mmap with PROT_WRITE of dma-buf FD coming from eglExportDMABUFImageMESA

[Daniel Vetter]

On Thu, Feb 02, 2017 at 05:19:58PM +0100, Volker Vogelhuber wrote:
> Hi,
> 
> I'm currently trying to mmap the memory of an OpenGL texture I've created by
> doing the following:
> 
> std::vector<EGLint> image_attribs = {
>         EGL_WIDTH,              static_cast<int>(m_texWidth & 0x7FFFFFFF),
>         EGL_HEIGHT,             static_cast<int>(m_texHeight & 0x7FFFFFFF),
>         EGL_DRM_BUFFER_USE_MESA, EGL_DRM_BUFFER_USE_SCANOUT_MESA,
>         EGL_DRM_BUFFER_FORMAT_MESA, EGL_DRM_BUFFER_FORMAT_ARGB32_MESA
>         EGL_NONE
>     };
> glGenTextures( 1, &texID );
> glBindTexture( GL_TEXTURE_2D, texID );
> eglImage = eglCreateDRMImageMESA( eglGetCurrentDisplay(), &image_attribs[0]
> );
> glEGLImageTargetTexture2DOES(GL_TEXTURE_2D, eglmage);
> eglExportDMABUFImageQueryMESA(eglGetCurrentDisplay(), eglImage, &fourcc,
> &nplanes, &modifiers)
> eglExportDMABUFImageMESA(eglGetCurrentDisplay(), eglImage, &fd, strides,
> offsets)
> 
> struct dma_buf_sync sync_args = { .flags = DMA_BUF_SYNC_START };
> ioctl( fd, DMA_BUF_IOCTL_SYNC, sync_args);
> mmap( NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0 );
> 
> For some reason mmap returns MAP_FAILED when using the PROT_WRITE flag,
> while
> using only PROT_READ seems to be fine. I'm currently using kernel 4.9.6 on
> an Intel CherryTrail (i915).
> 
> Is there an obvious reason why I'm not able to mmap the texture FD with
> PROT_WRITE flags?
> Reading https://01.org/blogs/2016/sharing-cpu-and-gpu-buffers-linux I would
> have guessed that
> this should be possible. Or is it only true for specific GEM buffers not
> textures created using
> eglCreateDRMImageMESA?

>From a quick look the mesa/libdrm (depending upon driver) handle2fd
implementations don't set the DRM_RDWR flag when asking PRIME for a
dma-buf fd, which means you fd doesn't have write permissions. Which means
mmap with writing allowed will fail.

Not sure how to fix that.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch