Re: [PATCH] Revert "dma-contiguous: do not allocate a single page from CMA area"

[Nicolin Chen <nicoleotsuka@gmail.com>]

On Tue, Feb 26, 2019 at 11:35:44PM +0000, Robin Murphy wrote:
> On 2019-02-26 8:23 pm, Nicolin Chen wrote:
> > This reverts commit d222e42e88168fd67e6d131984b86477af1fc256.
> > 
> > The original change breaks omap dss:
> >      omapdss_dispc 58001000.dispc:
> >          dispc_errata_i734_wa_init: dma_alloc_writecombine failed
> > 
> > Let's revert it first and then find a safer solution instead.
> 
> Ah, I think I see the problem - once arch/arm's __dma_alloc() has decided to
> use CMA (because dev_get_cma_area(dev) returns the global area), it then
> won't fall back to trying a regular page allocation if
> dma_alloc_from_contiguous() returns NULL. Thus anything on 32-bit Arm trying
> to allocate a single-page buffer in blockable context with a CMA-enabled
> config is just going to fail. Similarly, it looks like none of the
> DMA_ATTR_FORCE_CONTIGUOUS cases are prepared to handle this change either
> (amd_iommu appears technically affected, but is already using
> dma_alloc_from_contiguous() backwards compared to everyone else, hmm).

Yea, I searched the tree and got the same results.

> I guess the question is whether to add alloc_page()/free_page() fallbacks to
> those call sites, or stuff them directly into the CMA helpers here.

Probably would be safer/easier to do the later one I feel.