GSoC 2014 #0 status report - Improving Allwinner SoC support

[Maxime Ripard]

Hi,

On Thu, Jul 10, 2014 at 12:41:03AM -0300, Emilio López wrote:
> Hi everyone,
> 
> As some of you may know, I'm currently participating in Google
> Summer of Code under the Linux Foundation, working on a proposal
> titled "Improve Allwinner SoCs support on mainline Linux". There is
> a great quantity of devices out there that are powered by Allwinner
> processors, including but not limited to the various Cubieboards,
> OLinuXinos, STBs, Tablets and “Mini PCs”. However, to date, support
> on mainline Linux is not yet feature complete. My proposal on
> particular focuses on DMA and analog audio on the earlier SoCs, and
> improved A23 support.
> 
> The idea here is to make a weekly status report of the project. As
> we are starting mid-program, this one will be a bit different and
> I'll outline what has been worked on so far since the beginning.
> 
> To date, I have been mainly working on the DMA driver for sun4i,
> sun5i and sun7i. Despite having completely different drivers on SDK
> kernels, the hardware block looks and behaves the same, so we are
> using a single driver for these three SoC families. I have sent a
> series of patches[1] as well as a follow-up v2[2], and I will keep
> iterating over it until it's accepted. I have also sent two trivial
> patches[3][4] as a side-effect.

Yep, thanks a lot for that good work.

Hopefully, it will be merged rather soon, as it looks quite nice
already.

> The main challenges while writing this driver can be summarized as a
> lack of documentation. First of all, it took me a bit to get to know
> DMAEngine. As there is not much documentation on it, most of my
> learning took place by reading pre-existing drivers and consulting
> with my mentor. Secondly, as the Allwinner documentation is mostly a
> register list with bitfield details, I also had to read the SDK
> drivers and do some trial and error testing to discover magic values
> and understand others.

I feel your pain :)

> Lately, I have also been working on the audio part, now that I have
> a working DMA driver. After implementing cyclic DMA transfers and
> some clock code, and armed with a Buildroot image with mpg123 and an
> OpenBSD release track[5] in mp3 format, I've been trying to get some
> sound out of my Cubietruck's headphone jack, but without much
> success so far. I have verified my userspace stack and hardware by
> running these same binaries on top of the linux-sunxi 3.4 kernel,
> and it worked fine. I have since then been dumping relevant
> registers with devmem and comparing them, resolving issues as I see
> them - hopefully this will yield some audible results.

What have you been working on? A new driver from scratch, tried to
take Allwinner's code and then cleant it up, or used the recently
published driver Jon made?

> Interestingly enough, Allwinner themselves do not seem to be using
> cyclic DMA transfers on their driver[6]. I hope this is not a sign
> of a hardware bug that's not documented.

So they just implement a cyclic-like behaviour in software?

> To give some closure to this status report, I'd like to thank Maxime
> for his mentoring so far, Ezequiel for letting me pick his brain,
> and the Linux Foundation and Google for giving me this opportunity,
> as well as everyone on the kernel and sunxi communities who have
> come forward to review and test patches. You can expect a new report
> soon, in about a week's time or less.

Keep up the good work.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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