[Intel-gfx] [PATCH 24/29] drm/i915/gem: Delay context creation

Daniel Vetter daniel at ffwll.ch
Mon May 31 09:50:45 UTC 2021


On Thu, May 27, 2021 at 11:26:45AM -0500, Jason Ekstrand wrote:
> The current context uAPI allows for two methods of setting context
> parameters: SET_CONTEXT_PARAM and CONTEXT_CREATE_EXT_SETPARAM.  The
> former is allowed to be called at any time while the later happens as
> part of GEM_CONTEXT_CREATE.  Currently, everything settable via one is
> settable via the other.  While some params are fairly simple and setting
> them on a live context is harmless such the context priority, others are

such _as_ the

> far trickier such as the VM or the set of engines.  In order to swap out
> the VM, for instance, we have to delay until all current in-flight work
> is complete, swap in the new VM, and then continue.  This leads to a
> plethora of potential race conditions we'd really rather avoid.
> 
> In previous patches, we added a i915_gem_proto_context struct which is
> capable of storing and tracking all such create parameters.  This commit
> delays the creation of the actual context until after the client is done
> configuring it with SET_CONTEXT_PARAM.  From the perspective of the
> client, it has the same u32 context ID the whole time.  From the
> perspective of i915, however, it's an i915_gem_proto_context right up
> until the point where we attempt to do something which the proto-context
> can't handle at which point the real context gets created.

s/ at which point/. Then/

At least feels a bit like a run-on sentence :-)


> This is accomplished via a little xarray dance.  When GEM_CONTEXT_CREATE
> is called, we create a proto-context, reserve a slot in context_xa but
> leave it NULL, the proto-context in the corresponding slot in
> proto_context_xa.  Then, whenever we go to look up a context, we first
> check context_xa.  If it's there, we return the i915_gem_context and
> we're done.  If it's not, we look in proto_context_xa and, if we find it
> there, we create the actual context and kill the proto-context.
> 
> In order for this dance to work properly, everything which ever touches
> a proto-context is guarded by drm_i915_file_private::proto_context_lock,
> including context creation.  Yes, this means context creation now takes
> a giant global lock but it can't really be helped and that should never
> be on any driver's fast-path anyway.
> 
> Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_context.c   | 211 ++++++++++++++----
>  drivers/gpu/drm/i915/gem/i915_gem_context.h   |   3 +
>  .../gpu/drm/i915/gem/i915_gem_context_types.h |  54 +++++
>  .../gpu/drm/i915/gem/selftests/mock_context.c |   5 +-
>  drivers/gpu/drm/i915/i915_drv.h               |  24 +-
>  5 files changed, 239 insertions(+), 58 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> index 8288af0d33245..f7c83730ee07f 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> @@ -298,6 +298,42 @@ proto_context_create(struct drm_i915_private *i915, unsigned int flags)
>  	return err;
>  }
>  
> +static int proto_context_register_locked(struct drm_i915_file_private *fpriv,
> +					 struct i915_gem_proto_context *pc,
> +					 u32 *id)
> +{
> +	int ret;
> +	void *old;
> +
> +	lockdep_assert_held(&fpriv->proto_context_lock);
> +
> +	ret = xa_alloc(&fpriv->context_xa, id, NULL, xa_limit_32b, GFP_KERNEL);
> +	if (ret)
> +		return ret;
> +
> +	old = xa_store(&fpriv->proto_context_xa, *id, pc, GFP_KERNEL);
> +	if (xa_is_err(old)) {
> +		xa_erase(&fpriv->context_xa, *id);
> +		return xa_err(old);
> +	}
> +	GEM_BUG_ON(old);

I'd go with WARN_ON here. We just leak, and BUG_ON kills the box.
GEM_BUG_ON is for the additional gem consistency checks which are too
expensive to have enabled in production. Registering a proto context isn't
one of these things.

