Re: [PATCH v2 2/4] usb: usbfs: Use consistent mmap functions

[Ruihan Li <lrh2000@pku.edu.cn>]

On Mon, May 15, 2023 at 04:07:01PM +0000, David Laight wrote:
> 
> From: Ruihan Li
> > Sent: 15 May 2023 14:10
> > 
> > When hcd->localmem_pool is non-null, localmem_pool is used to allocate
> > DMA memory. In this case, the dma address will be properly returned (in
> > dma_handle), and dma_mmap_coherent should be used to map this memory
> > into the user space. However, the current implementation uses
> > pfn_remap_range, which is supposed to map normal pages.
> 
> I've an (out of tree) driver that does the same.
> Am I right in thinking that this does still work?

Generally, it still works most of the time, but it can break sometimes.
I am going to quote commit 2bef9aed6f0e ("usb: usbfs: correct
kernel->user page attribute mismatch"), which introduces
dma_mmap_coherent in usbdev_mmap, and says [1]:

	On some architectures (e.g. arm64) requests for
	IO coherent memory may use non-cachable attributes if
	the relevant device isn't cache coherent. If these
	pages are then remapped into userspace as cacheable,
	they may not be coherent with the non-cacheable mappings.

 [1] https://lore.kernel.org/all/20200504201348.1183246-1-jeremy.linton@arm.com/

I think it means that if your driver deals with devices that aren't
cache-coherent on arm64, using pfn_remap_range directly may cause
problems. Otherwise, you may need to check the arch-specific dma mmap
operation and see if it performs additional things that pfn_remap_range
does not (for the arm example, arm_iommu_mmap_attrs updates the
vm_page_prot field to make the pages non-cacheable if the device is not
cache-coherent [2]).

 [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/mm/dma-mapping.c?id=f1fcbaa18b28dec10281551dfe6ed3a3ed80e3d6#n1129

> 
> I can't change the driver to use dma_map_coherent() because it
> doesn't let me mmap from a page offset within a 16k allocation.
> 
> In this case the memory area is an 8MB shared transfer area to an
> FPGA PCIe target sparsely filled with 16kB allocation (max 512 allocs).
> The discontinuous physical memory blocks appear as logically
> contiguous to both the FPGA logic and when mapped to userspace.
> (But not to driver code.)
> 
> I don't really want to expose the 16k allocation size to userspace.
> If we need more than 8MB then the allocation size would need
> changing.
> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)

Thanks,
Ruihan Li