Re: [PATCH 21/28] drm: Kerneldoc for drm_mode_config_funcs

Thierry Reding
2015-12-07T13:14:48Z

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On Fri, Dec 04, 2015 at 09:46:02AM +0100, Daniel Vetter wrote:
[...]
> diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
[...]
> struct drm_mode_config_funcs {
> + /**
> + * @fb_create:
> + *
> + * Create a new framebuffer object. The core does basic checks on the
> + * requested metadata, but most of that is left to the driver. See
> + * struct &drm_mode_fb_cmd2 for details.
> + *
> + * RETURNS:
> + *
> + * A new framebuffer with an initial refernce count of 1 or a negative

"reference"

> + * error code encoded with ERR_PTR().
> + */
> struct drm_framebuffer *(*fb_create)(struct drm_device *dev,
> struct drm_file *file_priv,
> const struct drm_mode_fb_cmd2 *mode_cmd);
> +
> + /**
> + * @output_poll_changed:
> + *
> + * Callback used by helpers to inform the driver of output configuration
> + * changes.
> + *
> + * Drivers implementing fbdev emulation with the helpers can call
> + * drm_fb_helper_hotplug_changed from this hook to inform the fbdev
> + * helper of output changes.
> + *
> + * FIXME:
> + *
> + * Except that there's no vtable for device-level helper callbacks
> + * there's no reason this is a core function.
> + */
> void (*output_poll_changed)(struct drm_device *dev);
>
> + /**
> + * @atomic_check:
> + *
> + * This is the only hook to validate an atomic modeset update. This
> + * function must reject any modeset and state changes which the hardware
> + * or driver doesn't support. This includes but is of course not limited
> + * to:
> + *
> + * - Checking that the modes, framebuffers, scaling and placement
> + * requirements and so on are within the limits of the hardware.
> + *
> + * - Checking that any hidden shared resources are not oversubscribed.
> + * This can be shared PLLs, shared lanes, overall memory bandwidth,
> + * display fifo space (where shared between planes or maybe even
> + * CRTCs).
> + *
> + * - Checking that virtualized resources exported to userspace are not
> + * oversubscribed. For various reasons it can make sense to expose
> + * more planes, crtcs or encoders than which are physically there. One
> + * example is dual-pipe operations (which generally should be hidden
> + * from userspace if when lockstepped in hardware, otherwise exposed),

"hidden from userspace when lockstepped in hardware, exposed otherwise"?

> + * where a plane might need 1 hardware plane (if it's just on one
> + * pipe), 2 hardware planes (when it spans both pipes) or maybe even
> + * shared a hardware plane with a 2nd plane (if there's a compatible
> + * plane requested on the area handled by the other pipe).
> + *
> + * - Check that any transitional state is possible and that if
> + * requested, the update can indeed be done in the vblank period
> + * without temporarily disabling some functions.
> + *
> + * - Check any other constraints the driver or hardware might have.
> + *
> + * - This callback also needs to correctly fill out the &drm_crtc_state
> + * in this update to make sure that drm_atomic_crtc_needs_modeset()
> + * reflects the nature of the possible update and returns true if and
> + * only if the update cannot be applied without tearing within one
> + * vblank on that CRTC. The core uses that information to reject
> + * updates which require a full modeset (i.e. blanking the screen, or
> + * at least pausing updates for a substantial amount of time) if
> + * userspace has disallowed that in its request.
> + *
> + * - The driver also does not need to repeat basic input validation
> + * like done for the corresponding legacy entry points. The core does
> + * that before calling this hook.
> + *
> + * See the documentation of @atomic_commit for an exhaustive list of
> + * error conditions which are allowed to not be checked in this
> + * callback.

I think this is confusing. Perhaps "list of error conditions which don't
have to be checked at the ->atomic_check() stage."?

> + *
> + * See the documentation for struct &drm_atomic_state for how exactly
> + * an atomic modeset update is described.
> + *
> + * Drivers using the atomic helpers can implement this hook using
> + * drm_atomic_helper_check(), or one of the exported sub-functions of
> + * it.
> + *
> + * RETURNS:
> + *
> + * 0 on success or one of the below negative error codes:
> + *
> + * - -EINVAL, if any of the above constraints are violated.
> + *
> + * - -EDEADLK, when returned from an attempt to acquire an additional
> + * &drm_modeset_lock through drm_modeset_lock().
> + *
> + * - -ENOMEM, if allocating additional state sub-structures failed due
> + * to lack of memory.
> + *
> + * - -EINTR, -EAGAIN or -ERESTARTSYS, if the ioctl should be restarted.
> + * This can either be due to a pending signal, or because the driver
> + * needs to completely bail out to recover from an exceptional
> + * situation like a gpu hang. From a userspace point all errors are
> + * treated equally.
> + */
> int (*atomic_check)(struct drm_device *dev,
> struct drm_atomic_state *a);
> +
> + /**
> + * @atomic_commit:
> + *
> + * This is the only hook to commit an atomic modeset update. The core
> + * guarantees that @atomic_check has been called successfully before
> + * calling this function, and that nothing has been changed in the
> + * interim.
> + *
> + * See the documentation for struct &drm_atomic_state for how exactly
> + * an atomic modeset update is described.
> + *
> + * Drivers using the atomic helpers can implement this hook using
> + * drm_atomic_helper_commit(), or one of the exported sub-functions of
> + * it.
> + *
> + * Asynchronous commits (as indicated with the async parameter) must
> + * do any preparatory work which might result in an unsuccessful commit
> + * in the context of this callback. The only exception is hardware

