Re: [PATCH 8/8] WIP: add a dma_alloc_contiguous API

[Tomasz Figa]

Hi Christoph,

On Wed, Sep 30, 2020 at 06:09:17PM +0200, Christoph Hellwig wrote:
> Add a new API that returns a virtually non-contigous array of pages
> and dma address.  This API is only implemented for dma-iommu and will
> not be implemented for non-iommu DMA API instances that have to allocate
> contiguous memory.  It is up to the caller to check if the API is
> available.

Would you mind scheding some more light on what made the previous attempt
not work well? I liked the previous API because it was more consistent with
the regular dma_alloc_coherent().

> 
> The intent is that media drivers can use this API if either:

FWIW, the USB subsystem also has similar needs, and so do some DRM drivers
using DMA API rather than IOMMU API directly. Basically I believe that all
the users removed in your previous series relied on custom downstream
patches to make DMA_ATTR_NON_CONSISTENT work and could be finally made work
in upstream using this API.

> 
>  - no kernel mapping or only temporary kernel mappings are required.
>    That is as a better replacement for DMA_ATTR_NO_KERNEL_MAPPING
>  - a kernel mapping is required for cached and DMA mapped pages, but
>    the driver also needs the pages to e.g. map them to userspace.
>    In that sense it is a replacement for some aspects of the recently
>    removed and never fully implemented DMA_ATTR_NON_CONSISTENT

What's the expected allocation and mapping flow with the latter? Would that be

pages = dma_alloc_noncoherent(...)
vaddr = vmap(pages, ...);

?

Would one just use the usual dma_sync_for_{cpu,device}() for cache
invallidate/clean, while keeping the mapping in place?

Best regards,
Tomasz