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).

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.


Reported-by: Tony Lindgren <t...@atomide.com>
Signed-off-by: Nicolin Chen <nicoleots...@gmail.com>
Would you please test and verify? Thanks!

  kernel/dma/contiguous.c | 22 +++-------------------
  1 file changed, 3 insertions(+), 19 deletions(-)

diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
index 09074bd04793..b2a87905846d 100644
--- a/kernel/dma/contiguous.c
+++ b/kernel/dma/contiguous.c
@@ -186,32 +186,16 @@ int __init dma_contiguous_reserve_area(phys_addr_t size, 
phys_addr_t base,
   * This function allocates memory buffer for specified device. It uses
   * device specific contiguous memory area if available or the default
- * global one.
- *
- * However, it skips one-page size of allocations from the global area.
- * As the addresses within one page are always contiguous, so there is
- * no need to waste CMA pages for that kind; it also helps reduce the
- * fragmentations in the CMA area. So a caller should be the rebounder
- * in such case to allocate a normal page upon NULL return value.
- *
- * Requires architecture specific dev_get_cma_area() helper function.
+ * global one. Requires architecture specific dev_get_cma_area() helper
+ * function.
  struct page *dma_alloc_from_contiguous(struct device *dev, size_t count,
                                       unsigned int align, bool no_warn)
-       struct cma *cma;
        if (align > CONFIG_CMA_ALIGNMENT)
                align = CONFIG_CMA_ALIGNMENT;
- if (dev && dev->cma_area)
-               cma = dev->cma_area;
-       else if (count > 1)
-               cma = dma_contiguous_default_area;
-       else
-               return NULL;
-       return cma_alloc(cma, count, align, no_warn);
+       return cma_alloc(dev_get_cma_area(dev), count, align, no_warn);

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