From: Bart Van Assche <bvanassche@xxxxxxx> Sent: Thursday, August 22, 2024 12:29 PM > > On 8/22/24 11:37 AM, mhkelley58@xxxxxxxxx wrote: > > Linux device drivers may make DMA map/unmap calls in contexts that > > cannot block, such as in an interrupt handler. > > Although I really appreciate your work, what alternatives have been > considered? How many drivers perform DMA mapping from atomic context? > Would it be feasible to modify these drivers such that DMA mapping > always happens in a context in which sleeping is allowed? > I had assumed that allowing DMA mapping from interrupt context is a long-time fundamental requirement that can't be changed. It's been allowed at least for the past 20 years, as Linus added this statement to kernel documentation in 2005: The streaming DMA mapping routines can be called from interrupt context. But I don't have any idea how many drivers actually do that. There are roughly 1700 call sites in kernel code/drivers that call one of the dma_map_*() variants, so looking through them all doesn't seem feasible. From the limited samples I looked at, block device drivers typically do not call dma_map_*() from interrupt context, though they do call dma_unmap_*(). Network drivers _do_ call dma_map_*() from interrupt context, and that seems likely to be an artifact of the generic networking framework that the drivers fit into. I haven't looked at any other device types. Christoph Hellwig, or anyone else who knows the history and current reality better than I do, please jump in. :-) Michael
