Re: [PATCH 01/59] drm: Add devm_drm_dev_alloc macro

From: Daniel Vetter <daniel.vetter@ffwll.ch>
To: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Daniel Vetter <daniel.vetter@intel.com>,
	Sam Ravnborg <sam@ravnborg.org>
Subject: Re: [PATCH 01/59] drm: Add devm_drm_dev_alloc macro
Date: Tue, 21 Apr 2020 12:45:49 +0200	[thread overview]
Message-ID: <CAKMK7uH2vhrQ7eTTF1B+==UJS9ZxhDv2RDvR0ct4P0vVJobf=w@mail.gmail.com> (raw)
In-Reply-To: <4d5229c2-acb4-b76f-13c7-88a5f3de4760@suse.de>

On Mon, Apr 20, 2020 at 3:37 PM Thomas Zimmermann <tzimmermann@suse.de> wrote:
>
> Hi
>
> Am 15.04.20 um 09:39 schrieb Daniel Vetter:
> > 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.
> >
> > 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.
> >
> > v2:
> > - Actually explain what this is for in the commit message (Sam)
> > - Fix checkpatch issues (Sam)
> >
> > Acked-by: Noralf Trønnes <noralf@tronnes.org>
> > Cc: Noralf Trønnes <noralf@tronnes.org>
> > Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
> > Cc: Sam Ravnborg <sam@ravnborg.org>
> > Cc: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
> > Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

Thanks for taking a look, some questions on your suggestions below.

> Sorry for being late. A number of nits are listed below. In any case:
>
> Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
>
> Best regards
> Thomas
>
> > ---
> >  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..8e1813d2a12e 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)
>
> Maybe rename 'offset' of 'dev_offset' to make the relationship clear.

Hm, I see the point of this (and the dev_field below, although I'd go
with dev_member there for some consistency with other macros using
offset_of or container_of), but I'm not sure about the dev_ prefix.
Drivers use that sometimes for the struct device *, and usage for
struct drm_device * is also very inconsistent. I've seen ddev, drm,
dev and base (that one only for embedded structs ofc). So not sure
which prefix to pick, aside from dev_ seems the most confusing. Got
ideas?

> > +{
> > +     void *container;
> > +     struct drm_device *drm;
> > +     int ret;
> > +
> > +     container = kzalloc(size, GFP_KERNEL);
> > +     if (!container)
> > +             return ERR_PTR(-ENOMEM);
> > +
> > +     drm = container + offset;
>
> While convenient, I somewhat dislike the use of void* variables. I'd use
> unsigned char* for container and do an explicit cast to struct
> drm_device* here.

I thought ever since C89 the explicit recommendation for untyped
pointer math has been void *, and no longer char *, with the spec
being explicit that void * pointer math works exactly like char *. So
not clear on why you think char * is preferred here. I'm also not
aware of any other kernel code that casts to char * for untyped
pointer math. So unless you have some supporting evidence, I'll skip
this one, ok?

Thanks, Daniel

> > +     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..f07f15721254 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.
> > + *
> > + * 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
> > + * 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. 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) \
>
> I'd replace 'member' with 'dev_field' to make the relation ship clear.
>
> > +     ((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);
> >
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Felix Imendörffer
>

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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