> +
> +	return 0;
> +}
> +
> +static int proto_context_register(struct drm_i915_file_private *fpriv,
> +				  struct i915_gem_proto_context *pc,
> +				  u32 *id)
> +{
> +	int ret;
> +
> +	mutex_lock(&fpriv->proto_context_lock);
> +	ret = proto_context_register_locked(fpriv, pc, id);
> +	mutex_unlock(&fpriv->proto_context_lock);
> +
> +	return ret;
> +}
> +
>  static int set_proto_ctx_vm(struct drm_i915_file_private *fpriv,
>  			    struct i915_gem_proto_context *pc,
>  			    const struct drm_i915_gem_context_param *args)
> @@ -1448,12 +1484,12 @@ void i915_gem_init__contexts(struct drm_i915_private *i915)
>  	init_contexts(&i915->gem.contexts);
>  }
>  
> -static int gem_context_register(struct i915_gem_context *ctx,
> -				struct drm_i915_file_private *fpriv,
> -				u32 *id)
> +static void gem_context_register(struct i915_gem_context *ctx,
> +				 struct drm_i915_file_private *fpriv,
> +				 u32 id)
>  {
>  	struct drm_i915_private *i915 = ctx->i915;
> -	int ret;
> +	void *old;
>  
>  	ctx->file_priv = fpriv;
>  
> @@ -1462,19 +1498,12 @@ static int gem_context_register(struct i915_gem_context *ctx,
>  		 current->comm, pid_nr(ctx->pid));
>  
>  	/* And finally expose ourselves to userspace via the idr */
> -	ret = xa_alloc(&fpriv->context_xa, id, ctx, xa_limit_32b, GFP_KERNEL);
> -	if (ret)
> -		goto err_pid;
> +	old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
> +	GEM_BUG_ON(old);

Same song about WARN_ON here.

>  
>  	spin_lock(&i915->gem.contexts.lock);
>  	list_add_tail(&ctx->link, &i915->gem.contexts.list);
>  	spin_unlock(&i915->gem.contexts.lock);
> -
> -	return 0;
> -
> -err_pid:
> -	put_pid(fetch_and_zero(&ctx->pid));
> -	return ret;
>  }
>  
>  int i915_gem_context_open(struct drm_i915_private *i915,
> @@ -1484,9 +1513,12 @@ int i915_gem_context_open(struct drm_i915_private *i915,
>  	struct i915_gem_proto_context *pc;
>  	struct i915_gem_context *ctx;
>  	int err;
> -	u32 id;
>  
> -	xa_init_flags(&file_priv->context_xa, XA_FLAGS_ALLOC);
> +	mutex_init(&file_priv->proto_context_lock);
> +	xa_init_flags(&file_priv->proto_context_xa, XA_FLAGS_ALLOC);
> +
> +	/* 0 reserved for the default context */
> +	xa_init_flags(&file_priv->context_xa, XA_FLAGS_ALLOC1);
>  
>  	/* 0 reserved for invalid/unassigned ppgtt */
>  	xa_init_flags(&file_priv->vm_xa, XA_FLAGS_ALLOC1);
> @@ -1504,28 +1536,31 @@ int i915_gem_context_open(struct drm_i915_private *i915,
>  		goto err;
>  	}
>  
> -	err = gem_context_register(ctx, file_priv, &id);
> -	if (err < 0)
> -		goto err_ctx;
> +	gem_context_register(ctx, file_priv, 0);
>  
> -	GEM_BUG_ON(id);
>  	return 0;
>  
> -err_ctx:
> -	context_close(ctx);
>  err:
>  	xa_destroy(&file_priv->vm_xa);
>  	xa_destroy(&file_priv->context_xa);
> +	xa_destroy(&file_priv->proto_context_xa);
> +	mutex_destroy(&file_priv->proto_context_lock);
>  	return err;
>  }
>  
>  void i915_gem_context_close(struct drm_file *file)
>  {
>  	struct drm_i915_file_private *file_priv = file->driver_priv;
> +	struct i915_gem_proto_context *pc;
>  	struct i915_address_space *vm;
>  	struct i915_gem_context *ctx;
>  	unsigned long idx;
>  
> +	xa_for_each(&file_priv->proto_context_xa, idx, pc)
> +		proto_context_close(pc);
> +	xa_destroy(&file_priv->proto_context_xa);
> +	mutex_destroy(&file_priv->proto_context_lock);
> +
>  	xa_for_each(&file_priv->context_xa, idx, ctx)
>  		context_close(ctx);
>  	xa_destroy(&file_priv->context_xa);
> @@ -2480,12 +2515,73 @@ static bool client_is_banned(struct drm_i915_file_private *file_priv)
>  	return atomic_read(&file_priv->ban_score) >= I915_CLIENT_SCORE_BANNED;
>  }
>  
> +static inline struct i915_gem_context *
> +__context_lookup(struct drm_i915_file_private *file_priv, u32 id)
> +{
> +	struct i915_gem_context *ctx;
> +
> +	rcu_read_lock();
> +	ctx = xa_load(&file_priv->context_xa, id);
> +	if (ctx && !kref_get_unless_zero(&ctx->ref))
> +		ctx = NULL;
> +	rcu_read_unlock();
> +
> +	return ctx;
> +}
> +
> +struct i915_gem_context *
> +lazy_create_context_locked(struct drm_i915_file_private *file_priv,
> +			   struct i915_gem_proto_context *pc, u32 id)

