On Thu, Feb 02, 2017 at 05:19:58PM +0100, Volker Vogelhuber wrote:
>> 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.
Thanks for the reply. I guess you mean that line of code:
http://code.metager.de/source/xref/freedesktop/mesa/drm/intel/intel_bufmgr_gem.c#2679
Is there a good reason not to patch drm_intel_bo_gem_export_to_prime to call
drmPrimeHandleToFD with DRM_RDWR flag set as well? I just saw a similar discussion
under https://chromium-review.googlesource.com/c/323990/
Regards,
Volker
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