linux-kernel - Re: [REPOST][PATCH 1/2] driver core: Add new device_driver flag to allow binding via sysfs only

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Date:	Mon, 09 Dec 2013 20:12:45 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Kim Phillips <kim.phillips@...aro.org>, scottwood@...escale.com
CC:	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
	kvm@...r.kernel.org, R65777@...escale.com, B07421@...escale.com,
	B08248@...escale.com, christoffer.dall@...aro.org,
	alex.williamson@...hat.com, a.motakis@...tualopensystems.com,
	agraf@...e.de, B16395@...escale.com
Subject: Re: [REPOST][PATCH 1/2] driver core: Add new device_driver flag to
 allow binding via sysfs only

On 2013-12-09 19:58, Kim Phillips wrote:
> On Thu, 5 Dec 2013 16:38:15 -0600
> Scott Wood <scottwood@...escale.com> wrote:
> 
>> On Thu, 2013-12-05 at 17:45 +0000, Kim Phillips wrote:
>>> On Tue, 03 Dec 2013 16:34:33 +0100
>>> Jan Kiszka <jan.kiszka@...mens.com> wrote:
>>>
>>>> On 2013-12-03 13:34, Kim Phillips wrote:
>>>>> VFIO supports pass-through of devices to user space - for sake
>>>>> of illustration, say a PCI e1000 device:
>>>>>
>>>>> - the e1000 is first unbound from the PCI e1000 driver via sysfs
>>>>> - the vfio-pci driver is told via new_id that it now handles e1000 devices
>>>>> - the e1000 is explicitly bound to vfio-pci through sysfs
>>>>>
>>>>> However, now we have two drivers in the system that both handle e1000
>>>>> devices.  A hotplug event could then occur and it is ambiguous as to which
>>>>> driver will claim the device.  The desired semantics is that vfio-pci is
>>>>> only bound to devices by explicit request in sysfs.  This patch makes this
>>>>> possible by introducing a sysfs_bind_only flag in struct device_driver.
>>>>>
>>>>> Signed-off-by: Stuart Yoder <stuart.yoder@...escale.com>
>>>>> Signed-off-by: Kim Phillips <kim.phillips@...aro.org>
>>>>> ---
>>>>> rebased onto 3.13-rc2, and reposted from first submission which
>>>>> recieved no comments:
>>>>>
>>>>> https://lkml.org/lkml/2013/10/11/53
>>>>>
>>>>>  drivers/base/dd.c      | 5 ++++-
>>>>>  include/linux/device.h | 2 ++
>>>>>  2 files changed, 6 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
>>>>> index 0605176..b83b16d 100644
>>>>> --- a/drivers/base/dd.c
>>>>> +++ b/drivers/base/dd.c
>>>>> @@ -389,7 +389,7 @@ static int __device_attach(struct device_driver *drv, void *data)
>>>>>  {
>>>>>  	struct device *dev = data;
>>>>>  
>>>>> -	if (!driver_match_device(drv, dev))
>>>>> +	if (drv->sysfs_bind_only || !driver_match_device(drv, dev))
>>>>>  		return 0;
>>>>>  
>>>>>  	return driver_probe_device(drv, dev);
>>>>> @@ -476,6 +476,9 @@ static int __driver_attach(struct device *dev, void *data)
>>>>>   */
>>>>>  int driver_attach(struct device_driver *drv)
>>>>>  {
>>>>> +	if (drv->sysfs_bind_only)
>>>>> +		return 0;
>>>>> +
>>>>>  	return bus_for_each_dev(drv->bus, NULL, drv, __driver_attach);
>>>>>  }
>>>>>  EXPORT_SYMBOL_GPL(driver_attach);
>>>>> diff --git a/include/linux/device.h b/include/linux/device.h
>>>>> index 952b010..ed441d1 100644
>>>>> --- a/include/linux/device.h
>>>>> +++ b/include/linux/device.h
>>>>> @@ -200,6 +200,7 @@ extern struct klist *bus_get_device_klist(struct bus_type *bus);
>>>>>   * @owner:	The module owner.
>>>>>   * @mod_name:	Used for built-in modules.
>>>>>   * @suppress_bind_attrs: Disables bind/unbind via sysfs.
>>>>> + * @sysfs_bind_only: Only allow bind/unbind via sysfs.
>>>>>   * @of_match_table: The open firmware table.
>>>>>   * @acpi_match_table: The ACPI match table.
>>>>>   * @probe:	Called to query the existence of a specific device,
>>>>> @@ -233,6 +234,7 @@ struct device_driver {
>>>>>  	const char		*mod_name;	/* used for built-in modules */
>>>>>  
>>>>>  	bool suppress_bind_attrs;	/* disables bind/unbind via sysfs */
>>>>> +	bool sysfs_bind_only;		/* only allow bind/unbind via sysfs */
>>>>>  
>>>>>  	const struct of_device_id	*of_match_table;
>>>>>  	const struct acpi_device_id	*acpi_match_table;
>>>>
>>>> I think I only discussed this with Stuart in person at the KVM Forum:
>>>> Why not deriving the property "sysfs bind only" from the fact that a
>>>> device does wild-card binding? Are there use cases that benefit from
>>>> decoupling both features?
>>>
>>> you mean merge the two new flags sysfs_bind_only and platform driver's
>>> match_any_dev into one new single driver flag, right?  good question.
>>
>> What would combining them solve, other than making it more likely that
>> Greg complains about the wildcard because it would no longer be handled
>> at the bus level where all the other matching goes on?
>>
>> They are logically separate things.  That doesn't change just because we
>> currently plan to use them together.
> 
> Jan?  Given the above, what would be the advantage of merging
> sysfs_bind_only and (PCI drivers' PCI_ANY_ID and platform drivers'
> match_any_dev)?

That you cannot configure (likely) meaningless or even harmful (bind-any
+ auto-bind) configurations.

I didn't follow if Greg expressed his opinion on this or a similar
scenario before. If he prefers separate knobs for a certain reason, he
likely wins.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux
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