[RFC PATCH v4 0/5] ARM: Fix dma_alloc_coherent() and friends for NOMMU
Vladimir Murzin
vladimir.murzin at arm.com
Mon Jan 16 03:58:04 PST 2017
On 13/01/17 12:40, Robin Murphy wrote:
> On 13/01/17 09:12, Vladimir Murzin wrote:
>> 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?
>
> Ah yes, I overlooked that one. AFAICS, that's intended to be accounted
> for when calling dma_init_coherent_memory (i.e. phys_addr vs.
> device_addr), but that's a bit awkward for a global pool.
>
> How utterly disgusting do you think this (or some variant thereof) looks?
>
> /* Apply device-specific offset for the global pool */
> if (mem == dma_coherent_default_memory)
> *handle += dev->dma_pfn_offset << PAGE_SHIFT;
It'd work for default dma region, but IMO the issue is wider here... does
following look better?
diff --git a/drivers/base/dma-coherent.c b/drivers/base/dma-coherent.c
index b52ba27..22daa4c 100644
--- a/drivers/base/dma-coherent.c
+++ b/drivers/base/dma-coherent.c
@@ -27,6 +27,15 @@ static inline struct dma_coherent_mem *dev_get_coherent_memory(struct device *de
return dma_coherent_default_memory;
}
+static inline dma_addr_t dma_get_device_base(struct device *dev,
+ struct dma_coherent_mem * mem)
+{
+ if (!dev)
+ return mem->pfn_base << PAGE_SHIFT;
+
+ return (mem->pfn_base + dev->dma_pfn_offset) << PAGE_SHIFT;
+}
+
static bool dma_init_coherent_memory(
phys_addr_t phys_addr, dma_addr_t device_addr, size_t size, int flags,
struct dma_coherent_mem **mem)
@@ -92,13 +101,19 @@ static void dma_release_coherent_memory(struct dma_coherent_mem *mem)
static int dma_assign_coherent_memory(struct device *dev,
struct dma_coherent_mem *mem)
{
+ unsigned long dma_pfn_offset = mem->pfn_base - PFN_DOWN(mem->device_base);
+
if (!dev)
return -ENODEV;
if (dev->dma_mem)
return -EBUSY;
+ if (dev->dma_pfn_offset)
+ WARN_ON(dev->dma_pfn_offset != dma_pfn_offset);
+
dev->dma_mem = mem;
+ dev->dma_pfn_offset = dma_pfn_offset;
/* FIXME: this routine just ignores DMA_MEMORY_INCLUDES_CHILDREN */
return 0;
@@ -145,7 +160,7 @@ void *dma_mark_declared_memory_occupied(struct device *dev,
return ERR_PTR(-EINVAL);
spin_lock_irqsave(&mem->spinlock, flags);
- pos = (device_addr - mem->device_base) >> PAGE_SHIFT;
+ pos = PFN_DOWN(device_addr - dma_get_device_base(dev, mem));
err = bitmap_allocate_region(mem->bitmap, pos, get_order(size));
spin_unlock_irqrestore(&mem->spinlock, flags);
@@ -195,8 +210,9 @@ int dma_alloc_from_coherent(struct device *dev, ssize_t size,
/*
* Memory was found in the per-device area.
*/
- *dma_handle = mem->device_base + (pageno << PAGE_SHIFT);
+ *dma_handle = dma_get_device_base(dev, mem) + (pageno << PAGE_SHIFT);
*ret = mem->virt_base + (pageno << PAGE_SHIFT);
+
dma_memory_map = (mem->flags & DMA_MEMORY_MAP);
spin_unlock_irqrestore(&mem->spinlock, flags);
if (dma_memory_map)
Cheers
Vladimir
>
> Robin.
>
>> 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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