Re: [RFC 0/5] drivers: Add boot constraints core

[Chen-Yu Tsai]

On Fri, Jun 30, 2017 at 11:16 AM, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> On 29-06-17, 15:06, Enrico Weigelt, metux IT consult wrote:
>> On 29.06.2017 14:47, Viresh Kumar wrote:
>>
>> >No. Drivers are registered to the kernel (randomly, though we can know
>> >their order) and devices are registered separately (platform/amba
>> >devices get registered automatically with DT, hint:
>> >drivers/of/platform.c). The device core checks while registering
>> >devices/drivers if their drivers/devices are available or not. If
>> >yes, then the devices are probed using the drivers. Now the drivers
>> >must make sure all the dependencies are met at this point, else they
>> >can return -EPROBE_DEFER and the kernel will try probing them again.
>>
>> Could we somehow introduce an strict ordering ?
>
> The problem I am trying to solve isn't really related to ordering.
>
> Consider this for example:
>
> A supply shared between LCD and I2C controller (Not sure if such
> configurations are there in any of the hardware we have), where the
> same I2C controller is used to access the LCD controller's registers.
> Both are enabled at boot and the supply is configured to satisfy both.
> If the voltage requirements of the I2C controller are below that of
> LCD, then we can't decide on which one to probe first. We can't probe
> LCD first as its bus isn't active yt and if we probe I2C first, then
> it may take the supply down to a level that isn't acceptable for the
> LCD (which was on from boot).

AFAIK regulator constraints are supposed to satisfy all users of it.

>> Maybe by letting the device core know of the dependencies, before
>> individual probe()'s explicitly ask for them ?
>
> That's what we are sorting out in probe() and I am not sure if we need
> any more intelligence on that. Though, you may want to look at the
> "functional dependency" stuff, which can be of some help in such
> cases. Its mentioned in cover-letter as well.
>
>> >This should happen in probe, otherwise we are screwed.
>>
>> Yes, but the probe result may be deferred, so it's tried again in the
>> next round. Correct ?
>
> Right.
>
>> >But the kernel doesn't know how it is configured, there can be so many
>> >configurable parameters. The kernel needs to do it again by itself.
>>
>> Could it read back the config ?
>
> First, it may not always be possible to do that. And even if the
> kernel reads it all well, then it wouldn't know why things are
> configured the way they are. And trying to read the config in drivers
> is going to be so so hacky, that we wouldn't want to do it anyway. We
> need a clean way of doing this, so that the kernel knows of what's
> going on and that's what this series is targeting here.
>
>> By the way: I've got a similar problem w/ gpmc right now: uboot already
>> sets it up, but the kernel only knows about one CS (for the nand) and
>> screwes up the others (eg. fpga), so it cant access the fpga . Until
>> I've sorted out all the parameters for DT (unfortunately, only have the
>> raw register values), I'll have to rely on an userland test program
>> to set it all up ...
>>
>> >Let me try with an example. A regulator is shared between LCD and DMA
>> >controller.
>> >
>> >Operable ranges of the regulator: 1.8 - 3.0 V
>> >Range required by LCD: 2.0 - 3.0 V
>> >Range required by DMA: 1.8 - 2.5 V

So for the example here, the regulator constraint should be 2.5 - 3.0 V,
or the intersection of all voltage requirements.

ChenYu

>>
>> Would a config readback help here ?
>>
>> The regulator core then should know that we're already in proper
>> range for DMA and no need to touch the regulator.
>
> No body is going to allow that kind of hacky code to get merged :)
>
> --
> viresh
>
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