linux-kernel - Re: [RFC v2 4/5] drm/dsi: Add routine to unregister dsi device

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Date:	Mon, 2 Nov 2015 11:58:16 +0530
From:	Archit Taneja <architt@...eaurora.org>
To:	Andrzej Hajda <a.hajda@...sung.com>,
	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 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.

>
>>
>> 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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