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Previously, iris_bufmgr had the ability to maintain multiple
simultaneous memory mappings for a BO, one in WB mode (with CPU caches),
and another in WC (streaming) mode. Depending on the flags passed to
iris_bo_map(), we would select one mode or the other.
The rules for deciding which to use were:
- Systems with LLC always use WB mode because it's basically free
- Non-LLC systems used...
- WB maps for all BOs where snooping is enabled (which translates to
when BO_ALLOC_COHERENT is set at allocation time)
- WB maps for reads unless persistent, coherent, async, or raw.
- WC maps for everything else.
This patch simplifies the system by selecting a single mmap mode at
BO allocation time, and always using that. Each BO now has at most one
map at a time, rather than up to two (or three before we deleted GTT
map support in recent patches).
In practical terms, this eliminates the capability to use WB maps for
reads of non-snooped BOs on non-LLC systems. Such reads would now be
slow, uncached reads. However, iris_transfer_map recently began using
staging blits for such reads - so the GPU copies the data to a snooped
buffer which will be mapped WB. So, rather than incurring slow UC
reads, we really just take the hit of a blit, and retain fast reads.
The rest of the rules remain the same.
There are a few reasons for this:
1. TTM doesn't support mapping an object as both WB and WC. The
cacheability is treated as a property of the object, not the map.
The kernel is moving to use TTM as part of adding discrete local
memory support. So it makes sense to centralize on that model.
2. Mapping the same BO as both WB and WC is impossible to support on
some CPUs. It works on x86/x86_64, which was fine for integrated
GPUs, but it may become an issue for discrete graphics paired with
other CPUs (should that ever be a thing we need to support).
3. It's overall much simpler. We only have one bo->map field, and
manage to drop a significant amount of boilerplate.
One issue that arises is the interaction with the BO cache: BOs with
WB maps and WC maps will be lumped together into the same cache. This
means that a cached BO may have the wrong mmap mode. We check that,
and if it doesn't match, we unmap it, waiting until iris_bo_map is
called to restore one with the desired mode. This may underutilize
cache mappings slightly on non-LLC systems, but I don't expect it to
have a large impact.
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4747
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10941>
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`Mesa <https://mesa3d.org>`_ - The 3D Graphics Library ====================================================== Source ------ This repository lives at https://gitlab.freedesktop.org/mesa/mesa. Other repositories are likely forks, and code found there is not supported. Build & install --------------- You can find more information in our documentation (`docs/install.rst <https://mesa3d.org/install.html>`_), but the recommended way is to use Meson (`docs/meson.rst <https://mesa3d.org/meson.html>`_): .. code-block:: sh $ mkdir build $ cd build $ meson .. $ sudo ninja install Support ------- Many Mesa devs hang on IRC; if you're not sure which channel is appropriate, you should ask your question on `OFTC's #dri-devel <irc://irc.oftc.net/dri-devel>`_, someone will redirect you if necessary. Remember that not everyone is in the same timezone as you, so it might take a while before someone qualified sees your question. To figure out who you're talking to, or which nick to ping for your question, check out `Who's Who on IRC <https://dri.freedesktop.org/wiki/WhosWho/>`_. The next best option is to ask your question in an email to the mailing lists: `mesa-dev\@lists.freedesktop.org <https://lists.freedesktop.org/mailman/listinfo/mesa-dev>`_ Bug reports ----------- If you think something isn't working properly, please file a bug report (`docs/bugs.rst <https://mesa3d.org/bugs.html>`_). Contributing ------------ Contributions are welcome, and step-by-step instructions can be found in our documentation (`docs/submittingpatches.rst <https://mesa3d.org/submittingpatches.html>`_). Note that Mesa uses gitlab for patches submission, review and discussions.