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SubjectRe: [PATCH v4 0/22] On-demand device probing
On 9 September 2015 at 03:33, Rob Herring <> wrote:
> On 09/08/2015 02:30 AM, Tomeu Vizoso wrote:
>> On 7 September 2015 at 22:50, Rob Herring <> wrote:
>>> On Mon, Sep 7, 2015 at 7:23 AM, Tomeu Vizoso <> wrote:
>>>> Hello,
>>>> I have a problem with the panel on my Tegra Chromebook taking longer
>>>> than expected to be ready during boot (Stéphane Marchesin reported what
>>>> is basically the same issue in [0]), and have looked into ordered
>>>> probing as a better way of solving this than moving nodes around in the
>>>> DT or playing with initcall levels and linking order.
>>>> While reading the thread [1] that Alexander Holler started with his
>>>> series to make probing order deterministic, it occurred to me that it
>>>> should be possible to achieve the same by probing devices as they are
>>>> referenced by other devices.
>>>> This basically reuses the information that is already implicit in the
>>>> probe() implementations, saving us from refactoring existing drivers or
>>>> adding information to DTBs.
>>>> During review of v1 of this series Linus Walleij suggested that it
>>>> should be the device driver core to make sure that dependencies are
>>>> ready before probing a device. I gave this idea a try [2] but Mark Brown
>>>> pointed out to the logic duplication between the resource acquisition
>>>> and dependency discovery code paths (though I think it's fairly minor).
>>>> To address that code duplication I experimented with Arnd's devm_probe
>>>> [3] concept of having drivers declare their dependencies instead of
>>>> acquiring them during probe, and while it worked [4], I don't think we
>>>> end up winning anything when compared to just probing devices on-demand
>>>> from resource getters.
>>>> One remaining objection is to the "sprinkling" of calls to
>>>> of_device_probe() in the resource getters of each subsystem, but I think
>>>> it's the right thing to do given that the storage of resources is
>>>> currently subsystem-specific.
>>>> We could avoid the above by moving resource storage into the core, but I
>>>> don't think there's a compelling case for that.
>>>> I have tested this on boards with Tegra, iMX.6, Exynos, Rockchip and
>>>> OMAP SoCs, and these patches were enough to eliminate all the deferred
>>>> probes (except one in PandaBoard because omap_dma_system doesn't have a
>>>> firmware node as of yet).
>>>> Have submitted a branch [5] with only these patches on top of thursday's
>>>> linux-next to and I don't see any issues that could be
>>>> caused by them. For some reason it currently has more passes than the
>>>> version of -next it's based on!
>>>> With this series I get the kernel to output to the panel in 0.5s,
>>>> instead of 2.8s.
>>>> Regards,
>>>> Tomeu
>>>> [0]
>>>> [1]
>>>> [2]
>>>> [3]
>>>> [4]
>>>> [5]
>>>> [6]
>>>> [7]
>>>> Changes in v4:
>>>> - Added bus.pre_probe callback so the probes of Primecell devices can be
>>>> deferred if their device IDs cannot be yet read because of the clock
>>>> driver not having probed when they are registered. Maybe this goes
>>>> overboard and the matching information should be in the DT if there is
>>>> one.
>>> Seems overboard to me or at least a separate problem.
>> It's a separate problem but this was preventing the series from
>> working on a few boards.
> What is the failure? Not booting? Fixing not working would certainly not
> be overboard.

On the device I was testing on (qemu's vexpress-a15 machine) the
machine booted and I was able to open a ssh session, but serial was
broken among other AMBA devices:


Another way of avoiding this particular problem would be not delaying
the probe of devices in the configuration bus, by doing something like

diff --git a/drivers/bus/vexpress-config.c b/drivers/bus/vexpress-config.c
index 6575c0fe6a4e..eda293869cd3 100644
--- a/drivers/bus/vexpress-config.c
+++ b/drivers/bus/vexpress-config.c
@@ -181,7 +181,7 @@ static int vexpress_config_populate(struct
device_node *node)
if (WARN_ON(!parent))
return -ENODEV;

- return of_platform_populate(node, NULL, NULL, parent);
+ return of_platform_populate_early(node, NULL, NULL, parent);

static int __init vexpress_config_init(void)
But I think this would be papering over the underlying issue and it
would be better to have proper explicit dependencies.



>>> Most clocks have
>>> to be setup before the driver model simply because timers depend on
>>> clocks usually.
>> Yes, but in this case the apb clocks for the primecell devices are
>> implemented in a normal platform driver (vexpress_osc_driver), instead
>> of using CLK_OF_DECLARE.
> Okay.
> Rob
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