"The only exception are hardware errors"

> + * errors resulting in -EIO. But even in that case the driver must
> + * ensure that the display pipe is at least running, to avoid
> + * compositors crashing when pageflips don't work. Anything else,
> + * specifically committing the update to the hardware, should be done
> + * without blocking the caller. For updates which do not require a
> + * modeset this must be guaranteed.
> + *
> + * The driver must wait for any pending rendering to the new
> + * framebuffers to complete before executing the flip. It should also
> + * wait for any pending rendering from other drivers if the underlying
> + * buffer is a shared dma-buf. Asynchronous commits must not wait for
> + * rendering in the context of this callback.
> + *
> + * An application can request to be notified when the atomic commit has
> + * completed. These events are per-CRTC and can be distinguished by the
> + * CRTC index supplied in &drm_event to userspace.
> + *
> + * The drm core will supply a struct &drm_event in the event
> + * member of each CRTC's &drm_crtc_state structure. This can be handled by the
> + * drm_crtc_send_vblank_event() function, which the driver should call on
> + * the provided event upon completion of the atomic commit. Note that if
> + * the driver supports vblank signalling and timestamping the vblank
> + * counters and timestamps must agree with the ones returned from page
> + * flip events. With the current vblank helper infrastructure this can
> + * be achieved by holding a vblank reference while the page flip is
> + * pending, acquired through drm_crtc_vblank_get() and released with
> + * drm_crtc_vblank_put(). Drivers are free to implement their own vblank
> + * counter and timestamp tracking though, e.g. if they have accurate
> + * timestamp registers in hardware.
> + *
> + * NOTE:
> + *
> + * Drivers are not allowed to shut down any display pipe successfully
> + * enabled through an atomic commit on their own. Doing so can result in
> + * compositors crashing if a page flip is suddenly reject because the

"rejected"

> + * pipe is off.
> + *
> + * RETURNS:
> + *
> + * 0 on success or one of the below negative error codes:
> + *
> + * - -EBUSY, if an asynchronous updated is requested and there is

"update"

> + * currently an earlier updated still pending. Drivers are allowed to

"update", also I'd drop the "currently" and "still" because they are
somewhat redundant: "... and an earlier update is pending."

> + * support a queue of outstanding updates, but currently no driver
> + * supports that. Note that drivers must wait for preceding updates
> + * to complete if a synchronous update is requested, they are not
> + * allowed to fail the commit in that case.
> + *
> + * - -ENOMEM, if the driver failed to allocate memory. Specifically
> + * this can happen when trying to pin framebuffers, which must only
> + * be done when committing the state.
> + *
> + * - -ENOSPC, as a refinement of the more generic -ENOMEM to indicate
> + * that the driver has run out of vram, iommu space or similar gpu
> + * address space needed for framebuffer.
> + *
> + * - -EIO, if the hardware completely died.
> + *
> + * - -EINTR, -EAGAIN or -ERESTARTSYS, if the ioctl should be restarted.

s/ioctl/IOCTL/

> + * This can either be due to a pending signal, or because the driver
> + * needs to completely bail out to recover from an exceptional
> + * situation like a gpu hang. From a userspace point of view all errors are

s/gpu/GPU/

> + * treated equally.
> + *
> + * This list is exhaustive. Specifically this hook is not allowed to
> + * return -EINVAL (any invalid requests should be caught in
> + * @atomic_check) or -EDEADLK (this function must not acquire
> + * additional modeset locks). The core will also reject any async
> + * atomic flips with -EINVAL already (for matching semantics in this
> + * case with legacy page flips).

Can you elaborate on this last assertion? Why does the core reject async
atomic flips? I don't see where it does that.

> + */
> int (*atomic_commit)(struct drm_device *dev,
> struct drm_atomic_state *a,

Why is the state variable called "a" here? Why not "state"? Same for
->atomic_check() above.

> + /**
> + * @atomic_state_alloc:
> + *
> + * This optional hook can be used by drivers who want to subclass struct

"... drivers that want ..."

> + * &drm_atomic_state to be able to track their own driver-private global
> + * state easily. If this hook is implemented, drivers must also
> + * implement @atomic_state_clear and @atomic_state_free.
> + *
> + * RETURNS:
> + *
> + * A new &drm_atomic_state on success or NULL on failure.
> + */
> struct drm_atomic_state *(*atomic_state_alloc)(struct drm_device *dev);
> +
> + /**
> + * @atomic_state_clear:
> + *
> + * This hook must clear any driver private state duplicated into the
> + * passed-in &drm_atomic_state. This hook is called when the caller
> + * encountered a &drm_modeset_lock deadlock and needs to drop all
> + * already acquired locks as part of the deadlock avoidance dance
> + * implemented in drm_modeset_lock_backoff().
> + *
> + * Any duplicated state must be invalidated since a concurrent atomic
> + * update might change it, and the drm atomic interfaces always apply
> + * updates as relative changes to the current state.
> + *
> + * Drivers who implement this must call drm_atomic_state_default_clear()

"Drivers that implement ..."

> + * to clear common state.
> + */
> void (*atomic_state_clear)(struct drm_atomic_state *state);
> +
> + /**
> + * @atomic_state_free:
> + *
> + * This hook needs driver private resources and the &drm_atomic_state

Did you mean "This hook frees ..."?

> + * itself. Note that the core first calls drm_atomic_state_clear to

Parentheses after drm_atomic_state_clear?

Thierry

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