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

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Message-ID: <20090604073218.GT11363@kernel.dk>
Date:	Thu, 4 Jun 2009 09:32:18 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Leon Woestenberg <leon.woestenberg@...il.com>
Cc:	Steve Rottinger <steve@...tek.com>, linux-kernel@...r.kernel.org
Subject: Re: splice methods in character device driver

On Wed, Jun 03 2009, Leon Woestenberg wrote:
> 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.

That is what the ->map() hook is for.

> 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

This is what allows zero copy throughout the pipe line. ->steal(), if
sucesful, should pass ownership of that page to the caller. The previous
owner must no longer modify it.

> .map
> 
> unsure

See above :-)

-- 
Jens Axboe

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