[PATCH V6] drm/xe/mmap: Add mmap support for PCI memory barrier

[Mrozek, Michal]

> In order to avoid having userspace to use MI_MEM_FENCE, we are adding 
> a mechanism for userspace to generate a PCI memory barrier with low 
> overhead (avoiding IOCTL call as well as writing to VRAM will adds 
> some overhead).
> 
> This is implemented by memory-mapping a page as uncached that is 
> backed by MMIO on the dGPU and thus allowing userspace to do memory 
> write to the page without invoking an IOCTL.
> We are selecting the MMIO so that it is not accessible from the PCI 
> bus so that the MMIO writes themselves are ignored, but the PCI memory 
> barrier will still take action as the MMIO filtering will happen after the memory barrier effect.
> 
> When we detect special defined offset in mmap(), We are mapping 4K 
> page which contains the last of page of doorbell MMIO range to 
> userspace for same purpose.
> 
> For user to query special offset we are adding special flag in 
> mmap_offset ioctl which needs to be passed as follows, struct 
> drm_xe_gem_mmap_offset mmo = {
>         .handle = 0, /* this must be 0 */
>         .flags = DRM_XE_MMAP_OFFSET_FLAG_PCI_BARRIER,
> };
> igt_ioctl(fd, DRM_IOCTL_XE_GEM_MMAP_OFFSET, &mmo); map = mmap(NULL, 
> size, PROT_WRITE, MAP_SHARED, fd, mmo);
> 
> Note: Test coverage for this is added by IGT
>       https://patchwork.freedesktop.org/series/140368/  here.
>       UMD implementing test, once PR is ready will attach with
>       this patch.
> 
> V6(MAuld)
>   - Move physical mmap to fault handler
>   - Modify kernel-doc and attach UMD PR when ready
> V5(MAuld)
>   - Return invalid early in case of non 4K PAGE_SIZE
>   - Format kernel-doc and add note for 4K PAGE_SIZE HW limit
> V4(MAuld)
>   - Add kernel-doc for uapi change
>   - Restrict page size to 4K
> V3(MAuld)
>   - Remove offset defination from UAPI to be able to change later
>   - Edit commit message for special flag addition
> V2(MAuld)
>   - Add fault handler with dummy page to handle unplug device
>   - Add Build check for special offset to be below normal start page
>   - Test d3hot, mapping seems to be valid in d3hot as well
>   - Add more info to commit message
> 
> Cc: Matthew Auld <matthew.auld at intel.com>
> Cc: Michal Mrozek <michal.mrozek at intel.com>
> Signed-off-by: Tejas Upadhyay <tejas.upadhyay at intel.com>

Acked-by: Michal Mrozek <michal.mrozek at intel.com>