[Intel-gfx] [PATCH 02/44] drm: Add devm_drm_dev_alloc macro

[Daniel Vetter]

On Wed, Apr 08, 2020 at 08:57:14AM +0200, Sam Ravnborg wrote:
> Hi Daniel.
> 
> Finally managed to dive into this..
> 
> Maybe I need more coffee, it is still morning here.
> But alas this patch triggered a few comments.
> 
> 	Sam
> 
> On Fri, Apr 03, 2020 at 03:57:46PM +0200, Daniel Vetter wrote:
> > The kerneldoc is only added for this new function. Existing kerneldoc
> > and examples will be udated at the very end, since once all drivers
> > are converted over to devm_drm_dev_alloc we can unexport a lot of
> > interim functions and make the documentation for driver authors a lot
> > cleaner and less confusing. There will be only one true way to
> > initialize a drm_device at the end of this, which is going to be
> > devm_drm_dev_alloc.
> 
> This changelog entry does a poor job describing what the purpose of this
> change is.
> Try to read it outside context.

Something like:

Add a new macro helper to combine the usual init sequence in drivers,
consisting of a kzalloc + devm_drm_dev_init + drmm_add_final_kfree
triplet. This allows us to remove the rather unsightly
drmm_add_final_kfree from all currently merged drivers.

This good enough, as an intro paragraph?

> 
> 
> > 
> > Cc: Paul Kocialkowski <paul.kocialkowski at bootlin.com>
> > Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > ---
> >  drivers/gpu/drm/drm_drv.c | 23 +++++++++++++++++++++++
> >  include/drm/drm_drv.h     | 33 +++++++++++++++++++++++++++++++++
> >  2 files changed, 56 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> > index 1bb4f636b83c..9e60b784b3ac 100644
> > --- a/drivers/gpu/drm/drm_drv.c
> > +++ b/drivers/gpu/drm/drm_drv.c
> > @@ -739,6 +739,29 @@ int devm_drm_dev_init(struct device *parent,
> >  }
> >  EXPORT_SYMBOL(devm_drm_dev_init);
> >  
> > +void* __devm_drm_dev_alloc(struct device *parent, struct drm_driver *driver,
> > +			   size_t size, size_t offset)
> > +{
> > +	void *container;
> > +	struct drm_device *drm;
> > +	int ret;
> > +
> > +	container = kzalloc(size, GFP_KERNEL);
> > +	if (!container)
> > +		return ERR_PTR(-ENOMEM);
> > +
> > +	drm = container + offset;
> > +	ret = devm_drm_dev_init(parent, drm, driver);
> > +	if (ret) {
> > +		kfree(container);
> > +		return ERR_PTR(ret);
> > +	}
> > +	drmm_add_final_kfree(drm, container);
> > +
> > +	return container;
> > +}
> > +EXPORT_SYMBOL(__devm_drm_dev_alloc);
> > +
> >  /**
> >   * drm_dev_alloc - Allocate new DRM device
> >   * @driver: DRM driver to allocate device for
> > diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
> > index e7c6ea261ed1..26776be5a21e 100644
> > --- a/include/drm/drm_drv.h
> > +++ b/include/drm/drm_drv.h
> > @@ -626,6 +626,39 @@ int devm_drm_dev_init(struct device *parent,
> >  		      struct drm_device *dev,
> >  		      struct drm_driver *driver);
> >  
> > +void* __devm_drm_dev_alloc(struct device *parent, struct drm_driver *driver,
> > +			   size_t size, size_t offset);
> > +
> > +/**
> > + * devm_drm_dev_alloc - Resource managed allocation of a &drm_device instance
> > + * @parent: Parent device object
> > + * @driver: DRM driver
> > + * @type: the type of the struct which contains struct &drm_device
> > + * @member: the name of the &drm_device within @type.
> I am confused about the naming here.
> devm_ implies we allocate something with a lifetime equal that of a
> driver. So when the driver are gone what we allocate is also gone.
> Like everythign else devm_ prefixed.
> 
> But the lifetime of a drm_device is until the last userspace reference
> is gone (final drm_dev_put() is called).

The kerneldoc for this is largely copied from the existing
devm_drm_dev_init. And yes the lifetime is bound to the device, we do the
drm_dev_put() when that disappears. Now other users of drm_device might
still hold references and delay cleanup, but "cleanup is done as a devres
action" is very much what devm_ signifies.
"
> > + *
> > + * This allocates and initialize a new DRM device. No device registration is done.
> > + * Call drm_dev_register() to advertice the device to user space and register it
> > + * with other core subsystems. This should be done last in the device
> s/This/Calling drm_dev_register()/ will make this sentence a bit more
> explicit.
> 
> > + * initialization sequence to make sure userspace can't access an inconsistent
> > + * state.
> > + *
> > + * The initial ref-count of the object is 1. Use drm_dev_get() and
> > + * drm_dev_put() to take and drop further ref-counts.
> > + *
> > + * It is recommended that drivers embed &struct drm_device into their own device
> > + * structure.
> > + *
> > + * Note that this manages the lifetime of the resulting &drm_device
> > + * automatically using devres.
> Hmm, no this is managed by drmres???

Yup, the next sentence explains how. And note that we're already using
this in the form of devm_drm_dev_init. So not clear what's unclear here
...

Thanks for your comments.
-Daniel


> 
> 
> > + * The DRM device initialized with this function is
> > + * automatically put on driver detach using drm_dev_put().
> > + *
> > + * RETURNS:
> > + * Pointer to new DRM device, or ERR_PTR on failure.
> > + */
> > +#define devm_drm_dev_alloc(parent, driver, type, member) \
> > +	((type *) __devm_drm_dev_alloc(parent, driver, sizeof(type), \
> > +				       offsetof(type, member)))
> > +
> >  struct drm_device *drm_dev_alloc(struct drm_driver *driver,
> >  				 struct device *parent);
> >  int drm_dev_register(struct drm_device *dev, unsigned long flags);
> > -- 
> > 2.25.1
> > 
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch