Jérôme Pouiller <jerome.pouiller@xxxxxxxxxx> writes: > On Tuesday 22 December 2020 16:27:01 CET Greg Kroah-Hartman wrote: >> >> On Tue, Dec 22, 2020 at 05:10:11PM +0200, Kalle Valo wrote: >> > Jerome Pouiller <Jerome.Pouiller@xxxxxxxxxx> writes: >> > >> > > +/* >> > > + * Internal helpers. >> > > + * >> > > + * About CONFIG_VMAP_STACK: >> > > + * When CONFIG_VMAP_STACK is enabled, it is not possible to run DMA on stack >> > > + * allocated data. Functions below that work with registers (aka functions >> > > + * ending with "32") automatically reallocate buffers with kmalloc. However, >> > > + * functions that work with arbitrary length buffers let's caller to handle >> > > + * memory location. In doubt, enable CONFIG_DEBUG_SG to detect badly located >> > > + * buffer. >> > > + */ >> > >> > This sounds very hacky to me, I have understood that you should never >> > use stack with DMA. >> >> You should never do that because some platforms do not support it, so no >> driver should ever try to do that as they do not know what platform they >> are running on. > > Yes, I have learned this rule the hard way. > > There is no better way than a comment to warn the user that the argument > will be used with a DMA? A Sparse annotation, for example? I have not seen anything, but something like sparse annotation would be useful. Please let me know if you find anything like that. But I think that CONFIG_VMAP_STACK is irrelevant and the comment should be clarified that using stack memory must NOT be used for DMA operations in any circumstances. -- https://patchwork.kernel.org/project/linux-wireless/list/ https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches
