• Lyude Paul's avatar
    drm/i915: Call dma_set_max_seg_size() in i915_driver_hw_probe() · 32f0a982
    Lyude Paul authored
    Currently, we don't call dma_set_max_seg_size() for i915 because we
    intentionally do not limit the segment length that the device supports.
    However, this results in a warning being emitted if we try to map
    anything larger than SZ_64K on a kernel with CONFIG_DMA_API_DEBUG_SG
    enabled:
    
    [    7.751926] DMA-API: i915 0000:00:02.0: mapping sg segment longer
    than device claims to support [len=98304] [max=65536]
    [    7.751934] WARNING: CPU: 5 PID: 474 at kernel/dma/debug.c:1220
    debug_dma_map_sg+0x20f/0x340
    
    This was originally brought up on
    https://bugs.freedesktop.org/show_bug.cgi?id=108517 , and the consensus
    there was it wasn't really useful to set a limit (and that dma-debug
    isn't really all that useful for i915 in the first place). Unfortunately
    though, CONFIG_DMA_API_DEBUG_SG is enabled in the debug configs for
    various distro kernels. Since a WARN_ON() will disable automatic problem
    reporting (and cause any CI with said option enabled to start
    complaining), we really should just fix the problem.
    
    Note that as me and Chris Wilson discussed, the other solution for this
    would be to make DMA-API not make such assumptions when a driver hasn't
    explicitly set a maximum segment size. But, taking a look at the commit
    which originally introduced this behavior, commit 78c47830
    
    
    ("dma-debug: check scatterlist segments"), there is an explicit mention
    of this assumption and how it applies to devices with no segment size:
    
    	Conversely, devices which are less limited than the rather
    	conservative defaults, or indeed have no limitations at all
    	(e.g. GPUs with their own internal MMU), should be encouraged to
    	set appropriate dma_parms, as they may get more efficient DMA
    	mapping performance out of it.
    
    So unless there's any concerns (I'm open to discussion!), let's just
    follow suite and call dma_set_max_seg_size() with UINT_MAX as our limit
    to silence any warnings.
    
    Changes since v3:
    * Drop patch for enabling CONFIG_DMA_API_DEBUG_SG in CI. It looks like
      just turning it on causes the kernel to spit out bogus WARN_ONs()
      during some igt tests which would otherwise require teaching igt to
      disable the various DMA-API debugging options causing this. This is
      too much work to be worth it, since DMA-API debugging is useless for
      us. So, we'll just settle with this single patch to squelch WARN_ONs()
      during driver load for users that have CONFIG_DMA_API_DEBUG_SG turned
      on for some reason.
    * Move dma_set_max_seg_size() call into i915_driver_hw_probe() - Chris
      Wilson
    Signed-off-by: default avatarLyude Paul <lyude@redhat.com>
    Reviewed-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
    Cc: <stable@vger.kernel.org> # v4.18+
    Link: https://patchwork.freedesktop.org/patch/msgid/20190823205251.14298-1-lyude@redhat.com
    
    
    (cherry picked from commit acd674af95d3f627062007429b9c195c6b32361d)
    Signed-off-by: default avatarJani Nikula <jani.nikula@intel.com>
    32f0a982
i915_drv.c 89 KB