From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Date: Tue, 30 Jul 2019 06:26:51 +0000 Subject: Re: [PATCH 4/5] dma-mapping: provide a better default ->get_required_mask Message-Id: <20190730062651.GA29518@lst.de> List-Id: References: <20190725063401.29904-1-hch@lst.de> <20190725063401.29904-5-hch@lst.de> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Geert Uytterhoeven Cc: linux-xtensa@linux-xtensa.org, Michal Simek , Parisc List , Linux-sh list , Takashi Iwai , linuxppc-dev , the arch/x86 maintainers , linux-m68k , Linux Kernel Mailing List , Linux IOMMU , Robin Murphy , Christoph Hellwig , Linux ARM , Marek Szyprowski On Mon, Jul 29, 2019 at 11:57:19AM +0200, Geert Uytterhoeven wrote: > Hi Christoph, > > On Thu, Jul 25, 2019 at 8:35 AM Christoph Hellwig wrote: > > Most dma_map_ops instances are IOMMUs that work perfectly fine in 32-bits > > of IOVA space, and the generic direct mapping code already provides its > > own routines that is intelligent based on the amount of memory actually > > present. Wire up the dma-direct routine for the ARM direct mapping code > > as well, and otherwise default to the constant 32-bit mask. This way > > we only need to override it for the occasional odd IOMMU that requires > > 64-bit IOVA support, or IOMMU drivers that are more efficient if they > > can fall back to the direct mapping. > > As I know you like diving into cans of worms ;-) > > Does 64-bit IOVA support actually work in general? Or only on 64-bit > platforms, due to dma_addr_t to unsigned long truncation on 32-bit? Most IOMMUs use 32-bit IOVAs, and thus we default to the 32-bit mask because it is common and failsafe vs the normal linux assumptions. However the ia64 SGI SN2 platform, and the powerpc IBM ebus implementations seem to require a 64-bit mask already, so we keep that behavior as is.