Re: [PATCH 1/6] drm/rockchip: import dma_buf to gem

From: Mark yao
Date: Thu Jun 18 2015 - 22:51:25 EST

Thanks Russell
On 2015å06æ18æ 18:57, Russell King - ARM Linux wrote:
This is wrong.

First, if you can only cope with a single scatterlist entry, you need to
enforce that. You can do that in your gem_prime_import_sg_table() method
by checking sgt->nents.
I'm confuse that how to get coherent iova address from nocoherent scatterlist with iommu.
I saw the arm_iommu_map_sg, it says that:
The scatter gather list elements are merged together (if possible) and
tagged with the appropriate dma address and length.

I guess the map_sg maybe merge scatterlist into coherent iova address, but I don't know how to
check the "if possible", and the sgt->nents not be 1 when already map to coherent iova address.

checking sgt-nents = 1 can sure that iova is coherent, but when coherent iova address from
nocoherent scatterlist, I think sgt->nents maybe greater than 1, is there a method can deal it?

All above is my guess, maybe wrong, I only tested it with CMA buffer import.

Secondly, you're mapping an already mapped scatterlist - scatterlists
are mapped by the exporter inside dma_buf_map_attachment() for your

Right, found the dma_map_sg on dma_buf_map_attachment().

Thirdly, I hate drm_gem_prime_import() being used on ARM... it forces
drivers to do something very buggy: the DMA buffer is mapped for DMA
when the buffer is imported. If the buffer is a write-combine or cached
buffer, writes to the buffer after the import will not become visible to
the display hardware until sometime later (when they're evicted from the
caches and/or pushed out of the bus structure.) The DMA mapping should
be performed as close to the start of DMA as possible. However, this is
a long-standing issue I have with dma_buf itself and is not something you
should be too concerned with in your patch. Just bear it in mind if you
start to see corruption of imported buffers - the answer is not more
dma_map_sg() calls, but to get dma_buf fixed.
Yeah, Got it.

ïark Yao

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