[PATCH 00/19] drm/i915/guc: Refactor ADS access to use dma_buf_map

Lucas De Marchi
2022-01-26T20:36:41Z

While porting i915 to arm64 we noticed some issues accessing lmem.
Some writes were getting corrupted and the final state of the buffer
didn't have exactly what we wrote. This became evident when enabling
GuC submission: depending on the number of engines the ADS struct was
being corrupted and GuC would reject it, refusin to initialize.

From Documentation/core-api/bus-virt-phys-mapping.rst:

This memory is called "PCI memory" or "shared memory" or "IO memory" or
whatever, and there is only one way to access it: the readb/writeb and
related functions. You should never take the address of such memory, because
there is really nothing you can do with such an address: it's not
conceptually in the same memory space as "real memory" at all, so you cannot
just dereference a pointer. (Sadly, on x86 it **is** in the same memory space,
so on x86 it actually works to just deference a pointer, but it's not
portable).

When reading or writing words directly to IO memory, in order to be portable
the Linux kernel provides the abstraction detailed in section "Differences
between I/O access functions" of Documentation/driver-api/device-io.rst.

This limits our ability to simply overlay our structs on top a buffer
and directly access it since that buffer may come from IO memory rather than
system memory. Hence the approach taken in intel_guc_ads.c needs to be
refactored. This is not the only place in i915 that neeed to be changed, but
the one causing the most problems, with a real reproducer. This first set of
patch focuses on fixing the gem object to pass the ADS

After the addition of a few helpers in the dma_buf_map API, most of
intel_guc_ads.c can be converted to use it. The exception is the regset
initialization: we'd incur into a lot of extra indirection when
reading/writting each register. So the regset is converted to use a
temporary buffer allocated on probe, which is then copied to its
final location when finishing the initialization or on gt reset.

Testing on some discrete cards, after this change we can correctly pass the
ADS struct to GuC and have it initialized correctly.

thanks
Lucas De Marchi

Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Cc: linux-kernel@vger.kernel.org
Cc: Christian König <christian.koenig@amd.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>

Lucas De Marchi (19):
dma-buf-map: Add read/write helpers
dma-buf-map: Add helper to initialize second map
drm/i915/gt: Add helper for shmem copy to dma_buf_map
drm/i915/guc: Keep dma_buf_map of ads_blob around
drm/i915/guc: Add read/write helpers for ADS blob
drm/i915/guc: Convert golden context init to dma_buf_map
drm/i915/guc: Convert policies update to dma_buf_map
drm/i915/guc: Convert engine record to dma_buf_map
dma-buf-map: Add wrapper over memset
drm/i915/guc: Convert guc_ads_private_data_reset to dma_buf_map
drm/i915/guc: Convert golden context prep to dma_buf_map
drm/i915/guc: Replace check for golden context size
drm/i915/guc: Convert mapping table to dma_buf_map
drm/i915/guc: Convert capture list to dma_buf_map
drm/i915/guc: Prepare for error propagation
drm/i915/guc: Use a single pass to calculate regset
drm/i915/guc: Convert guc_mmio_reg_state_init to dma_buf_map
drm/i915/guc: Convert __guc_ads_init to dma_buf_map
drm/i915/guc: Remove plain ads_blob pointer

drivers/gpu/drm/i915/gt/shmem_utils.c | 32 ++
drivers/gpu/drm/i915/gt/shmem_utils.h | 3 +
drivers/gpu/drm/i915/gt/uc/intel_guc.h | 14 +-
drivers/gpu/drm/i915/gt/uc/intel_guc_ads.c | 374 +++++++++++-------
drivers/gpu/drm/i915/gt/uc/intel_guc_ads.h | 3 +-
.../gpu/drm/i915/gt/uc/intel_guc_submission.c | 11 +-
include/linux/dma-buf-map.h | 127 ++++++
7 files changed, 405 insertions(+), 159 deletions(-)

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2.35.0