linux-kernel - Re: [PATCH 1/1] Userspace I/O (UIO): Add support for userspace DMA

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Message-ID: <208aa0f00812051632h38fc0a5g58d233190436cc90@mail.gmail.com>
Date:	Fri, 5 Dec 2008 16:32:30 -0800
From:	"Edward Estabrook" <edward.estabrook.lkml@...il.com>
To:	"Hans J. Koch" <hjk@...utronix.de>
Cc:	"Peter Zijlstra" <peterz@...radead.org>,
	edward_estabrook@...lent.com, linux-kernel@...r.kernel.org,
	gregkh@...e.de, edward.estabrook@...il.com,
	hugh <hugh@...itas.com>, linux-mm <linux-mm@...ck.org>,
	"Thomas Gleixner" <tglx@...utronix.de>
Subject: Re: [PATCH 1/1] Userspace I/O (UIO): Add support for userspace DMA

> Well, UIO already rapes the mmap interface by using the "offset" parameter to
> pass in the number of the mapping.

Exactly.

> But I'll NAK the current concept, too. It's a UIO kernel driver's task to tell
> userspace which memory a device has to offer. The UIO core prevents userspace
> as much as possible from mapping anything different. And it should stay that
> way.

The ultimate purpose (I thought) of the UIO driver is to simplify
driver development
by pushing device control into userspace.  There is a very real need
for efficient
dynamic control over the DMA allocation of a device.  Why not 'allow' this to
happen in userspace if it can be done safely and without breaking anything else?

Remember that for devices employing ring buffers it is not a question of
'how much memory a device has to offer' but rather 'how much system
memory would the
driver like to configure that device to use'.

I don't want to stop my DMA engine and reload the driver to create
more buffers (and I don't
want to pre-allocate more than I need as contingency).

Cheers,
Ed
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