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.
