[Intel-gfx] [PATCH v7 6/7] drm/i915: refactor duplicate object vmap functions (the final rework?)

[Dave Gordon]

On 08/03/16 09:43, Tvrtko Ursulin wrote:
>
> On 02/03/16 15:40, Dave Gordon wrote:
>> On 02/03/16 12:08, Chris Wilson wrote:
>>> On Tue, Mar 01, 2016 at 04:33:58PM +0000, Dave Gordon wrote:
>>>> This is essentially Chris Wilson's patch of a similar name, reworked on
>>>> top of Alex Dai's recent patch:
>>>> | drm/i915: Add i915_gem_object_vmap to map GEM object to virtual space
>>>>
>>>> Chris' original commentary said:
>>>> | We now have two implementations for vmapping a whole object, one for
>>>> | dma-buf and one for the ringbuffer. If we couple the vmapping into
>>>> | the obj->pages lifetime, then we can reuse an obj->vmapping for both
>>>> | and at the same time couple it into the shrinker.
>>>> |
>>>> | v2: Mark the failable kmalloc() as __GFP_NOWARN (vsyrjala)
>>>> | v3: Call unpin_vmap from the right dmabuf unmapper
>>>>
>>>> v4: reimplements the same functionality, but now as wrappers round the
>>>>      recently-introduced i915_gem_object_vmap_range() from Alex's patch
>>>>      mentioned above.
>>>>
>>>> v5: separated from two minor but unrelated changes [Tvrtko Ursulin];
>>>>      this is the third and most substantial portion.
>>>>
>>>>      Decided not to hold onto vmappings after the pin count goes to
>>>> zero.
>>>>      This may reduce the benefit of Chris' scheme a bit, but does avoid
>>>>      any increased risk of exhausting kernel vmap space on 32-bit
>>>> kernels
>>>>      [Tvrtko Ursulin]. Potentially, the vunmap() could be deferred
>>>> until
>>>>      the put_pages() stage if a suitable notifier were written, but
>>>> we're
>>>>      not doing that here. Nonetheless, the simplification of both
>>>> dmabuf
>>>>      and ringbuffer code makes it worthwhile in its own right.
>>>>
>>>> v6: change BUG_ON() to WARN_ON(). [Tvrtko Ursulin]
>>>>
>>>> Signed-off-by: Dave Gordon <david.s.gordon at intel.com>
>>>> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>>> Cc: Chris Wilson <chris at chris-wilson.co.uk>
>>>> Cc: Alex Dai <yu.dai at intel.com>
>>>> ---
>>>>   drivers/gpu/drm/i915/i915_drv.h         | 22 ++++++++++++++-----
>>>>   drivers/gpu/drm/i915/i915_gem.c         | 39
>>>> +++++++++++++++++++++++++++++++++
>>>>   drivers/gpu/drm/i915/i915_gem_dmabuf.c  | 36
>>>> ++++--------------------------
>>>>   drivers/gpu/drm/i915/intel_ringbuffer.c |  9 ++++----
>>>>   4 files changed, 65 insertions(+), 41 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/i915_drv.h
>>>> b/drivers/gpu/drm/i915/i915_drv.h
>>>> index b3ae191..f1ad3b3 100644
>>>> --- a/drivers/gpu/drm/i915/i915_drv.h
>>>> +++ b/drivers/gpu/drm/i915/i915_drv.h
>>>> @@ -2172,10 +2172,7 @@ struct drm_i915_gem_object {
>>>>           struct scatterlist *sg;
>>>>           int last;
>>>>       } get_page;
>>>> -
>>>> -    /* prime dma-buf support */
>>>> -    void *dma_buf_vmapping;
>>>> -    int vmapping_count;
>>>> +    void *vmapping;
>>>>
>>>>       /** Breadcrumb of last rendering to the buffer.
>>>>        * There can only be one writer, but we allow for multiple
>>>> readers.
>>>> @@ -2980,7 +2977,22 @@ static inline void
>>>> i915_gem_object_pin_pages(struct drm_i915_gem_object *obj)
>>>>   static inline void i915_gem_object_unpin_pages(struct
>>>> drm_i915_gem_object *obj)
>>>>   {
>>>>       BUG_ON(obj->pages_pin_count == 0);
>>>> -    obj->pages_pin_count--;
>>>> +    if (--obj->pages_pin_count == 0 && obj->vmapping) {
>>>> +        /*
>>>> +         * Releasing the vmapping here may yield less benefit than
>>>> +         * if we kept it until put_pages(), but on the other hand
>>>
>>> Yields no benefit. Makes the patch pointless.
>>> Plus there is also pressure to enable WC vmaps.
>>> -Chris
>>
>> The patch is not pointless -- at the very least, it:
>> + reduces the size of "struct drm_i915_gem_object" (OK, only by 4 bytes)
>> + replaces special-function code for dmabufs with more generic code that
>> can be reused for other objects (for now, ringbuffers; next GuC-shared
>> objects -- see Alex's patch "drm/i915/guc: Simplify code by keeping vmap
>> of guc_client object" which will eliminate lot of short-term
>> kmap_atomics with persistent kmaps).
>> + provides a shorthand for the sequence of { get_pages(), pin_pages(),
>> vmap() } so we don't have to open-code it (and deal with all the error
>> paths) in several different places
>>
>> Thus there is an engineering benefit even if this version doesn't
>> provide any performance benefit. And if, as the next step, you want to
>> extend the vmap lifetime, you just have to remove those few lines in
>> i915_gem_object_unpin_pages() and incorporate the notifier that you
>> prototyped earlier -- if it actually provides any performance boost.
>
> So Chris do you ack on this series on the basis of the above - that it
> consolidates the current code and following GuC patch will be another
> user of the pin_vmap API?
>
> Regards,
> Tvrtko

I see that Chris has posted a patch to add a vmap notifier, although it 
hasn't yet got its R-B. So I suggest we merge this patch series now, and 
then update it by moving the vunmap() into put_pages() when Chris has 
the notifier finalised. IIRC you wanted Daniel to merge the new DRM bits 
(patches 3 and 7, which already have their R-Bs) ?

Or we can merge 1-5+7, all of which already have R-Bs, and I can turn
6 into a GuC-private version, without the benefit of simplifying and 
unifying the corresponding object-mapping management in the DMAbuf and 
ringbuffer code.

Or I can repost just the bits that don't rely on drm_malloc_gfp() and 
exclude the final patch so that we can move ahead on the bits we 
actually want for improving the performance of the GuC interface and 
reducing the number of kmap_atomic calls elsewhere, and then the omitted 
bits can be added back once drm_malloc_gfp() has been merged upstream 
and the notifier is working.

.Dave.