# Re: remove NULL struct device support in the DMA API

- From: Christoph Hellwig \<hch@lst.de\>
- Date: 2019-04-03 19:42:35 
- List: linux-fbdev
- Page: https://ratatoskr.run/linux-fbdev/2019/04/5559882
- Thread: https://ratatoskr.run/linux-doc/2019/03/5545246/t.md

```text
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.
```