Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hello,

On 2015-01-27 09:25, Sumit Semwal wrote:
> Add some helpers to share the constraints of devices while attaching
> to the dmabuf buffer.
>
> At each attach, the constraints are calculated based on the following:
> - max_segment_size, max_segment_count, segment_boundary_mask from
>     device_dma_parameters.
>
> In case the attaching device's constraints don't match up, attach() fails.
>
> At detach, the constraints are recalculated based on the remaining
> attached devices.
>
> Two helpers are added:
> - dma_buf_get_constraints - which gives the current constraints as calculated
>        during each attach on the buffer till the time,
> - dma_buf_recalc_constraints - which recalculates the constraints for all
>        currently attached devices for the 'paranoid' ones amongst us.
>
> The idea of this patch is largely taken from Rob Clark's RFC at
> https://lkml.org/lkml/2012/7/19/285, and the comments received on it.
>
> Cc: Rob Clark <robdclark@gmail.com>
> Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>

The code looks okay, although it will probably will work well only with 
typical
cases like 'contiguous memory needed' or 'no constraints at all' (iommu).

Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>

> ---
> v3:
> - Thanks to Russell's comment, remove dma_mask and coherent_dma_mask from
>    constraints' calculation; has a nice side effect of letting us use
>    device_dma_parameters directly to list constraints.
> - update the debugfs output to show constraint info as well.
>    
> v2: split constraints-sharing and allocation helpers
>
>   drivers/dma-buf/dma-buf.c | 126 +++++++++++++++++++++++++++++++++++++++++++++-
>   include/linux/dma-buf.h   |   7 +++
>   2 files changed, 132 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 5be225c2ba98..f363f1440803 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -264,6 +264,66 @@ static inline int is_dma_buf_file(struct file *file)
>   	return file->f_op == &dma_buf_fops;
>   }
>   
> +static inline void init_constraints(struct device_dma_parameters *cons)
> +{
> +	cons->max_segment_count = (unsigned int)-1;
> +	cons->max_segment_size = (unsigned int)-1;
> +	cons->segment_boundary_mask = (unsigned long)-1;
> +}
> +
> +/*
> + * calc_constraints - calculates if the new attaching device's constraints
> + * match, with the constraints of already attached devices; if yes, returns
> + * the constraints; else return ERR_PTR(-EINVAL)
> + */
> +static int calc_constraints(struct device *dev,
> +			    struct device_dma_parameters *calc_cons)
> +{
> +	struct device_dma_parameters cons = *calc_cons;
> +
> +	cons.max_segment_count = min(cons.max_segment_count,
> +					dma_get_max_seg_count(dev));
> +	cons.max_segment_size = min(cons.max_segment_size,
> +					dma_get_max_seg_size(dev));
> +	cons.segment_boundary_mask &= dma_get_seg_boundary(dev);
> +
> +	if (!cons.max_segment_count ||
> +	    !cons.max_segment_size ||
> +	    !cons.segment_boundary_mask) {
> +		pr_err("Dev: %s's constraints don't match\n", dev_name(dev));
> +		return -EINVAL;
> +	}
> +
> +	*calc_cons = cons;
> +
> +	return 0;
> +}
> +
> +/*
> + * recalc_constraints - recalculates constraints for all attached devices;
> + *  useful for detach() recalculation, and for dma_buf_recalc_constraints()
> + *  helper.
> + *  Returns recalculated constraints in recalc_cons, or error in the unlikely
> + *  case when constraints of attached devices might have changed.
> + */
> +static int recalc_constraints(struct dma_buf *dmabuf,
> +			      struct device_dma_parameters *recalc_cons)
> +{
> +	struct device_dma_parameters calc_cons;
> +	struct dma_buf_attachment *attach;
> +	int ret = 0;
> +
> +	init_constraints(&calc_cons);
> +
> +	list_for_each_entry(attach, &dmabuf->attachments, node) {
> +		ret = calc_constraints(attach->dev, &calc_cons);
> +		if (ret)
> +			return ret;
> +	}
> +	*recalc_cons = calc_cons;
> +	return 0;
> +}
> +
>   /**
>    * dma_buf_export_named - Creates a new dma_buf, and associates an anon file
>    * with this buffer, so it can be exported.
> @@ -313,6 +373,9 @@ struct dma_buf *dma_buf_export_named(void *priv, const struct dma_buf_ops *ops,
>   	dmabuf->ops = ops;
>   	dmabuf->size = size;
>   	dmabuf->exp_name = exp_name;
> +
> +	init_constraints(&dmabuf->constraints);
> +
>   	init_waitqueue_head(&dmabuf->poll);
>   	dmabuf->cb_excl.poll = dmabuf->cb_shared.poll = &dmabuf->poll;
>   	dmabuf->cb_excl.active = dmabuf->cb_shared.active = 0;
> @@ -422,7 +485,7 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>   					  struct device *dev)
>   {
>   	struct dma_buf_attachment *attach;
