[gentoo-dev] linux/dma-buf.h mysteriously missing

I was packaging chromium-59.0.3053.3 . I worked around the problem
described here, but I'd like to find the right long term solution.

I was hitting the following compile error:

../../ui/gfx/linux/client_native_pixmap_dmabuf.cc:39:27: fatal error:
linux/dma-buf.h: No such file or directory
 #include <linux/dma-buf.h>

Despite having sys-kernel/linux-headers-4.10 installed, I do not have
that header. Interestingly, it is present in the tarball
(gentoo-headers-base-4.10.tar.xz), but doesn't seem to get installed.

This is the workaround I applied:
<https://gitweb.gentoo.org/repo/gentoo.git/tree/www-client/chromium/files/chromium-dma-buf-r1.patch?id=f3d721fbb4127f17e836a1842cfdd2bf76f0d398>

This is part of the chromium code in question:

<https://cs.chromium.org/chromium/src/ui/gfx/linux/client_native_pixmap_dmabuf.cc?q=dma-buf.h+package:%5Echromium$&l=39&dr=C>

#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 6, 0)
#include <linux/types.h>

struct local_dma_buf_sync {
  __u64 flags;
};

#define LOCAL_DMA_BUF_SYNC_READ (1 << 0)
#define LOCAL_DMA_BUF_SYNC_WRITE (2 << 0)
#define LOCAL_DMA_BUF_SYNC_RW \
  (LOCAL_DMA_BUF_SYNC_READ | LOCAL_DMA_BUF_SYNC_WRITE)
#define LOCAL_DMA_BUF_SYNC_START (0 << 2)
#define LOCAL_DMA_BUF_SYNC_END (1 << 2)

#define LOCAL_DMA_BUF_BASE 'b'
#define LOCAL_DMA_BUF_IOCTL_SYNC \
  _IOW(LOCAL_DMA_BUF_BASE, 0, struct local_dma_buf_sync)

#else
#include <linux/dma-buf.h>
#endif

Paweł