My bikeshed would call this finalize_create_context_locked or something
like that ... At least I'm thinking of this more as "finializing the
process of creating a context" and less of "creating context on-demand".
The latter is e.g. what we're doing with the default engine set. Different
beasts conceptually.

> +{
> +	struct i915_gem_context *ctx;
> +	void *old;
> +
> +	lockdep_assert_held(&file_priv->proto_context_lock);
> +
> +	ctx = i915_gem_create_context(file_priv->dev_priv, pc);
> +	if (IS_ERR(ctx))
> +		return ctx;
> +
> +	gem_context_register(ctx, file_priv, id);
> +
> +	old = xa_erase(&file_priv->proto_context_xa, id);
> +	GEM_BUG_ON(old != pc);
> +	proto_context_close(pc);
> +
> +	/* One for the xarray and one for the caller */
> +	return i915_gem_context_get(ctx);
> +}
> +
> +struct i915_gem_context *
> +i915_gem_context_lookup(struct drm_i915_file_private *file_priv, u32 id)
> +{
> +	struct i915_gem_proto_context *pc;
> +	struct i915_gem_context *ctx;
> +
> +	ctx = __context_lookup(file_priv, id);
> +	if (ctx)
> +		return ctx;
> +
> +	mutex_lock(&file_priv->proto_context_lock);
> +	/* Try one more time under the lock */
> +	ctx = __context_lookup(file_priv, id);
> +	if (!ctx) {
> +		pc = xa_load(&file_priv->proto_context_xa, id);
> +		if (!pc)
> +			ctx = ERR_PTR(-ENOENT);
> +		else
> +			ctx = lazy_create_context_locked(file_priv, pc, id);
> +	}
> +	mutex_unlock(&file_priv->proto_context_lock);
> +
> +	return ctx;
> +}
> +
>  int i915_gem_context_create_ioctl(struct drm_device *dev, void *data,
>  				  struct drm_file *file)
>  {
>  	struct drm_i915_private *i915 = to_i915(dev);
>  	struct drm_i915_gem_context_create_ext *args = data;
> -	struct i915_gem_context *ctx;
>  	struct create_ext ext_data;
>  	int ret;
>  	u32 id;
> @@ -2517,28 +2613,21 @@ int i915_gem_context_create_ioctl(struct drm_device *dev, void *data,
>  					   create_extensions,
>  					   ARRAY_SIZE(create_extensions),
>  					   &ext_data);
> -		if (ret) {
> -			proto_context_close(ext_data.pc);
> -			return ret;
> -		}
> +		if (ret)
> +			goto err_pc;
>  	}
>  
> -	ctx = i915_gem_create_context(i915, ext_data.pc);
> -	proto_context_close(ext_data.pc);
> -	if (IS_ERR(ctx))
> -		return PTR_ERR(ctx);
> -
> -	ret = gem_context_register(ctx, ext_data.fpriv, &id);
> +	ret = proto_context_register(ext_data.fpriv, ext_data.pc, &id);
>  	if (ret < 0)
> -		goto err_ctx;
> +		goto err_pc;
>  
>  	args->ctx_id = id;
>  	drm_dbg(&i915->drm, "HW context %d created\n", args->ctx_id);
>  
>  	return 0;
>  
> -err_ctx:
> -	context_close(ctx);
> +err_pc:
> +	proto_context_close(ext_data.pc);
>  	return ret;
>  }
>  
> @@ -2547,6 +2636,7 @@ int i915_gem_context_destroy_ioctl(struct drm_device *dev, void *data,
>  {
>  	struct drm_i915_gem_context_destroy *args = data;
>  	struct drm_i915_file_private *file_priv = file->driver_priv;
> +	struct i915_gem_proto_context *pc;
>  	struct i915_gem_context *ctx;
>  
>  	if (args->pad != 0)
> @@ -2555,11 +2645,21 @@ int i915_gem_context_destroy_ioctl(struct drm_device *dev, void *data,
>  	if (!args->ctx_id)
>  		return -ENOENT;
>  
> +	mutex_lock(&file_priv->proto_context_lock);

