On Mon, Oct 5, 2020 at 10:26 AM Christoph Hellwig [off-list ref] wrote:

On Fri, Oct 02, 2020 at 05:50:40PM +0000, Tomasz Figa wrote:

quoted

Hi Christoph,

On Wed, Sep 30, 2020 at 06:09:17PM +0200, Christoph Hellwig wrote:

quoted

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.

quoted

quoted

 - 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.

quoted

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