linux-kernel - Re: [PATCH V5 2/2] mfd: stmpe: Update DT support in stmpe driver

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Date:	Thu, 6 Dec 2012 07:28:22 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Lee Jones <lee.jones@...aro.org>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	rabin.vincent@...ricsson.com, shiraz.hashim@...com,
	devicetree-discuss@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	spear-devel@...t.st.com, linus.walleij@...aro.org,
	Vipul Kumar Samar <vipulkumar.samar@...com>
Subject: Re: [PATCH V5 2/2] mfd: stmpe: Update DT support in stmpe driver

First of all, thanks for explaining :)

On 6 December 2012 04:12, Grant Likely <grant.likely@...retlab.ca> wrote:
> On Sat, 1 Dec 2012 00:33:46 +0530, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> This first tries to match the table my patch added, _BUT_ the string will
>> never match as we had "st,stmpe810" in table and "stmpe810" in dev.
>
> of_driver_match_device() matches against the compatible list in
> dev->of_node, not against the device name. So, if the compatible
> property has a string that is in the table, then it really should match
> against it.

How could i misread it? Yes you are correct.

>> static int i2c_device_probe(struct device *dev)
>> {
>>         .....
>>       status = driver->probe(client, i2c_match_id(driver->id_table, client));
>
> Here things are a bit wonky. Even if matched against the table, it is

table means of_device_id table ?

> possible that it also matches against i2c_match_id() and that data is
> passed to the driver.

It is a possibility or guarantee ? And so whatever device name we got from
modalias routine, should match with the names in driver->id_table.

> But regardless, it is the responsiblity of the probe function to go and
> look if of_driver_match_device() matches against anything if it cares
> about the of_match_table entries (for instance, if there is extra data
> attached).

Ok, so filling .data field in of_device_id[] is not required for our case as
we aren't doing anything special in our drivers.

>> Which will again match the legacy table to find correct struct i2c_device_id *id
>> to pass to probe().
>>
>> So, the final question: WTF is of_match_table for?
>
> A bit of history is valuable here. The whole of_modalias_node() thing is
> really just a best-effort heuristic for figuring out which driver
> *might* work against a device described in the device tree. It won't
> work in all circumstances (and it was created at a time when there was
> resistance to adding DT knowledge to drivers). An of_match_table is the
> robust way of identifying specific devices and allows for matching
> against any entry in the compatible list.

Got it.
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