I think a comment here that we need to hold the proto mutex even for
finalized context to avoid races with finalization would be nice.

>  	ctx = xa_erase(&file_priv->context_xa, args->ctx_id);
> -	if (!ctx)
> +	pc = xa_erase(&file_priv->proto_context_xa, args->ctx_id);
> +	mutex_unlock(&file_priv->proto_context_lock);
> +
> +	if (!ctx && !pc)
>  		return -ENOENT;
> +	GEM_WARN_ON(ctx && pc);
> +
> +	if (pc)
> +		proto_context_close(pc);
> +
> +	if (ctx)
> +		context_close(ctx);
>  
> -	context_close(ctx);
>  	return 0;
>  }
>  
> @@ -2692,16 +2792,41 @@ int i915_gem_context_setparam_ioctl(struct drm_device *dev, void *data,
>  {
>  	struct drm_i915_file_private *file_priv = file->driver_priv;
>  	struct drm_i915_gem_context_param *args = data;
> +	struct i915_gem_proto_context *pc;
>  	struct i915_gem_context *ctx;
> -	int ret;
> +	int ret = 0;
>  
> -	ctx = i915_gem_context_lookup(file_priv, args->ctx_id);
> -	if (IS_ERR(ctx))
> -		return PTR_ERR(ctx);
> +	ctx = __context_lookup(file_priv, args->ctx_id);
> +	if (ctx)
> +		goto set_ctx_param;
>  
> -	ret = ctx_setparam(file_priv, ctx, args);
> +	mutex_lock(&file_priv->proto_context_lock);
> +	ctx = __context_lookup(file_priv, args->ctx_id);

Not unconditionally taking the mutex here feels a bit like overkill? Do we
really need that fast path?

> +	if (ctx)
> +		goto unlock;
> +
> +	pc = xa_load(&file_priv->proto_context_xa, args->ctx_id);
> +	if (!pc) {
> +		ret = -ENOENT;
> +		goto unlock;
> +	}
> +
> +	/* FIXME: We should consider disallowing SET_CONTEXT_PARAM for most
> +	 * things on future platforms.  Clients should be using
> +	 * CONTEXT_CREATE_EXT_PARAM instead.
> +	 */

I think the way to do that is to finalize the context creation from
CONTEXT_CREATE_EXT on these platforms. That plugs this hole for good and
by design. Maybe on gen13+ or something like that. Or whatever it is we're
using for enumerating generations now.

> +	ret = set_proto_ctx_param(file_priv, pc, args);
> +
> +unlock:
> +	mutex_unlock(&file_priv->proto_context_lock);
> +
> +set_ctx_param:
> +	if (!ret && ctx)

I don't think you need to check for ret here? It's not set by any path
leading to here where ctx != NULL.

