Re: convert sg to block layer helpers - v5

[Mike Christie <michaelc@cs.wisc.edu>]

Douglas Gilbert wrote:
> Mike,
> I see you are removing the scatter_elem_sz parameter.
> What decides the scatter gather element size? Can it
> be greater than PAGE_SIZE?

Oh yeah, sorry I should have documented that.

I just made the code try to allocate as large a element as possible.
So the code looks at q->max_segment_size and tries to allocate segments
that large initially. If that is too large then it will drop down by
half like what sg.c used to do when it could not allocate large segments.

I will add the param back if you want. I had thought it was a workaound
due to the segment size of a device not being exported.

> 
> 
> *** Generalizing the idea of a mmap-ed reserve buffer to
> something the user had more control over could be very
> powerful.
> For example allowing two file descriptors (to different
> devices) in the same process to share the same mmap-ed
> area. This would allow a device to device copy to DMA into
> and out of the same memory, potentially with large per command
> transfers and with no per command scatter gather build and
> tear down. Basically a zero copy copy with minimal CPU
> overhead.
> 

I was thinking of something similar but not based on mmap. I have been
trying to figure out a way to do sg io splice. I do not care what
interface or method is used, I think it would be useful.

I know we talked about the mmap approach a little, but I do not remember
if we talked about how to tell both fds that they are going to use the
same buffer. Would we need a modification to the sg header or would we
need to add in a new IOCTL which would tell sg.c to share the buffer
between two fds?