linux-kernel - Re: splice methods in character device driver

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Date:	Wed, 3 Jun 2009 23:32:56 +0200
From:	Leon Woestenberg <leon.woestenberg@...il.com>
To:	Steve Rottinger <steve@...tek.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org
Subject: Re: splice methods in character device driver

Hello all,

On Wed, May 13, 2009 at 6:59 PM, Steve Rottinger <steve@...tek.com> wrote:
> is passing in the pages into splice_to_pipe.  The pages are associated
> with a PCI BAR, not main memory.  I'm wondering if this could be a problem?
>
Good question; my newbie answer would be the pages need to be mapped
in kernel space.

I have a similar use case but with memory being DMA'd to host main
memory (instead of the data sitting in your PCI device) in a character
device driver. The driver is a complete rewrite from scratch from
what's currently sitting-butt-ugly in staging/altpcichdma.c
so-please-don't-look-there.

I have already implemented zero-latency overlapping transfers in the
DMA engine (i.e. it never sits idle if async I/O is performed through
threads), now it would be really cool to add zero-copy.

What is it my driver is expected to do?

.splice_read:

- Allocate a bunch of single pages
- Create a scatter-gather list
- "stuff the data pages in question into a struct page *pages[]." a la
"fs/splice.c:vmsplice_to_pipe()"
- Start the DMA from the device to the pages (i.e. the transfer)
- Return.

.splice_write:

- Create a scatter-gather list

interrupt handler / DMA service routine:
- device book keeping
- wake_up_interruptible(transfer_queue)

.confirm():

"then you need to provide a suitable ->confirm() hook that can wait on
this IO to complete if needed."
- wait_on_event_interruptibe(transfer_queue)

.release():

- release the pages

.steal():

unsure

.map

unsure

Regards,
-- 
Leon
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