Also mildly unhappy about the control flow here, we could simplify it if
we don't do the lockless faspath.

> +		ret = ctx_setparam(file_priv, ctx, args);
> +
> +	if (ctx)
> +		i915_gem_context_put(ctx);
>  
> -	i915_gem_context_put(ctx);
>  	return ret;
>  }
>  
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.h b/drivers/gpu/drm/i915/gem/i915_gem_context.h
> index b5c908f3f4f22..20411db84914a 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.h
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.h
> @@ -133,6 +133,9 @@ int i915_gem_context_setparam_ioctl(struct drm_device *dev, void *data,
>  int i915_gem_context_reset_stats_ioctl(struct drm_device *dev, void *data,
>  				       struct drm_file *file);
>  
> +struct i915_gem_context *
> +i915_gem_context_lookup(struct drm_i915_file_private *file_priv, u32 id);
> +
>  static inline struct i915_gem_context *
>  i915_gem_context_get(struct i915_gem_context *ctx)
>  {
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> index 2ac341f805c8f..b673061f4f5ba 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> @@ -122,6 +122,60 @@ struct i915_gem_proto_engine {
>   * an i915_gem_context.  This is used to gather parameters provided either
>   * through creation flags or via SET_CONTEXT_PARAM so that, when we create
>   * the final i915_gem_context, those parameters can be immutable.
> + *
> + * The context uAPI allows for two methods of setting context parameters:
> + * SET_CONTEXT_PARAM and CONTEXT_CREATE_EXT_SETPARAM.  The former is
> + * allowed to be called at any time while the later happens as part of
> + * GEM_CONTEXT_CREATE.  When these were initially added, Currently,
> + * everything settable via one is settable via the other.  While some
> + * params are fairly simple and setting them on a live context is harmless
> + * such the context priority, others are far trickier such as the VM or the
> + * set of engines.  To avoid some truly nasty race conditions, we don't
> + * allow setting the VM or the set of engines on live contexts.
> + *
> + * The way we dealt with this without breaking older userspace that sets
> + * the VM or engine set via SET_CONTEXT_PARAM is to delay the creation of
> + * the actual context until after the client is done configuring it with
> + * SET_CONTEXT_PARAM.  From the perspective of the client, it has the same
> + * u32 context ID the whole time.  From the perspective of i915, however,
> + * it's an i915_gem_proto_context right up until the point where we attempt
> + * to do something which the proto-context can't handle at which point the
> + * real context gets created.
> + *
> + * This is accomplished via a little xarray dance.  When GEM_CONTEXT_CREATE
> + * is called, we create a proto-context, reserve a slot in context_xa but
> + * leave it NULL, the proto-context in the corresponding slot in
> + * proto_context_xa.  Then, whenever we go to look up a context, we first
> + * check context_xa.  If it's there, we return the i915_gem_context and
> + * we're done.  If it's not, we look in proto_context_xa and, if we find it
> + * there, we create the actual context and kill the proto-context.
> + *
> + * At the time we made this change (April, 2021), we did a fairly complete
> + * audit of existing userspace to ensure this wouldn't break anything:
> + *
> + *  - Mesa/i965 didn't use the engines or VM APIs at all
> + *
> + *  - Mesa/ANV used the engines API but via CONTEXT_CREATE_EXT_SETPARAM and
> + *    didn't use the VM API.
> + *
> + *  - Mesa/iris didn't use the engines or VM APIs at all
> + *
> + *  - The open-source compute-runtime didn't yet use the engines API but
> + *    did use the VM API via SET_CONTEXT_PARAM.  However, CONTEXT_SETPARAM
> + *    was always the second ioctl on that context, immediately following
> + *    GEM_CONTEXT_CREATE.
> + *
> + *  - The media driver sets engines and bonding/balancing via
> + *    SET_CONTEXT_PARAM.  However, CONTEXT_SETPARAM to set the VM was
> + *    always the second ioctl on that context, immediately following
> + *    GEM_CONTEXT_CREATE and setting engines immediately followed that.
> + *
> + * In order for this dance to work properly, any modification to an
> + * i915_gem_proto_context that is exposed to the client via
> + * drm_i915_file_private::proto_context_xa must be guarded by
> + * drm_i915_file_private::proto_context_lock.  The exception is when a
> + * proto-context has not yet been exposed such as when handling
> + * CONTEXT_CREATE_SET_PARAM during GEM_CONTEXT_CREATE.
>   */
>  struct i915_gem_proto_context {
>  	/** @vm: See i915_gem_context::vm */
> diff --git a/drivers/gpu/drm/i915/gem/selftests/mock_context.c b/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> index cbeefd060e97b..61aaac4a334cf 100644
> --- a/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> +++ b/drivers/gpu/drm/i915/gem/selftests/mock_context.c
> @@ -81,6 +81,7 @@ void mock_init_contexts(struct drm_i915_private *i915)
>  struct i915_gem_context *
>  live_context(struct drm_i915_private *i915, struct file *file)
>  {
> +	struct drm_i915_file_private *fpriv = to_drm_file(file)->driver_priv;
>  	struct i915_gem_proto_context *pc;
>  	struct i915_gem_context *ctx;
>  	int err;
> @@ -97,10 +98,12 @@ live_context(struct drm_i915_private *i915, struct file *file)
>  
>  	i915_gem_context_set_no_error_capture(ctx);
>  
> -	err = gem_context_register(ctx, to_drm_file(file)->driver_priv, &id);
> +	err = xa_alloc(&fpriv->context_xa, &id, NULL, xa_limit_32b, GFP_KERNEL);
>  	if (err < 0)
>  		goto err_ctx;
>  
> +	gem_context_register(ctx, fpriv, id);
> +
>  	return ctx;
>  
>  err_ctx:
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index d7bd732ceacfc..8b00292e1ae56 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -201,6 +201,16 @@ struct drm_i915_file_private {
>  		struct rcu_head rcu;
>  	};
>  
> +	/** @proto_context_lock: Guards all i915_gem_proto_context operations
> +	 *
> +	 * See i915_gem_proto_context.

