On Fri, May 17, 2013 at 01:33:45PM +0200, Jean-Francois Moine wrote:
> I quickly compared your dove drm driver and ours (Sebastian and me):
>
> - CMA helper
>
> You don't use DRM_KMS_CMA_HELPER and DRM_GEM_CMA_HELPER which would
> simplify some code.
Looking at the CMA helper code in DRM, it makes some fundamental errors:
1. It assumes the returned DMA address is a physical address. This
is not necessarily the case (there are a number of ARM platforms
where this is most definitely not true.) So:
cma_obj->vaddr = dma_alloc_writecombine(drm->dev, size,
&cma_obj->paddr, GFP_KERNEL | __GFP_NOWARN);
...
ret = remap_pfn_range(vma, vma->vm_start, cma_obj->paddr >> PAGE_SHIFT,
vma->vm_end - vma->vm_start, vma->vm_page_prot);
is extremely broken.
2. If you use the CMA helper, then all your GEM objects must be CMA
objects. You can't combine different types of objects (eg, SHM-backed
objects) with CMA objects. This will be a massive performance
regression with GPUs which can handle scatter-gathered SHM objects
such as the Vivante GPU on the Armada 510.
So, for me, the last point especially is a very strong argument not to
use the DRM CMA helper. Yes, I could switch to using the DMA coherent/
writecombine allocation API and thus make use of CMA, and that's
something I will be looking at.
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