Re: [PATCH v5 2/5] driver core: Functional dependencies tracking support

[Geert Uytterhoeven]

Hi Laurent,

On Fri, Nov 11, 2016 at 1:08 AM, Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
> On Thursday 10 Nov 2016 23:40:54 Greg Kroah-Hartman wrote:
>> On Thu, Nov 10, 2016 at 11:04:07PM +0100, Luis R. Rodriguez wrote:
>> Don't try to make this more difficult than it is please. DEFER works
>> today really really well, and it's really really simple.
>> Inter-dependancy of modules and devices connected to each other are two
>> different things, be careful about this.
>
> One issue we don't address today is handling of optional dependencies. A
> simple example is an SPI controller that can use a DMA engine or work in PIO
> mode. At probe time the driver will request a DMA channel if the platform
> (ACPI, DT, platform data) specifies that DMA is available. This can fail for
> various reasons, one of them being that the DMA engine driver hasn't probed
> the DMA device yet. In that case the SPI controller driver will continue in
> PIO mode, ignoring the DMA engine that will later be probed. We can't defer
> probing of the SPI controller as the DMA engine driver might never get loaded,
> which would result in the SPI controller probe being deferred forever.
>
> One solution for this type of dependency issue would be to notify the SPI
> controller driver after probe that the DMA channel is now available. I'd like
> to avoid that though, as it would drastically increase the complexity of lots
> of drivers and create lots of race conditions.

Alternatively, the driver can request optional DMA resources at open time
instead of at probe time.
E.g. sh_sci does that in its uart_ops.startup() callback.

Regardless of that, DMA can fail temporarily for other reasons
(e.g. running out of channels).

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds