linux-kernel - Re: [PATCH v6 2/4] driver core: add deferring probe reason to devices

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Date:   Thu, 2 Jul 2020 08:57:55 +0200
From:   Andrzej Hajda <a.hajda@...sung.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Grygorii Strashko <grygorii.strashko@...com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Jonas Karlman <jonas@...boo.se>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH v6 2/4] driver core: add deferring probe reason to
 devices_deferred property


On 30.06.2020 20:00, Dmitry Torokhov wrote:
> On Tue, Jun 30, 2020 at 8:42 AM Andrzej Hajda <a.hajda@...sung.com> wrote:
>>
>> On 30.06.2020 10:59, Grygorii Strashko wrote:
>>> Hi
>>>
>>> On 29/06/2020 14:28, Andrzej Hajda wrote:
>>>> Hi Grygorii,
>>>>
>>>> (...)
>>>>
>>>>>>     /*
>>>>>>      * deferred_devs_show() - Show the devices in the deferred probe
>>>>>> pending list.
>>>>>>      */
>>>>>> @@ -221,7 +241,8 @@ static int deferred_devs_show(struct seq_file *s,
>>>>>> void *data)
>>>>>>         mutex_lock(&deferred_probe_mutex);
>>>>>>           list_for_each_entry(curr, &deferred_probe_pending_list,
>>>>>> deferred_probe)
>>>>>> -        seq_printf(s, "%s\n", dev_name(curr->device));
>>>>>> +        seq_printf(s, "%s\t%s", dev_name(curr->device),
>>>>>> + curr->device->p->deferred_probe_reason ?: "\n");
>>>>>>           mutex_unlock(&deferred_probe_mutex);
>>>>>>
>>>>> Sry, may be i missing smth, but shouldn't it be optional
>>>>> (CONFIG_DEBUG_FS is probably too generic).
>>>>>
>>>> I am not sure what exactly are you referring to, but this patch does not
>>>> add new property, it just extends functionality of existing one.
>>> Sry, needed to be more specific.
>>>
>>> You've added  device_set_deferred_probe_reson(dev, &vaf);
>>> which expected to be used on every EPROBE_DEFER in dev_err_probe() in
>>> combination with
>>>
>>> +       } else {
>>> +               device_set_deferred_probe_reson(dev, &vaf);
>>>                  dev_dbg(dev, "error %d: %pV", err, &vaf);
>>>
>>> ^^ dev_dbg() does not add any runtime overhead during boot unless enabled
>>> +       }
>>>
>>> But:
>>>
>>> +void device_set_deferred_probe_reson(const struct device *dev, struct
>>> va_format *vaf)
>>> +{
>>> +       const char *drv = dev_driver_string(dev);
>>> +
>>> +       mutex_lock(&deferred_probe_mutex);
>>> +
>>> +       kfree(dev->p->deferred_probe_reason);
>>> +       dev->p->deferred_probe_reason = kasprintf(GFP_KERNEL, "%s:
>>> %pV", drv, vaf);
>>> +
>>> +       mutex_unlock(&deferred_probe_mutex);
>>> +}
>>>
>>> ^^ Adds locking, kfree() and kasprintf() for every deferred probe
>>> during boot and can't be disabled.
>>>
>>> Right?
>>
>> Right, but usually the burden should be insignificant in comparison to
>> probe time, so I do not think it is worth optimizing.
> I do not think this is going to take. You are suggesting that we
> modify pretty much every driver to supply this deferral reason, and I
> doubt it will happen. Can we put this burden on providers that raise
> the deferral?


I wouldn't say they raise the deferral, they just inform resource is not 
yet available. Only device driver, and only in its probe function can 
"raise the deferral".


>   I.e. majority of code are using devm API now, so we most
> likely know the device for which deferral is being raised. We can have
> a list of deferral reasons and their devices and when in device code
> once probe is done we could try reconciling it with the deferred
> devicelist, and this would mean you only need to implement this in
> gpiolib, regulator core, clocks core, etc.


This patchset tries to solve simple issue - replace multiple lines of 
code present in multiple probe functions (additionally fixing lot of 
them) with single call and then enhance it little bit, nothing more.

What you are proposing is blurry at the moment for me, provider does not 
know if consumer want to defer,  or will continue working without 
missing resource, moreover some consumers can acquire resources after 
probe - again no probe deferral. Even if it will be done (it can be, for 
example by creating probe version of all resource get functions), it 
will require much more changes but finally it will look like:

res = devm_get_resource_from_probe(....)

if (IS_ERR(res))

     return PTR_ERR(res);

vs:

res = devm_get_resource(...)

if (IS_ERR(res))

     return dev_err_probe(dev, PTR_ERR(res), ...);


Regards

Andrzej


>
> Thanks.
>