Please add locking rules here, like "This is always held whenever we
manipulate any proto context, including finalizing it on first actual use
of the GEM context".

> +	 */
> +	struct mutex proto_context_lock;
> +
> +	/** @proto_context_xa: xarray of i915_gem_proto_context */

Pls fix hyperlinks.

Also please put your nice explainer from the commit message here ...


> +	struct xarray proto_context_xa;
> +
> +	/** @context_xa: xarray of fully created i915_gem_context */

... and reference it with a "See @proto_context_xa" here.

Maybe also reference i915_gem_context_lookup() from these so readers of
the code can easily find all the pieces of this magic.

Also mention here that write access to @context_xa is protected by
@proto_context_lock. It must be held to avoid races with finalization of
proto context in e.g. i915_gem_context_destroy_ioctl(), and this wasn't
obvious to me at all.


>  	struct xarray context_xa;
>  	struct xarray vm_xa;
>  
> @@ -1857,20 +1867,6 @@ struct drm_gem_object *i915_gem_prime_import(struct drm_device *dev,
>  
>  struct dma_buf *i915_gem_prime_export(struct drm_gem_object *gem_obj, int flags);
>  
> -static inline struct i915_gem_context *
> -i915_gem_context_lookup(struct drm_i915_file_private *file_priv, u32 id)
> -{
> -	struct i915_gem_context *ctx;
> -
> -	rcu_read_lock();
> -	ctx = xa_load(&file_priv->context_xa, id);
> -	if (ctx && !kref_get_unless_zero(&ctx->ref))
> -		ctx = NULL;
> -	rcu_read_unlock();
> -
> -	return ctx ? ctx : ERR_PTR(-ENOENT);
> -}
> -
>  static inline struct i915_address_space *
>  i915_gem_vm_lookup(struct drm_i915_file_private *file_priv, u32 id)
>  {
> -- 
> 2.31.1

Nothing big looks wrong, with the nits addressed:

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> 

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


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