[RFC PATCH v4 0/5] ARM: Fix dma_alloc_coherent() and friends for NOMMU
Vladimir Murzin
vladimir.murzin at arm.com
Fri Jan 13 01:12:19 PST 2017
On 12/01/17 18:07, Robin Murphy wrote:
> On 12/01/17 17:15, Vladimir Murzin wrote:
>> On 12/01/17 17:04, Robin Murphy wrote:
>>> On 12/01/17 16:52, Vladimir Murzin wrote:
>>>> On 12/01/17 10:55, Benjamin Gaignard wrote:
>>>>> 2017-01-12 11:35 GMT+01:00 Benjamin Gaignard <benjamin.gaignard at linaro.org>:
>>>>>> 2017-01-11 15:34 GMT+01:00 Vladimir Murzin <vladimir.murzin at arm.com>:
>>>>>>> On 11/01/17 13:17, Benjamin Gaignard wrote:
>>>>>>>> 2017-01-10 15:18 GMT+01:00 Vladimir Murzin <vladimir.murzin at arm.com>:
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> It seem that addition of cache support for M-class cpus uncovered
>>>>>>>>> latent bug in DMA usage. NOMMU memory model has been treated as being
>>>>>>>>> always consistent; however, for R/M classes of cpu memory can be
>>>>>>>>> covered by MPU which in turn might configure RAM as Normal
>>>>>>>>> i.e. bufferable and cacheable. It breaks dma_alloc_coherent() and
>>>>>>>>> friends, since data can stuck in caches now or be buffered.
>>>>>>>>>
>>>>>>>>> This patch set is trying to address the issue by providing region of
>>>>>>>>> memory suitable for consistent DMA operations. It is supposed that
>>>>>>>>> such region is marked by MPU as non-cacheable. Robin suggested to
>>>>>>>>> advertise such memory as reserved shared-dma-pool, rather then using
>>>>>>>>> homebrew command line option, and extend dma-coherent to provide
>>>>>>>>> default DMA area in the similar way as it is done for CMA (PATCH
>>>>>>>>> 2/5). It allows us to offload all bookkeeping on generic coherent DMA
>>>>>>>>> framework, and it is seems that it might be reused by other
>>>>>>>>> architectures like c6x and blackfin.
>>>>>>>>>
>>>>>>>>> Dedicated DMA region is required for cases other than:
>>>>>>>>> - MMU/MPU is off
>>>>>>>>> - cpu is v7m w/o cache support
>>>>>>>>> - device is coherent
>>>>>>>>>
>>>>>>>>> In case one of the above conditions is true dma operations are forced
>>>>>>>>> to be coherent and wired with dma_noop_ops.
>>>>>>>>>
>>>>>>>>> To make life easier NOMMU dma operations are kept in separate
>>>>>>>>> compilation unit.
>>>>>>>>>
>>>>>>>>> Since the issue was reported in the same time as Benjamin sent his
>>>>>>>>> patch [1] to allow mmap for NOMMU, his case is also addressed in this
>>>>>>>>> series (PATCH 1/5 and PATCH 3/5).
>>>>>>>>>
>>>>>>>>> Thanks!
>>>>>>>>
>>>>>>>> I have tested this v4 on my setup (stm32f4, no cache, no MPU) and unfortunately
>>>>>>>> it doesn't work with my drm/kms driver.
>>>>>>>
>>>>>>> I guess the same is for fbmem, but would be better to have confirmation since
>>>>>>> amba-clcd I use has not been ported to drm/kms (yet), so I can't test.
>>>>>>>
>>>>>>>> I haven't any errors but nothing is displayed unlike what I have when
>>>>>>>> using current dma-mapping
>>>>>>>> code.
>>>>>>>> I guess the issue is coming from dma-noop where __get_free_pages() is
>>>>>>>> used instead of alloc_pages()
>>>>>>>> in dma-mapping.
>>>>>>>
>>>>>>> Unless I've missed something bellow is a call stack for both
>>>>>>>
>>>>>>> #1
>>>>>>> __alloc_simple_buffer
>>>>>>> __dma_alloc_buffer
>>>>>>> alloc_pages
>>>>>>> split_page
>>>>>>> __dma_clear_buffer
>>>>>>> memset
>>>>>>> page_address
>>>>>>>
>>>>>>> #2
>>>>>>> __get_free_pages
>>>>>>> alloc_pages
>>>>>>> page_address
>>>>>>>
>>>>>>> So the difference is that nommu case in dma-mapping.c memzeros memory, handles
>>>>>>> DMA_ATTR_NO_KERNEL_MAPPING and does optimisation of memory usage.
>>>>>>>
>>>>>>> Is something from above critical for your driver?
>>>>>>
>>>>>> I have removed all the diff (split_page, __dma_clear_buffer, memset)
>>>>>> from #1 and it is still working.
