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SubjectRe: [PATCH 5/5] tracing: add binary buffer files for use with splice
On Tue, 03 Mar 2009 21:49:26 -0500 Steven Rostedt <rostedt@goodmis.org> wrote:

> +static ssize_t
> +tracing_buffers_read(struct file *filp, char __user *ubuf,
> + size_t count, loff_t *ppos)
> +{
> + struct ftrace_buffer_info *info = filp->private_data;
> + unsigned int pos;
> + ssize_t ret;
> + size_t size;
> +
> + /* Do we have previous read data to read? */
> + if (info->read < PAGE_SIZE)
> + goto read;
> +
> + info->read = 0;
> +
> + ret = ring_buffer_read_page(info->tr->buffer,
> + &info->spare,
> + count,
> + info->cpu, 0);
> + if (ret < 0)
> + return 0;
> +
> + pos = ring_buffer_page_len(info->spare);
> +
> + if (pos < PAGE_SIZE)
> + memset(info->spare + pos, 0, PAGE_SIZE - pos);
> +
> +read:
> + size = PAGE_SIZE - info->read;
> + if (size > count)
> + size = count;
> +
> + ret = copy_to_user(ubuf, info->spare + info->read, size);
> + if (ret)
> + return -EFAULT;

Conventionally a read() system call will return the number of bytes
copied, and will only return -EFOO if the number of bytes copied was
zero.

Lots of parts of the kernel break this, but it's usually device drivers
and scruffy pseudo files, in which case a partial file read doesn't
make much sense. This doesn't make the broken behaviour right, but at
least we have a bit of a weaselly excuse in that case.

> + *ppos += size;
> + info->read += size;
> +
> + return size;
> +}


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