Re: Memory sharing issue by application on V4L2 based device driver

thank you for your comments.

your second comment has no any problem as I said before, user virtual
addess could be translated in page unit. but the problem, as you said,
is that when cpu access to the memory in user mode, the memory
allocated by malloc, page fault occurs so we can't find pfn to user
virtual address. I missed that. but I think we could resolve this one.

as before, user application allocates memory through malloc function
and then send it to device driver(using userptr feature). if the pfn
is null when device driver translated user virtual address in page
unit then it allocates phsical memory in page unit using some
interface such as alloc_page() and then mapping them. when pfn is
null, to check it and allocate physical memory in page unit could be
processed by videobuf2.

of course, videobuf2 has no any duty considered for system mmu. so
videobuf2 just provides callback for 3rd party and any platform with
system mmu such as Samsung SoC C210 implements the function(allocating
physical memory and mapping it) and registers it to callback of
videobuf2. by doing so, I think your first comment could be cleared.

please, feel free to give me your opinion and pointing out.

thank you.

2011년 1월 7일 오전 8:57, Jonghun Han [off-list ref]님의 말:

Hello,

There are two reasons why malloc isn't suitable for it.

The first is that malloc doesn't allocate memory when malloc is called.
So driver or vb2 cannot find PFN for it in the VIDIOC_QBUF.

The second is that malloc uses 4KB page allocation.
SYS.MMU(IO-MMU) can handle scattered memory. But it has a penalty when TLB
miss is occurred.
So as possible as physically contiguous pages are needed for performance
enhancement.

So new allocator which can clear two main issues is needed.

Best regards,

quoted

-----Original Message-----
From: linux-media-owner@vger.kernel.org [mailto:linux-media-
owner@vger.kernel.org] On Behalf Of InKi Dae
Sent: Thursday, January 06, 2011 10:25 PM
To: linux-media@vger.kernel.org
Subject: Memory sharing issue by application on V4L2 based device driver

with

quoted

system mmu.

Hello, all.

I'd like to discuss memory sharing issue by application on v4l2 based

device driver

quoted

with system mmu and get some advices about that.

Now I am working on Samsung SoC C210 platform and this platform has some
multimedia devices with system mmu such as fimc, and mfc also we have
implemented device drivers for them. those drivers are based on V4L2

framework

quoted

with videobuf2. for system mmu of each device, we used VCM(Virtual

Contiguous

quoted

Memory) framework.

Simply, VCM framework provides  physical memory, device virtual memory
allocation and memory mapping between them. when device driver is

initialized or

quoted

operated by user application, each driver allocates physical memory and

device

quoted

virtual memory and then mapping using VCM interface.

refer to below link for more detail.
http://www.spinics.net/lists/linux-media/msg26548.html

Physical memory access process is as the following.
           DVA                          PA
device --------------> system mmu ------------------> physical memory

DVA : device virtual address.
PA : physical address.

like this, device virtual address should be set to buffer(source or
destination) register of multimedia device.

the problem is that application want to share own memory with any device

driver to

quoted

avoid memory copy. in other words, user-allocated memory could be source

or

quoted

destination memory of multimedia device driver.


let's see the diagram below.

               user application

                     |
                     |
                     |
                     |
                     |  1. UVA(allocated by malloc)
                     |
                     |
                   ＼|/                   2. UVA(in page unit)

       -----> multimedia device driver -------------------> videobuf2
       |
       |        |     ^                                         |
       |        |     |                                         |
       |        |     -------------------------------------------
       |        |                    3. PA(in page unit)
       |        |
       |        | 4. PA(in page unit)
6. DVA  |        |
       |        |
       |        |
       |      ＼|/
       |
       |       Virtual Contiguous Memory ---------
       |                                         |
       |           |     ^                       |
       |           |     |                       | 5. map PA to DVA
       |           |     |                       |
       |           |     |                       |
       -------------     -------------------------

PA : physical address.
UVA : user virtual address.
DVA : device virtual address.

1. user application allocates user space memory through malloc function

and

quoted

sending it to multimedia device driver based on v4l2 framework through

userptr

quoted

feature.

2, 3. multimedia device driver gets translated physical address from
videobuf2 framework in page unit.

4, 5. multimedia device driver gets allocated device virtual address and

mapping it

quoted

to physical address and then mapping them through VCM interface.

6. multimedia device driver sets device virtual address from VCM to

buffer register.

quoted

the diagram above is fully theoretical so I wonder that this way is

reasonable and

quoted

has some problems also what should be considered.

thank you for your interesting.

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