Hi Inki,
On 2017-11-14 00:46, Inki Dae wrote:
2017년 11월 13일 23:28에 Marek Szyprowski 이(가) 쓴 글:
On 2017-11-13 02:24, Inki Dae wrote:
2017년 11월 10일 16:35에 Marek Szyprowski 이(가) 쓴 글:
On 2017-11-10 04:04, Inki Dae wrote:
2017년 11월 01일 01:28에 Marek Szyprowski 이(가) 쓴 글:
When no IOMMU is available, all GEM buffers allocated by Exynos DRM driver
are contiguous, because of the underlying dma_alloc_attrs() function
provides only such buffers. In such case it makes no sense to keep
What if user disabled CMA support? In this case, it guarantees also to allocate physically contiguous memory?
I think it depends on CMA support so wouldn't be true.
dma_alloc_attrs() always guarantees the contiguity of the allocated memory
in DMA address space. When NOIOMMU is available, this mean that the allocated
buffer is contiguous in the physical memory. When CMA is disabled,
dma_alloc_attrs() uses alloc_pages() allocator. The drawback of alloc_pages()
is that it easily fails if physical memory is fragmented and it cannot
allocate memory larger than MAX_ORDER (4MiB in case of ARM32bit).
You are right. Without IOMMU suppot alloc_pages always tryies to allocate contiguous memory,
order = get_order(size);
page = alloc_pages(..., order);
if (!page)
return NULL;
...
Right
Real problem I think is that user don't know whether the gem buffer allocated with CONTIG or NONCONTIG flag can be used as a SCANOUT buffer.
So user can request a page flip with NONCONTIG buffer to kernel which doesn't support IOMMU.
And another is that user may want to use NONCONTIG buffer for another purpose, not scanout.
So if we enforce on using CONTIG buffer on kernel without IOMMU support, then it wouldn't be really what user intended.
When IOMMU support is disabled, ANY buffer allocated with dma_alloc_attrs()
will be contiguous, so I see no point propagating incorrect flag for it.
The only way to create a NONCONTIG buffer with IOMMU disabled is to import
it from other driver and this path is already handled correctly.
My idea is to provide a new flag - i.e., EXYNOS_BO_SCANOUT - which can allocate a buffer with a different allocation type - CONTIG or NONCONTIG - according to IOMMU support.
And any page flip request with NONCONTIG buffer to kernel without IOMMU support should fail and it has to return a error with a proper error message.
I don't think that we need it. With the proposed patch the same userspace will
work fine in both cases IOMMU enabled and disabled, even if it allocate GEM
with NONCONTIG flag. We can assume that CONTIG is a special case of NONCONTIG
then.
The problem is really not whether user space will work fine or not but is that this may be not what user space wanted.
I.e., user space expects the allocated buffer is NONCONTIG buffer because user space requested NONCONTIG buffer but kernel internally, this is changed to CONTIG buffer.
What's the problem when kernel allocated contiguous buffer but user
requested non-contiguous? Contiguous is a subclass of non-contiguous
in general. There is no driver nor scenario which will not work because
of such change (any code for handing n-segments should be fine with only
1 segment). In some conditions, by luck, kernel might allocate
a contiguous buffer even with IOMMU enabled. When we know that the
buffer is contiguous, then flag it as such.
I'd like to say what I experienced before here. I'd ever modified in-house kernel with similar issue that with IOMMU sometimes page fault happended. So tempararily I always made gem allocation request from user space to allocate CONTIG buffer and it worked well without page fault. Several days ago, a Platform guy reported one issue that gem allocation request failed even through it has free memory enough. The issus was as you guess fragementation issue and I talked to him memory fragmentation happended.
However, that guy didn't understand why memory fragementation happended because he definitely requested NONCONTIG buffer allocation. Thus, if I provided a some hit - gem allocation way is changed to CONTIG due to some reason - to user space then he could understand the fragmentation issue without contacting me. This patch could fix the X Server issue but the X Server never know that the allocated buffer is contiguous because you changed the allocation type witout user's knowledge.
Okay, I will post a v2 with a warning.
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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