mmap with PROT_WRITE of dma-buf FD coming from eglExportDMABUFImageMESA

[Volker Vogelhuber <v.vogelhuber@xxxxxxxxxxxxxxxxxxx>]

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?

Regards,
    Volker

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