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

[Christoph Hellwig]

On Fri, Oct 02, 2020 at 05:50:40PM +0000, Tomasz Figa wrote:
> 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 problem is that with a dma_alloc_noncoherent that can return pages
not in the kernel mapping we can't just use virt_to_page to fill in
scatterlists or mmap the buffer to userspace, but would need new helpers
and another two methods.

> >  - 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, ...);
> 
> ?

Yes.  Witht the vmap step optional for replacements of
DMA_ATTR_NO_KERNEL_MAPPING, which is another nightmare to deal with.

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

Yes.  And make sure the API isn't implemented when VIVT caches are
used, but that isn't really different from the current interface.