Re: [PATCH v3 1/4] drm: Add drm_crtc_has_vblank()

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 22.01.20 um 09:31 schrieb Daniel Vetter:
> On Mon, Jan 20, 2020 at 01:20:48PM +0100, Thomas Zimmermann wrote:
>> The new interface drm_crtc_has_vblank() return true if vblanking has
>> been initialized for a certain CRTC, or false otherwise. This function
>> will be useful for initializing CRTC state.
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>> ---
>>  drivers/gpu/drm/drm_vblank.c | 21 +++++++++++++++++++++
>>  include/drm/drm_vblank.h     |  1 +
>>  2 files changed, 22 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
>> index 1659b13b178c..c20102899411 100644
>> --- a/drivers/gpu/drm/drm_vblank.c
>> +++ b/drivers/gpu/drm/drm_vblank.c
>> @@ -501,6 +501,27 @@ int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs)
>>  }
>>  EXPORT_SYMBOL(drm_vblank_init);
>>  
>> +/**
>> + * drm_crtc_has_vblank - test if vblanking has been initialized for
>> + *                       a CRTC
>> + * @crtc: the CRTC
>> + *
>> + * Drivers may call this function to test if vblank support is
>> + * initialized for a CRTC. For most hardware this means that vblanking
>> + * can also be enabled on the CRTC.
>> + *
>> + * Returns:
>> + * True if vblanking has been initialized for the given CRTC, false
>> + * otherwise.
>> + */
>> +bool drm_crtc_has_vblank(const struct drm_crtc *crtc)
> 
> So making this specific to a CRTC sounds like a good idea. But it's not
> the reality, drm_vblank.c assumes that either everything or nothing
> supports vblanks.
> 
> The reason for dev->num_crtcs is historical baggage, it predates kms by a
> few years. For kms drivers the only two valid values are either 0 or
> dev->mode_config.num_crtcs. Yes that's an entire different can of worms
> that's been irking me since forever (ideally drm_vblank_init would somehow
> loose the num_crtcs argument for kms drivers, but some drivers call this
> before they've done all the drm_crtc_init calls so it's complicated).

Maybe as a first step, drm_vblank_init() could use
dev->mode_config.num_crtcs if the supplied number of CRTCs is zero.

> 
> Hence drm_dev_has_vblank as I suggested. That would also allow you to
> replace a bunch of if (dev->num_crtcs) checks in drm_vblank.c, which
> should help quite a bit in code readability.

OK, but I still don't understand why this interface is better overall.
We don't loose anything by passing in the crtc instead of the device
structure. And if there's ever a per-crtc vblank initialization, we'd
have the interface in place already. The tests with "if
(dev->num_crtcs)" could probably be removed in most places in any case.

We should also consider forking the vblank code for non-KMS drivers.
While working in this, I found the support for legacy drivers is getting
in the way at times. With such a fork, legacy drivers could continue
using struct drm_vblank_crtc, while modern drivers could maybe store
vblank state directly in struct drm_crtc.

Anyway, all this is for another patch. Unless you change your mind, I'll
replace drm_crtc_has_vblank() with drm_dev_has_vblank() for the
patchset's next iteration.

Best regards
Thomas

> 
> Cheers, Daniel
> 
>> +{
>> +	struct drm_device *dev = crtc->dev;
>> +
>> +	return crtc->index < dev->num_crtcs;
>> +}
>> +EXPORT_SYMBOL(drm_crtc_has_vblank);
>> +
>>  /**
>>   * drm_crtc_vblank_waitqueue - get vblank waitqueue for the CRTC
>>   * @crtc: which CRTC's vblank waitqueue to retrieve
>> diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
>> index c16c44052b3d..531a6bc12b7e 100644
>> --- a/include/drm/drm_vblank.h
>> +++ b/include/drm/drm_vblank.h
>> @@ -206,6 +206,7 @@ struct drm_vblank_crtc {
>>  };
>>  
>>  int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs);
>> +bool drm_crtc_has_vblank(const struct drm_crtc *crtc);
>>  u64 drm_crtc_vblank_count(struct drm_crtc *crtc);
>>  u64 drm_crtc_vblank_count_and_time(struct drm_crtc *crtc,
>>  				   ktime_t *vblanktime);
>> -- 
>> 2.24.1
>>
> 

-- 
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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