From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH 08/18] dma-mapping: add a new dma_alloc_noncoherent API Date: Fri, 25 Sep 2020 18:17:54 +0200 Message-ID: <20200925161754.GA18721@lst.de> References: <20200915155122.1768241-1-hch@lst.de> <20200915155122.1768241-9-hch@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: To: Robin Murphy Cc: Christoph Hellwig , Mauro Carvalho Chehab , Thomas Bogendoerfer , "James E.J. Bottomley" , Joonyoung Shim , Seung-Woo Kim , Ben Skeggs , Marek Szyprowski , Tomasz Figa , Matt Porter , iommu@lists.linux-foundation.org, alsa-devel@alsa-project.org, linux-samsung-soc@vger.kernel.org, linux-scsi@vger.kernel.org, linux-parisc@vger.kernel.org, linux-doc@vger.kernel.org, nouveau@lists.freedesktop.org, linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org, linux-mm@kvack.org, Stefan Richter , netdev List-Id: nouveau.vger.kernel.org On Fri, Sep 25, 2020 at 12:15:37PM +0100, Robin Murphy wrote: > On 2020-09-15 16:51, Christoph Hellwig wrote: > [...] >> +These APIs allow to allocate pages in the kernel direct mapping that are >> +guaranteed to be DMA addressable. This means that unlike dma_alloc_coherent, >> +virt_to_page can be called on the resulting address, and the resulting > > Nit: if we explicitly describe this as if it's a guarantee that can be > relied upon... > >> +struct page can be used for everything a struct page is suitable for. > > [...] >> +This routine allocates a region of bytes of consistent memory. It >> +returns a pointer to the allocated region (in the processor's virtual address >> +space) or NULL if the allocation failed. The returned memory may or may not >> +be in the kernels direct mapping. Drivers must not call virt_to_page on >> +the returned memory region. > > ...then forbid this document's target audience from relying on it, > something seems off. At the very least it's unhelpfully unclear :/ > > Given patch #17, I suspect that the first paragraph is the one that's no > longer true. Yes. dma_alloc_pages is the replacement for allocations that need the direct mapping. I'll send a patch to document dma_alloc_pages and fixes this up