about mmap dma-buf and sync

Michael Spang spang at chromium.org
Mon Aug 24 10:56:14 PDT 2015

On Mon, Aug 24, 2015 at 1:42 PM, Thomas Hellstrom <thellstrom at vmware.com> wrote:
> On 08/24/2015 07:12 PM, Daniel Stone wrote:
>> Hi,
>> On 24 August 2015 at 18:10, Thomas Hellstrom <thellstrom at vmware.com> wrote:
>>> On 08/24/2015 07:04 PM, Daniel Stone wrote:
>>>> On 24 August 2015 at 17:56, Thomas Hellstrom <thellstrom at vmware.com> wrote:
>>>>> On 08/24/2015 05:52 PM, Daniel Stone wrote:
>>>>>> I still don't think this ameliorates the need for batching: consider
>>>>>> the case where you update two disjoint screen regions and want them
>>>>>> both flushed. Either you issue two separate sync calls (which can be
>>>>>> disadvantageous performance-wise on some hardware / setups), or you
>>>>>> accumulate the regions and only flush later. So either two ioctls (one
>>>>>> in the style of dirtyfb and one to perform the sync/flush; you can
>>>>>> shortcut to assume the full buffer was damaged if the former is
>>>>>> missing), or one like this:
>>>>>> struct dma_buf_sync_2d {
>>>>>>         enum dma_buf_sync_flags flags;
>>>>>>         __u64 stride_bytes;
>>>>>>         __u32 bytes_per_pixel;
>>>>>>         __u32 num_regions;
>>>>>>         struct {
>>>>>>                 __u64 x;
>>>>>>                 __u64 y;
>>>>>>                 __u64 width;
>>>>>>                 __u64 height;
>>>>>>         } regions[];
>>>>>> };
>>>>> Fine with me, although perhaps bytes_per_pixel is a bit redundant?
>>>> Redundant how? It's not implicit in stride.
>>> For flushing purposes, isn't it possible to cover all cases by assuming
>>> bytes_per_pixel=1? Not that it matters much.
>> Sure, though in that case best to replace x with line_byte_offset or
>> something, because otherwise I guarantee you everyone will get it
>> wrong, and it'll be a pain to track down. Like how I managed to
>> misread it now. :)
> OK, yeah you have a point. IMO let's go for your proposal.
> Tiago, is this OK with you?

Is there any obstacle to making the above API a new syscall?

The reason we have issues with ioctls is because it's not possible to
whitelist them properly with seccomp BPF - there's no single namespace
for ioctls.

If this API is merged as a ioctl, we may not be able to actually use
it. Which is a bit unfortunate when it has been proposed with the
chrome renderer use case in mind.


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