver.

I agree about closing the gap and using DT as much as ever possible.
Perhaps we need to discuss a bit more about how to proceed.

In my mind this all boils down to two separate issues:

1) How to close the gap:

Our current plan is to copy the left over non-DT devices from legacy
board code to our DT board support. This can be done with or without
AUXDATA, exactly how to do it is a separate (but perhaps important?)
issue. When the DT board support is in OK state then we remove the
legacy board code. How long time that takes depends on the level of
board support. For the boards and devices I listed above I believe it
can be done in less than a week. If you want us to do this differently
then please advice us about your preference.

Regarding AUXDATA, it seems mainly useful for simple devices that have
no dependencies on other parts of the system. Our remaining portion of
non-DT device drivers are however all relatively complex devices with
many dependencies and sometimes odd topology. On such device drivers
our prototyping showed that AUXDATA mainly came with drawbacks of
having to commit to bindings prematurely but we could not really see
any upside. We would like to hear more from you how you think we
should proceed.


2) How to handle continuous driver development and integration

As you have seen, most of our simple device drivers have already been
successfully converted to DT. What remains are more complex devices.

Since we do upstream first we rely on being able to develop device
driver code incrementally over several kernel releases. To allow early
use, development and testing we also integrate the device drivers
continuously. We have been doing this for many years now and it seems
to work really well for us. That device drivers are reused and
improved over several years is quite common.

So far we have been developing basic support via platform device
interfaces and directly exposed that to our users via board code
written in C. Over time we start feeling confident that we have good
enough support level and understanding of the hardware for a
particular driver. When that point has been reached then we define
bindings to enable DT support. From there we probably keep on
iterating even more. This is what we currently do.

In mach-shmobile you see a lot of board changes coming from us when we
are performing continuous driver integration. I would like to ask you
for advice if you can recommend us any other development model that
may fit better with the ARM SoC way of doing things.

Thanks,

/ magnus