linux-kernel - Re: remove NULL struct device support in the DMA API

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Message-ID: <20190403194220.GA30376@lst.de>
Date:   Wed, 3 Apr 2019 21:42:20 +0200
From:   Christoph Hellwig <hch@....de>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>, x86@...nel.org,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-doc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: remove NULL struct device support in the DMA API

On Wed, Apr 03, 2019 at 07:26:40PM +0100, Russell King - ARM Linux admin wrote:
> On Thu, Mar 21, 2019 at 03:52:28PM -0700, Christoph Hellwig wrote:
> > We still have a few drivers which pass a NULL struct device pointer
> > to DMA API functions, which generally is a bad idea as the API
> > implementations rely on the device not only for ops selection, but
> > also the dma mask and various other attributes, and many implementations
> > have been broken for NULL device support for a while.
> 
> I think I must be missing something, but...
> 
> My understanding is that ISA DMA is normally limited to 24 bits of
> address

Yes.

> - indeed, the x86 version only programs 24 bits of DMA address.
> Looking through this series, it appears that the conversions mean that
> the DMA mask for ISA becomes the full all-ones DMA mask, which would
> of course lead to memory corruption if only 24 bits of the address end
> up being programmed into the hardware.

In the generic dma mapping code no struct device has always meant a
32-bit DMA mask - take a look at the dma_get_mask() function.

> Maybe you could say why you think this series is safe in regard to ISA
> DMA?

ISA DMA has always been rather painful in a myriad of ways, and the
DMA API so far hasn't helped, given that we don't do bounce buffering
for the 24-bit limit, but just the higher limits.  So far even if you
do use the DMA API and pass a device ISA DMA so far always meant
that the higher layers had to assure things are addressable, either
by using GFP_DMA allocation in the drivers, or mid-layer hacks like
the unchecked_isa_dma flag in SCSI and/or BLK_BOUNCE_ISA in the
block layer.

This series doesn't change those facts at all.  I have some half
started series to clean some of this up but it isn't high up on
the priority list.