Re: [PATCH 5/6] drm/i915: Support for pread/pwrite from/to non shmem backed objects

[Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>]

On Wed, 2015-12-09 at 19:39 +0000, Dave Gordon wrote:
> On 09/12/15 16:15, Tvrtko Ursulin wrote:
> >
> > Hi,
> >
> > On 09/12/15 12:46, ankitprasad.r.sharma@intel.com wrote:
> >> From: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
> >>
> >> This patch adds support for extending the pread/pwrite functionality
> >> for objects not backed by shmem. The access will be made through
> >> gtt interface. This will cover objects backed by stolen memory as well
> >> as other non-shmem backed objects.
> >>
> >> v2: Drop locks around slow_user_access, prefault the pages before
> >> access (Chris)
> >>
> >> v3: Rebased to the latest drm-intel-nightly (Ankit)
> >>
> >> v4: Moved page base & offset calculations outside the copy loop,
> >> corrected data types for size and offset variables, corrected if-else
> >> braces format (Tvrtko/kerneldocs)
> >>
> >> v5: Enabled pread/pwrite for all non-shmem backed objects including
> >> without tiling restrictions (Ankit)
> >>
> >> v6: Using pwrite_fast for non-shmem backed objects as well (Chris)
> >>
> >> v7: Updated commit message, Renamed i915_gem_gtt_read to
> >> i915_gem_gtt_copy,
> >> added pwrite slow path for non-shmem backed objects (Chris/Tvrtko)
> >>
> >> v8: Updated v7 commit message, mutex unlock around pwrite slow path for
> >> non-shmem backed objects (Tvrtko)
> >>
> >> Testcase: igt/gem_stolen
> >>
> >> Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
> >> ---
> >>   drivers/gpu/drm/i915/i915_gem.c | 151
> >> +++++++++++++++++++++++++++++++++-------
> >>   1 file changed, 127 insertions(+), 24 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/i915_gem.c
> >> b/drivers/gpu/drm/i915/i915_gem.c
> >> index ed97de6..68ed67a 100644
> >> --- a/drivers/gpu/drm/i915/i915_gem.c
> >> +++ b/drivers/gpu/drm/i915/i915_gem.c
> >> @@ -614,6 +614,99 @@ shmem_pread_slow(struct page *page, int
> >> shmem_page_offset, int page_length,
> >>       return ret ? - EFAULT : 0;
> >>   }
> >>
> >> +static inline uint64_t
> >> +slow_user_access(struct io_mapping *mapping,
> >> +         uint64_t page_base, int page_offset,
> >> +         char __user *user_data,
> >> +         int length, bool pwrite)
> >> +{
> >> +    void __iomem *vaddr_inatomic;
> >> +    void *vaddr;
> >> +    uint64_t unwritten;
> >> +
> >> +    vaddr_inatomic = io_mapping_map_wc(mapping, page_base);
> >> +    /* We can use the cpu mem copy function because this is X86. */
> >> +    vaddr = (void __force *)vaddr_inatomic + page_offset;
> >> +    if (pwrite)
> >> +        unwritten = __copy_from_user(vaddr, user_data, length);
> >> +    else
> >> +        unwritten = __copy_to_user(user_data, vaddr, length);
> >> +
> >> +    io_mapping_unmap(vaddr_inatomic);
> >> +    return unwritten;
> >> +}
> >> +
> >> +static int
> >> +i915_gem_gtt_copy(struct drm_device *dev,
> >> +           struct drm_i915_gem_object *obj, uint64_t size,
> >> +           uint64_t data_offset, uint64_t data_ptr)
> >> +{
> >> +    struct drm_i915_private *dev_priv = dev->dev_private;
> >> +    char __user *user_data;
> >> +    uint64_t remain;
> >> +    uint64_t offset, page_base;
> >> +    int page_offset, page_length, ret = 0;
> >> +
> >> +    ret = i915_gem_obj_ggtt_pin(obj, 0, PIN_MAPPABLE);
