Re: [PATCH 1/2] drm: add cache support for arm64

[Mark Rutland]

On Thu, Aug 08, 2019 at 09:58:27AM +0200, Christoph Hellwig wrote:
> On Wed, Aug 07, 2019 at 05:49:59PM +0100, Mark Rutland wrote:
> > For arm64, we can tear down portions of the linear map, but that has to
> > be done explicitly, and this is only possible when using rodata_full. If
> > not using rodata_full, it is not possible to dynamically tear down the
> > cacheable alias.
> 
> Interesting.  For this or next merge window I plan to add support to the
> generic DMA code to remap pages as uncachable in place based on the
> openrisc code.  AÑ far as I can tell the requirement for that is
> basically just that the kernel direct mapping doesn't use PMD or bigger
> mapping so that it supports changing protection bits on a per-PTE basis.
> Is that the case with arm64 + rodata_full?

Yes, with the added case that on arm64 we can also have contiguous
entries at the PTE level, which we also have to disable.

Our kernel page table creation code does that for rodata_full or
DEBUG_PAGEALLOC. See arch/arm64/mmu.c, in map_mem(), where we pass
NO_{BLOCK,CONT}_MAPPINGS down to our pagetable creation code.

Thanks,
Mark.