Re: [PATCH v2 04/22] drm: Don't test for IRQ support in VBLANK ioctls

From: Thomas Zimmermann <tzimmermann@suse.de>
To: Liviu Dudau <liviu.dudau@arm.com>
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Subject: Re: [PATCH v2 04/22] drm: Don't test for IRQ support in VBLANK ioctls
Date: Wed, 23 Jun 2021 08:43:07 +0200	[thread overview]
Message-ID: <f7e72a3c-df86-2d4b-2caa-bf91442290a9@suse.de> (raw)
In-Reply-To: <20210622152504.2sw6khajwydsoaqa@e110455-lin.cambridge.arm.com>

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Hi Liviu

Am 22.06.21 um 17:25 schrieb Liviu Dudau:
> Hello,
> 
> On Tue, Jun 22, 2021 at 04:09:44PM +0200, Thomas Zimmermann wrote:
>> For KMS drivers, replace the IRQ check in VBLANK ioctls with a check for
>> vblank support. IRQs might be enabled wthout vblanking being supported.
>>
>> This change also removes the DRM framework's only dependency on IRQ state
>> for non-legacy drivers. For legacy drivers with userspace modesetting,
>> the original test remains in drm_wait_vblank_ioctl().
>>
>> v2:
>> 	* keep the old test for legacy drivers in
>> 	  drm_wait_vblank_ioctl() (Daniel)
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>> ---
>>   drivers/gpu/drm/drm_irq.c    | 10 +++-------
>>   drivers/gpu/drm/drm_vblank.c | 13 +++++++++----
>>   2 files changed, 12 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
>> index c3bd664ea733..1d7785721323 100644
>> --- a/drivers/gpu/drm/drm_irq.c
>> +++ b/drivers/gpu/drm/drm_irq.c
>> @@ -74,10 +74,8 @@
>>    * only supports devices with a single interrupt on the main device stored in
>>    * &drm_device.dev and set as the device paramter in drm_dev_alloc().
>>    *
>> - * These IRQ helpers are strictly optional. Drivers which roll their own only
>> - * need to set &drm_device.irq_enabled to signal the DRM core that vblank
>> - * interrupts are working. Since these helpers don't automatically clean up the
>> - * requested interrupt like e.g. devm_request_irq() they're not really
>> + * These IRQ helpers are strictly optional. Since these helpers don't automatically
>> + * clean up the requested interrupt like e.g. devm_request_irq() they're not really
>>    * recommended.
>>    */
>>   
>> @@ -91,9 +89,7 @@
>>    * and after the installation.
>>    *
>>    * This is the simplified helper interface provided for drivers with no special
>> - * needs. Drivers which need to install interrupt handlers for multiple
>> - * interrupts must instead set &drm_device.irq_enabled to signal the DRM core
>> - * that vblank interrupts are available.
>> + * needs.
>>    *
>>    * @irq must match the interrupt number that would be passed to request_irq(),
>>    * if called directly instead of using this helper function.
>> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
>> index 3417e1ac7918..a98a4aad5037 100644
>> --- a/drivers/gpu/drm/drm_vblank.c
>> +++ b/drivers/gpu/drm/drm_vblank.c
>> @@ -1748,8 +1748,13 @@ int drm_wait_vblank_ioctl(struct drm_device *dev, void *data,
>>   	unsigned int pipe_index;
>>   	unsigned int flags, pipe, high_pipe;
>>   
>> -	if (!dev->irq_enabled)
>> -		return -EOPNOTSUPP;
>> +	if  (drm_core_check_feature(dev, DRIVER_MODESET)) {
>> +		if (!drm_dev_has_vblank(dev))
>> +			return -EOPNOTSUPP;
>> +	} else {
>> +		if (!dev->irq_enabled)
>> +			return -EOPNOTSUPP;
>> +	}
> 
> For a system call that is used quite a lot by userspace we have increased the code size
> in a noticeable way. Can we not cache it privately?

I'm not quite sure that I understand your concern. The additionally 
called functions are trivial one-liners; probably inlined anyway.

However, irq_enabled is only relevant for legacy drivers and will 
eventually disappear behind CONFIG_DRM_LEGACY. We can rewrite the test 
like this:

ifdef CONFIG_DRM_LEGACY
   if (unlikely(check_feature(dev, DRIVER_LEGACY))) {
     if (!irq_enabled)
       return;
   } else
#endif
   {
     if (!has_vblank_support(dev))
       return;
   }

As CONFIG_DRM_LEGACY is most likely disabled on concurrent systems, we'd 
get a single test for the modern drivers. If DRM_LEGACYis on, the 
compiler at least knows that the else branch is preferred.

Best regards
Thomas

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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