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

[Tomasz Figa]

On Mon, Oct 5, 2020 at 10:26 AM Christoph Hellwig <hch@xxxxxx> wrote:
>
> 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.

Okay, thanks. Let's see if we can make necessary changes to the videobuf2.

+Sergey Senozhatsky for awareness too.

Best regrards,
Tomasz