> -	int ret;
> +	int ret = 0;
>   
>   	if (WARN_ON(!dmabuf || !dev))
>   		return ERR_PTR(-EINVAL);
> @@ -436,6 +499,9 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>   
>   	mutex_lock(&dmabuf->lock);
>   
> +	if (calc_constraints(dev, &dmabuf->constraints))
> +		goto err_constraints;
> +
>   	if (dmabuf->ops->attach) {
>   		ret = dmabuf->ops->attach(dmabuf, dev, attach);
>   		if (ret)
> @@ -448,6 +514,7 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>   
>   err_attach:
>   	kfree(attach);
> +err_constraints:
>   	mutex_unlock(&dmabuf->lock);
>   	return ERR_PTR(ret);
>   }
> @@ -470,6 +537,8 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
>   	if (dmabuf->ops->detach)
>   		dmabuf->ops->detach(dmabuf, attach);
>   
> +	recalc_constraints(dmabuf, &dmabuf->constraints);
> +
>   	mutex_unlock(&dmabuf->lock);
>   	kfree(attach);
>   }
> @@ -770,6 +839,56 @@ void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
>   }
>   EXPORT_SYMBOL_GPL(dma_buf_vunmap);
>   
> +/**
> + * dma_buf_get_constraints - get the *current* constraints of the dmabuf,
> + *  as calculated during each attach(); returns error on invalid inputs
> + *
> + * @dmabuf:		[in]	buffer to get constraints of
> + * @constraints:	[out]	current constraints are returned in this
> + */
> +int dma_buf_get_constraints(struct dma_buf *dmabuf,
> +			    struct device_dma_parameters *constraints)
> +{
> +	if (WARN_ON(!dmabuf || !constraints))
> +		return -EINVAL;
> +
> +	mutex_lock(&dmabuf->lock);
> +	*constraints = dmabuf->constraints;
> +	mutex_unlock(&dmabuf->lock);
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_get_constraints);
> +
> +/**
> + * dma_buf_recalc_constraints - *recalculate* the constraints for the buffer
> + *  afresh, from the list of currently attached devices; this could be useful
> + *  cross-check the current constraints, for exporters that might want to be
> + *  'paranoid' about the device constraints.
> + *
> + *  returns error on invalid inputs
> + *
> + * @dmabuf:		[in]	buffer to get constraints of
> + * @constraints:	[out]	recalculated constraints are returned in this
> + */
> +int dma_buf_recalc_constraints(struct dma_buf *dmabuf,
> +			    struct device_dma_parameters *constraints)
> +{
> +	struct device_dma_parameters calc_cons;
> +	int ret = 0;
> +
> +	if (WARN_ON(!dmabuf || !constraints))
> +		return -EINVAL;
> +
> +	mutex_lock(&dmabuf->lock);
> +	ret = recalc_constraints(dmabuf, &calc_cons);
> +	if (!ret)
> +		*constraints = calc_cons;
> +
> +	mutex_unlock(&dmabuf->lock);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_recalc_constraints);
> +
>   #ifdef CONFIG_DEBUG_FS
>   static int dma_buf_describe(struct seq_file *s)
>   {
> @@ -801,6 +920,11 @@ static int dma_buf_describe(struct seq_file *s)
>   				buf_obj->file->f_flags, buf_obj->file->f_mode,
>   				file_count(buf_obj->file),
>   				buf_obj->exp_name);
> +		seq_printf(s, "\tConstraints: Seg Count: %08u, Seg Size: %08u",
> +				buf_obj->constraints.max_segment_count,
> +				buf_obj->constraints.max_segment_size);
> +		seq_printf(s, " seg boundary mask: %08lx\n",
> +				buf_obj->constraints.segment_boundary_mask);
>   
>   		seq_puts(s, "\tAttached Devices:\n");
>   		attach_count = 0;
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index 694e1fe1c4b4..489ad9b2e5ae 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -34,6 +34,7 @@
>   #include <linux/wait.h>
>   
>   struct device;
> +struct device_dma_parameters;
>   struct dma_buf;
>   struct dma_buf_attachment;
>   
> @@ -116,6 +117,7 @@ struct dma_buf_ops {
>    * @ops: dma_buf_ops associated with this buffer object.
>    * @exp_name: name of the exporter; useful for debugging.
>    * @list_node: node for dma_buf accounting and debugging.
> + * @constraints: calculated constraints of attached devices.
>    * @priv: exporter specific private data for this buffer object.
>    * @resv: reservation object linked to this dma-buf
>    */
> @@ -130,6 +132,7 @@ struct dma_buf {
>   	void *vmap_ptr;
>   	const char *exp_name;
>   	struct list_head list_node;
> +	struct device_dma_parameters constraints;
>   	void *priv;
>   	struct reservation_object *resv;
>   
> @@ -211,4 +214,8 @@ void *dma_buf_vmap(struct dma_buf *);
>   void dma_buf_vunmap(struct dma_buf *, void *vaddr);
>   int dma_buf_debugfs_create_file(const char *name,
>   				int (*write)(struct seq_file *));
> +
> +int dma_buf_get_constraints(struct dma_buf *, struct device_dma_parameters *);
> +int dma_buf_recalc_constraints(struct dma_buf *,
> +					struct device_dma_parameters *);
>   #endif /* __DMA_BUF_H__ */

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland