Message-id: <56373DFD.9050303@samsung.com>
Date:	Mon, 02 Nov 2015 11:42:05 +0100
From:	Andrzej Hajda <a.hajda@...sung.com>
To:	Archit Taneja <architt@...eaurora.org>,
	dri-devel@...ts.freedesktop.org
Cc:	linux-kernel@...r.kernel.org, airlied@...ux.ie, daniel@...ll.ch,
	treding@...dia.com, l.stach@...gutronix.de, robh@...nel.org,
	linux-arm-msm@...r.kernel.org, jani.nikula@...ux.intel.com
Subject: Re: [RFC v2 4/5] drm/dsi: Add routine to unregister dsi device

On 11/02/2015 07:28 AM, Archit Taneja wrote:
>
> On 10/30/2015 07:51 PM, Andrzej Hajda wrote:
>> On 10/06/2015 11:24 AM, Archit Taneja wrote:
>>> A driver calling mipi_dsi_device_new might want to unregister the device
>>> once it's done. It might also require it in an error handling path in
>>> case something didn't go right.
>>>
>>> When the dsi host driver calls mipi_dsi_host_unregister, the devices
>>> created by both DT and and without DT will be removed. This does leave
>>> the possibility of the host removing the dsi device without the
>>> peripheral driver being aware of it. I don't know a good way to solve
>>> this. Some suggestions here would be of help too.
>> The 2nd paragraph is not relevant here. It is another issue. Some comments
>> about it:
> Yes, it's probably not the best to put it in the commit message of this
> patch.
>
>> I am not sure, but I guess device should not be removed if it is refcounted
>> properly, it will be just detached from the driver, bus and system (whatever it
>> means:) ).
>> It does not mean it will be usable and probably some races can occur anyway.
>> I guess i2c and other buses have the same problem, am I right?
> I was concerned about one particular sequence:
>
> 1) DSI host driver calls mipi_dsi_host_unregister: All dsi devices would 
> be unregistered.
>
> 2) dsi device driver calls mipi_dsi_device_unregister: This will try to
> unregister our dsi device
>
> The problem here is that the device will cease to exist after step (1)
> itself, because the refcount of our device will never be 2.
>
> mipi_dsi_host_register() will only register devices represented in DT,
> not the one the drivers register manually.
>
> In other words, the dsi pointer in our driver will point to nothing 
> valid after mipi_dsi_host_unregister is called.
>
> As you said, I guess this exists in other buses too, and it's the
> drivers job to not use them.

I think the whole problem is due to fact we try to use devices
as interfaces to some bus hosts (DSI in our case), these devices
are owned by bus host and we cannot control their lifetime from other code.
The best solution IMO would be to create separate lightweight framework
as I suggested in previous discussion[1]. It should be cleaner solution
without any 'dummy' proxy devices.
But even in this case we would need some callbacks to notify that the provider
is about to be removed.

2nd 'solution' is to leave it as is and pretend everything is OK,
as in case of other frameworks :)

Maybe it would be possible 3rd solution - we could use probe and remove callbacks
from dsi driver to notify clients about adding/removal of dsi device to/from bus.
So during dummy dsi dev creation we would provide some callbacks which would be
called
by dummy dsi driver probe/remove to notifiy client it can start/stop using dsi
device.
This crazy construction probably can work but looks insane :)

[1]: http://www.spinics.net/lists/linux-arm-msm/msg16945.html

Regards
Andrzej

>
>>> Signed-off-by: Archit Taneja <architt@...eaurora.org>
>>> ---
>>>   drivers/gpu/drm/drm_mipi_dsi.c | 7 +++++++
>>>   include/drm/drm_mipi_dsi.h     | 2 ++
>>>   2 files changed, 9 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/drm_mipi_dsi.c b/drivers/gpu/drm/drm_mipi_dsi.c
>>> index db6130a..cbb7373 100644
>>> --- a/drivers/gpu/drm/drm_mipi_dsi.c
>>> +++ b/drivers/gpu/drm/drm_mipi_dsi.c
>>> @@ -183,6 +183,13 @@ err:
>>>   }
>>>   EXPORT_SYMBOL(mipi_dsi_device_new);
>>>
>>> +void mipi_dsi_device_unregister(struct mipi_dsi_device *dsi)
>>> +{
>>> +	if (dsi)
>>> +		device_unregister(&dsi->dev);
>>> +}
>>> +EXPORT_SYMBOL(mipi_dsi_device_unregister);
>>> +
>> I guess NULL check can be removed and the whole function can be inlined.
> Yeah, this check won't help anyway.
>
> I think I'll mention that drivers should use this only in error
> handling paths, and not in the driver's remove() op.
>
> I'll also change this to inlined.
>
> Archit
>
>> Regards
>> Andrzej
>>>   static struct mipi_dsi_device *
>>>   of_mipi_dsi_device_add(struct mipi_dsi_host *host, struct device_node *node)
>>>   {
>>> diff --git a/include/drm/drm_mipi_dsi.h b/include/drm/drm_mipi_dsi.h
>>> index 93dec7b..68f49f4 100644
>>> --- a/include/drm/drm_mipi_dsi.h
>>> +++ b/include/drm/drm_mipi_dsi.h
>>> @@ -197,6 +197,8 @@ ssize_t mipi_dsi_generic_read(struct mipi_dsi_device *dsi, const void *params,
>>>
>>>   struct mipi_dsi_device *mipi_dsi_device_new(struct mipi_dsi_host *host,
>>>   					    struct mipi_dsi_device_info *info);
>>> +void mipi_dsi_device_unregister(struct mipi_dsi_device *dsi);
>>> +
>>>   /**
>>>    * enum mipi_dsi_dcs_tear_mode - Tearing Effect Output Line mode
>>>    * @MIPI_DSI_DCS_TEAR_MODE_VBLANK: the TE output line consists of V-Blanking
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