>>>>>> DMA_ATTR_NO_KERNEL_MAPPING flag is not set when allocating the buffer.
>>>>>>
>>>>>> I have investigated more and found that dma-noop doesn't take care of
>>>>>> "dma-ranges" property which is set in DT.
>>>>>> I believed that is the root cause of my problem with your patches.
>>>>>
>>>>> After testing changing virt_to_phys to virt_to_dma in dma-noop.c fix the issue
>>>>> modetest and fbdemo are now still functional.
>>>>>
>>>>
>>>> Thanks for narrowing it down! I did not noticed that stm32f4 remap its memory,
>>>> so dma-ranges property is in use.
>>>>
>>>> It looks like virt_to_dma is ARM specific, so I probably have to discard idea
>>>> of reusing dma-noop-ops and switch logic into dma-mapping-nommu.c based on
>>>> is_device_dma_coherent(dev) check.
>>>
>>> dma_pfn_offset is a member of struct device, so it should be OK for
>>> dma_noop_ops to also make reference to it (and assume it's zero if not
>>> explicitly set).
>>>
>>>> Meanwhile, I'm quite puzzled on how such memory remaping should work together
>>>> with reserved memory. It seem it doesn't account dma-ranges while reserving
>>>> memory (it is too early) nor while allocating/mapping/etc.
>>>
>>> The reserved memory is described in terms of CPU physical addresses, so
>>> a device offset shouldn't matter from that perspective. It only comes
>>> into play at the point you generate the dma_addr_t to hand off to the
>>> device - only then do you need to transform the CPU physical address of
>>> the allocated/mapped page into the device's view of that page (i.e.
>>> subtract the offset).
>>
>> Thanks for explanation! So dma-coherent.c should be modified, right? I see
>> that some architectures provide phys_to_dma/dma_to_phys helpers primary for
>> swiotlb, is it safe to reuse them given that default implementation is
>> provided? Nothing under Documentation explains how they supposed to be used,
>> sorry if asking stupid question.
>
> Those are essentially SWIOTLB-specific, so can't be universally relied
> upon. I think something like this ought to suffice:
Yup, but what about dma-coherent.c? Currently it has
int dma_alloc_from_coherent(struct device *dev, ssize_t size,
dma_addr_t *dma_handle, void **ret)
{
...
*dma_handle = mem->device_base + (pageno << PAGE_SHIFT);
*ret = mem->virt_base + (pageno << PAGE_SHIFT);
...
}
In case reserved memory is described in terms of CPU phys addresses, would not
we need to take into account dma_pfn_offset? What I'm missing?
Thanks
Vladimir
>
> ---8<---
> diff --git a/lib/dma-noop.c b/lib/dma-noop.c
> index 3d766e78fbe2..fbb1b37750d5 100644
> --- a/lib/dma-noop.c
> +++ b/lib/dma-noop.c
> @@ -8,6 +8,11 @@
> #include <linux/dma-mapping.h>
> #include <linux/scatterlist.h>
>
> +static dma_addr_t dma_noop_dev_offset(struct device *dev)
> +{
> + return (dma_addr_t)dev->dma_pfn_offset << PAGE_SHIFT;
> +}
> +
> static void *dma_noop_alloc(struct device *dev, size_t size,
> dma_addr_t *dma_handle, gfp_t gfp,
> unsigned long attrs)
> @@ -16,7 +21,7 @@ static void *dma_noop_alloc(struct device *dev, size_t
> size,
>
> ret = (void *)__get_free_pages(gfp, get_order(size));
> if (ret)
> - *dma_handle = virt_to_phys(ret);
> + *dma_handle = virt_to_phys(ret) - dma_noop_dev_offset(dev);
> return ret;
> }
>
> @@ -32,7 +37,7 @@ static dma_addr_t dma_noop_map_page(struct device
> *dev, struct page *page,
> enum dma_data_direction dir,
> unsigned long attrs)
> {
> - return page_to_phys(page) + offset;
> + return page_to_phys(page) + offset - dma_noop_dev_offset(dev);
> }
>
> static int dma_noop_map_sg(struct device *dev, struct scatterlist *sgl,
> int nents,
> @@ -47,7 +52,8 @@ static int dma_noop_map_sg(struct device *dev, struct
> scatterlist *sgl, int nent
>
> BUG_ON(!sg_page(sg));
> va = sg_virt(sg);
> - sg_dma_address(sg) = (dma_addr_t)virt_to_phys(va);
> + sg_dma_address(sg) = (dma_addr_t)virt_to_phys(va) -
> + dma_noop_dev_offset(dev);
> sg_dma_len(sg) = sg->length;
> }
> --->8---
>
> intentionally whitespace-damaged by copy-pasting off my terminal to
> emphasise how utterly untested it is ;)
>
> Robin.
>
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