Date:	Mon, 06 Aug 2012 12:29:34 +0200
From:	Tomasz Stanislawski <t.stanislaws@...sung.com>
To:	Rob Clark <rob.clark@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-mm@...ck.org, linaro-mm-sig@...ts.linaro.org,
	dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
	patches@...aro.org, linux@....linux.org.uk, arnd@...db.de,
	jesse.barker@...aro.org, m.szyprowski@...sung.com, daniel@...ll.ch,
	sumit.semwal@...com, maarten.lankhorst@...onical.com,
	Rob Clark <rob@...com>
Subject: Re: [PATCH 2/2] dma-buf: add helpers for attacher dma-parms

Hi Rob,

On 07/19/2012 06:23 PM, Rob Clark wrote:
> From: Rob Clark <rob@...com>
> 
> Add some helpers to iterate through all attachers and get the most
> restrictive segment size/count/boundary.
> 
> Signed-off-by: Rob Clark <rob@...com>
> ---
>  drivers/base/dma-buf.c  |   63 +++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/dma-buf.h |   19 ++++++++++++++
>  2 files changed, 82 insertions(+)
> 
> diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
> index 24e88fe..757ee20 100644
> --- a/drivers/base/dma-buf.c
> +++ b/drivers/base/dma-buf.c
> @@ -192,6 +192,69 @@ void dma_buf_put(struct dma_buf *dmabuf)
>  EXPORT_SYMBOL_GPL(dma_buf_put);
>  
>  /**
> + * dma_buf_max_seg_size - helper for exporters to get the minimum of
> + * all attached device's max segment size
> + */
> +unsigned int dma_buf_max_seg_size(struct dma_buf *dmabuf)
> +{
> +	struct dma_buf_attachment *attach;
> +	unsigned int max = (unsigned int)-1;
> +
> +	if (WARN_ON(!dmabuf))
> +		return 0;

Maybe you should change return type to 'int' and return -EINVAL here?

> +
> +	mutex_lock(&dmabuf->lock);
> +	list_for_each_entry(attach, &dmabuf->attachments, node)
> +		max = min(max, dma_get_max_seg_size(attach->dev));
> +	mutex_unlock(&dmabuf->lock);
> +
> +	return max;
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_max_seg_size);
> +
> +/**
> + * dma_buf_max_seg_count - helper for exporters to get the minimum of
> + * all attached device's max segment count
> + */
> +unsigned int dma_buf_max_seg_count(struct dma_buf *dmabuf)
> +{
> +	struct dma_buf_attachment *attach;
> +	unsigned int max = (unsigned int)-1;
> +
> +	if (WARN_ON(!dmabuf))
> +		return 0;

maybe return -EINVAL here?

> +
> +	mutex_lock(&dmabuf->lock);
> +	list_for_each_entry(attach, &dmabuf->attachments, node)
> +		max = min(max, dma_get_max_seg_count(attach->dev));

I think that there is a bug here.
Assume that there are two deices on the list, one using unlimited number of
segments (value 0), the second one needs a contiguous buffer (value 1).
The result of the function is 0 = min(0, 2).

The return value 0 indicates that the unlimited number of sg segments is
accepted what is *wrong* because the correct value should be 1.

I recommend to change the semantics for unlimited number of segments
from 'value 0' to:

#define DMA_SEGMENTS_COUNT_UNLIMITED ((unsigned long)INT_MAX)

Using INT_MAX will allow using safe conversions between signed and
unsigned integers.

> +	mutex_unlock(&dmabuf->lock);
> +
> +	return max;
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_max_seg_count);
> +
> +/**
> + * dma_buf_get_seg_boundary - helper for exporters to get the most
> + * restrictive segment alignment of all the attached devices
> + */
> +unsigned int dma_buf_get_seg_boundary(struct dma_buf *dmabuf)
> +{
> +	struct dma_buf_attachment *attach;
> +	unsigned int mask = (unsigned int)-1;
> +
> +	if (WARN_ON(!dmabuf))
> +		return 0;
> +
> +	mutex_lock(&dmabuf->lock);
> +	list_for_each_entry(attach, &dmabuf->attachments, node)
> +		mask &= dma_get_seg_boundary(attach->dev);
> +	mutex_unlock(&dmabuf->lock);
> +
> +	return mask;
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_get_seg_boundary);
> +
> +/**
>   * dma_buf_attach - Add the device to dma_buf's attachments list; optionally,
>   * calls attach() of dma_buf_ops to allow device-specific attach functionality
>   * @dmabuf:	[in]	buffer to attach device to.
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index eb48f38..9533b9b 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -167,6 +167,10 @@ int dma_buf_fd(struct dma_buf *dmabuf, int flags);
>  struct dma_buf *dma_buf_get(int fd);
>  void dma_buf_put(struct dma_buf *dmabuf);
>  
> +unsigned int dma_buf_max_seg_size(struct dma_buf *dmabuf);
> +unsigned int dma_buf_max_seg_count(struct dma_buf *dmabuf);
> +unsigned int dma_buf_get_seg_boundary(struct dma_buf *dmabuf);

Instead of adding an army of new handlers you could provide a single helper:

int dma_buf_get_parameters(struct dma_buf *dmabuf,
	struct device_dma_parameters *params);

This function will fill *params with lowest common DMA requirements for
all devices on attachment list. Return value can be used to diagnose
errors like incorrectly initialized dma_buf pointer
(like no attachments on an attachment list).

Moreover, there will be no need to add a new handler every time
device_dma_parameters is extended.

Regards,
Tomasz Stanislawski

> +
>  struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *,
>  					enum dma_data_direction);
>  void dma_buf_unmap_attachment(struct dma_buf_attachment *, struct sg_table *,
> @@ -220,6 +224,21 @@ static inline void dma_buf_put(struct dma_buf *dmabuf)
>  	return;
>  }
>  
> +static inline unsigned int dma_buf_max_seg_size(struct dma_buf *dmabuf)
> +{
> +	return 0;
> +}
> +
> +static inline unsigned int dma_buf_max_seg_count(struct dma_buf *dmabuf)
> +{
> +	return 0;
> +}
> +
> +static inline unsigned int dma_buf_get_seg_boundary(struct dma_buf *dmabuf)
> +{
> +	return 0;
> +}
> +
>  static inline struct sg_table *dma_buf_map_attachment(
>  	struct dma_buf_attachment *attach, enum dma_data_direction write)
>  {
> 

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