> >> +    if (ret)
> >> +        goto out;
> >> +
> >> +    ret = i915_gem_object_set_to_gtt_domain(obj, false);
> >> +    if (ret)
> >> +        goto out_unpin;
> >> +
> >> +    ret = i915_gem_object_put_fence(obj);
> >> +    if (ret)
> >> +        goto out_unpin;
> >> +
> >> +    user_data = to_user_ptr(data_ptr);
> >> +    remain = size;
> >> +    offset = i915_gem_obj_ggtt_offset(obj) + data_offset;
> >> +
> >> +    mutex_unlock(&dev->struct_mutex);
> >> +    if (likely(!i915.prefault_disable))
> >> +        ret = fault_in_multipages_writeable(user_data, remain);
> >> +
> >> +    /*
> >> +     * page_offset = offset within page
> >> +     * page_base = page offset within aperture
> >> +     */
> >> +    page_offset = offset_in_page(offset);
> >> +    page_base = offset & PAGE_MASK;
> >> +
> >> +    while (remain > 0) {
> >> +        /* page_length = bytes to copy for this page */
> >> +        page_length = remain;
> >> +        if ((page_offset + remain) > PAGE_SIZE)
> >> +            page_length = PAGE_SIZE - page_offset;
> >> +
> >> +        /* This is a slow read/write as it tries to read from
> >> +         * and write to user memory which may result into page
> >> +         * faults
> >> +         */
> >> +        ret = slow_user_access(dev_priv->gtt.mappable, page_base,
> >> +                       page_offset, user_data,
> >> +                       page_length, false);
> >> +
> >> +        if (ret) {
> >> +            ret = -EFAULT;
> >> +            break;
> >> +        }
> >> +
> >> +        remain -= page_length;
> >> +        user_data += page_length;
> >> +        page_base += page_length;
> >> +        page_offset = 0;
> >> +    }
> >> +
> >> +    mutex_lock(&dev->struct_mutex);
> >> +
> >> +out_unpin:
> >> +    i915_gem_object_ggtt_unpin(obj);
> >> +out:
> >> +    return ret;
> >> +}
> >> +
> >>   static int
> >>   i915_gem_shmem_pread(struct drm_device *dev,
> >>                struct drm_i915_gem_object *obj,
> >> @@ -737,17 +830,14 @@ i915_gem_pread_ioctl(struct drm_device *dev,
> >> void *data,
> >>           goto out;
> >>       }
> >>
> >> -    /* prime objects have no backing filp to GEM pread/pwrite
> >> -     * pages from.
> >> -     */
> >> -    if (!obj->base.filp) {
> >> -        ret = -EINVAL;
> >> -        goto out;
> >> -    }
> >> -
> >>       trace_i915_gem_object_pread(obj, args->offset, args->size);
> >>
> >> -    ret = i915_gem_shmem_pread(dev, obj, args, file);
> >> +    /* pread for non shmem backed objects */
> >> +    if (!obj->base.filp && obj->tiling_mode == I915_TILING_NONE)
> >> +        ret = i915_gem_gtt_copy(dev, obj, args->size,
> >> +                    args->offset, args->data_ptr);
> >> +    else
> >> +        ret = i915_gem_shmem_pread(dev, obj, args, file);
> >
> > Hm, it will end up calling i915_gem_shmem_pread for non-shmem backed
> > objects if tiling is set. Sounds wrong to me unless I am missing something?
> 
> Which GEM objects have obj->base.filp set? Is it ONLY regular gtt-type 
> objects? What about (phys, stolen, userptr, dmabuf, ...?) Which of these 
> is the alternate path going to work with?
Only shmem backed objects have obj->base.filp set, filp pointing to the
shmem file. For all other non-shmem backed objects (stolen, userptr,
dmabuf) we use the alternate path.

-Ankit

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