linux-kernel - Re: [PATCH] x86/dma: make a zero length mapping length not dma capable

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Message-Id: <20110927233330N.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 27 Sep 2011 23:47:03 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	bigeasy@...utronix.de
Cc:	fujita.tomonori@....ntt.co.jp, linux-kernel@...r.kernel.org,
	balbi@...com
Subject: Re: [PATCH] x86/dma: make a zero length mapping length not dma
 capable

On Tue, 20 Sep 2011 19:44:25 +0200
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> The following ooops happens during a call to dma_map_single():
> 
> | ------------[ cut here ]------------
> | kernel BUG at lib/swiotlb.c:409!
> | invalid opcode: 0000 [#1] SMP
> | CPU 1
> |
> | RIP: 0010:[<ffffffff811a00c9>]  [<ffffffff811a00c9>] swiotlb_tbl_map_single+0x87/0x1e0
> | Call Trace:
> |  [<ffffffff813170e0>] ? printk+0x43/0x48
> |  [<ffffffff8100d027>] ? load_TLS+0x7/0xa
> |  [<ffffffff811a0261>] ? map_single+0x3f/0x3f
> |  [<ffffffff811a02be>] ? swiotlb_map_page+0x5d/0xcd
> |  [<ffffffff811a0261>] ? map_single+0x3f/0x3f
> |  [<ffffffffa02946ca>] ? dwc3_map_buffer_to_dma+0x8a/0xe2 [dwc3]
> 
> As you know from the line 409 the size parameter was 0. This happens
> because the upper layer sends a zero length packet which the dwc3 driver
> tried to map.
> The memory is not really used and the hardware should not really issue a
> dma request however we must setup a dma descriptor which contains an
> address field and a size field which is set to 0 for the zero length
> packet to be sent. So the unconditional mapping was kinda convenient to
> do. ARM does not complain about length == 0, the swiotlb which is
> used on x86 makes trouble. Looking at other udc drivers, they either
> ignore this case or workaround it by incrementing the size by 1 which
> looks nasty.
> For now I fixed this by not mapping zero packets at all. Anything what I
> should expect from the DMA API? I.e. should it be okay to call
> dma_map_single() with size = 0?

I don't think so. I know that some dma API implementations are ok with
it and some are not though. Drivers should calling dma_map_* with a
size that they do DMA with.

Accepting zero means that you can't catch a bug that passes zero. If
calling dma_map_* with zero size returns an address that nicely leads
to a system crash when a driver starts DMA (like calling kmalloc with
zero), then accepting zero size would be ok. But I don't think that
we can implement something like that.

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