On 25/04/17 07:26 PM, Ville Syrjälä wrote: > On Tue, Apr 25, 2017 at 10:12:37AM +0900, Michel Dänzer wrote: >> On 24/04/17 10:03 PM, Ville Syrjälä wrote: >>> On Mon, Apr 24, 2017 at 03:57:02PM +0900, Michel Dänzer wrote: >>>> On 22/04/17 07:05 PM, Ville Syrjälä wrote: >>>>> On Fri, Apr 21, 2017 at 06:14:31PM +0200, Gerd Hoffmann wrote: >>>>>> Hi, >>>>>> >>>>>>>> My personal opinion is that formats in drm_fourcc.h should be >>>>>>>> independent of the CPU byte order and the function >>>>>>>> drm_mode_legacy_fb_format() and drivers depending on that incorrect >>>>>>>> assumption be fixed instead. >>>>>>> >>>>>>> The problem is this isn't a kernel-internal thing any more. With the >>>>>>> addition of the ADDFB2 ioctl the fourcc codes became part of the >>>>>>> kernel/userspace abi ... >>>>>> >>>>>> Ok, added some printk's to the ADDFB and ADDFB2 code paths and tested a >>>>>> bit. Apparently pretty much all userspace still uses the ADDFB ioctl. >>>>>> xorg (modesetting driver) does. gnome-shell in wayland mode does. >>>>>> Seems the big transition to ADDFB2 didn't happen yet. >>>>>> >>>>>> I guess that makes changing drm_mode_legacy_fb_format + drivers a >>>>>> reasonable option ... >>>>> >>>>> Yeah, I came to the same conclusion after chatting with some >>>>> folks on irc. >>>>> >>>>> So my current idea is that we change any driver that wants to follow the >>>>> CPU endianness >>>> >>>> This isn't really optional for various reasons, some of which have been >>>> covered in this discussion. >>>> >>>> >>>>> to declare support for big endian formats if the CPU is >>>>> big endian. Presumably these are mostly the virtual GPU drivers. >>>>> >>>>> Additonally we'll make the mapping performed by drm_mode_legacy_fb_format() >>>>> driver controlled. That way drivers that got changed to follow CPU >>>>> endianness can return a framebuffer that matches CPU endianness. And >>>>> drivers that expect the GPU endianness to not depend on the CPU >>>>> endianness will keep working as they do now. The downside is that users >>>>> of the legacy addfb ioctl will need to magically know which endianness >>>>> they will get, but that is apparently already the case. And users of >>>>> addfb2 will keep on specifying the endianness explicitly with >>>>> DRM_FORMAT_BIG_ENDIAN vs. 0. >>>> >>>> I'm afraid it's not that simple. >>>> >>>> The display hardware of older (pre-R600 generation) Radeon GPUs does not >>>> support the "big endian" formats directly. In order to allow userspace >>>> to access pixel data in native endianness with the CPU, we instead use >>>> byte-swapping functionality which only affects CPU access. >>> >>> OK, I'm getting confused. Based on our irc discussion I got the >>> impression you don't byte swap CPU accesses. >> >> Sorry for the confusion. The radeon kernel driver does support >> byte-swapping for CPU access to VRAM with pre-R600 GPUs, and this is >> used for fbdev emulation. What I meant on IRC is that the xf86-video-ati >> radeon driver doesn't make use of this, mostly because it only applies >> while a BO is in VRAM, and userspace can't control when that's the case >> (while a BO isn't being scanned out). > > So that was my other question. So if someone just creates a bo, I presume > ttm can more or less move it between system memory and vram at any > time. So if we then mmap the bo, does it mean the CPU will see the bytes > in different order depending on where the bo happens to live at > the time the CPU access happens? If either of the RADEON_TILING_SWAP_16/32BIT flags was set when the BO was created, yes. That's why the xf86-video-ati radeon driver doesn't use this functionality. > And how would that work wih dumb bos? radeon_mode_dumb_create doesn't set the RADEON_TILING_SWAP_16/32BIT flags, so no byte swapping is performed for dumb BOs even in VRAM. -- Earthling Michel Dänzer | http://www.amd.com Libre software enthusiast | Mesa and X developer
