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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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