Re: [PATCH] drm/ttm: fix two regressions since move_notify changes

[Thomas Hellstrom <thomas@shipmail.org>]

On 01/25/2012 07:12 PM, Dave Airlie wrote:
> On Wed, Jan 25, 2012 at 5:19 PM, Thomas Hellstrom<thomas@shipmail.org>  wrote:
>> On 01/25/2012 04:37 PM, Jerome Glisse wrote:
>>> On Wed, Jan 25, 2012 at 10:21 AM, Ben Skeggs<skeggsb@gmail.com>    wrote:
>>>> On Wed, 2012-01-25 at 15:33 +0100, Thomas Hellstrom wrote:
>>>>> On 01/25/2012 10:41 AM, Ben Skeggs wrote:
>>>>>> My main concern is that we blindly and unnecessarily set up GPU
>>>>>> bindings and
>>>>>> end up with unnecessary code in TTM, and furthermore that we
>>>>>> communicate
>>>>>> that bad practice to future driver writers.
>>>>>> This "unnecessary code" is like 5 lines of cleanup if something fails,
>>>>>> hardly anything to be jumping up and down about :)
>>>>> It's just not TTM's business, unless the GPU maps are mappable by the
>>>>> CPU as well.
>>>>> Also, What if the mapping setup in move_notify() fails?
>>>> It can't fail, and well, in nouveau's implementation it never will.
>>>> It's simply a "fill the ptes for all the vmas currently associated with
>>>> a bo".
>>>>
>>>> And well, it's about as much TTM's business as VRAM aperture allocation
>>>> is..  I don't see the big deal, if you wan't to do it a different way in
>>>> your driver, there's nothing stopping you.  It's a lot of bother for
>>>> essentially zero effort in TTM..
>>>>
>>>>>>> Thomas, what do you suggest to move forward with this? Both of these
>>>>>>> bugs are serious regressions that make nouveau unusable with the
>>>>>>> current
>>>>>>> 3.3-rc series. Ben.
>>>>>>>
>>>>>>> My number one choice would of course be to have the drivers set up
>>>>>>> their
>>>>>>> private GPU mappings after a
>>>>>>> successful validate call, as I originally suggested and you agreed to.
>>>>>>>
>>>>>>> If that's not possible (I realize it's late in the release series),
>>>>>>> I'll
>>>>>>> ack this patch if you and Jerome agree not to block
>>>>>>> attempts to move in that direction for future kernel releases.
>>>>>> I can't say I'm entirely happy with the plan honestly.  To me, it still
>>>>>> seems more efficient to handle this when a move happens (comparatively
>>>>>> rare) and "map new backing storage into every vm that has a reference"
>>>>>> than to (on every single buffer of every single "exec" call) go "is
>>>>>> this
>>>>>> buffer mapped into this channel's vm? yes, ok; no, lets go map it".
>>>>>>
>>>>>> I'm not even sure how exactly I plan on storing this mapping
>>>>>> efficiently
>>>>>> yet.. Scanning the BO's linked list of VMs it's mapped into for "if
>>>>>> (this_vma == chan->vma)" doesn't exactly sound performant.
>>>>> As previously suggested, in the simplest case a bo could have a 'needs
>>>>> remap' flag
>>>>> that is set on gpu map teardown on move_notify(), and when this flag is
>>>>> detected in validate,
>>>>> go ahead and set up all needed maps and clear that flag.
>>>>>
>>>>> This is the simplest case and more or less equivalent to the current
>>>>> solution, except
>>>>> maps aren't set up unless needed by at least one channel and there is a
>>>>> clear way
>>>>> to handle errors when GPU maps are set up.
>>>> Yes, right.  That can be done, and gives exactly the same functionality
>>>> as I *already* achieve with move_notify() but apparently have to change
>>>> just because you've decided nouveau/radeon are both doing the
>>>> WrongThing(tm).
>>>>
>>>> Anyhow, I care more that 3.3 works than how it works.  So, whatever.  If
>>>> I must agree to this in order to get a regression fix in, then I guess I
>>>> really have no choice in the matter.
>>>>
>>>> Ben.
>>>>
>>>>> A simple and straightforward fix that leaves the path open (if so
>>>>> desired) to
>>>>> handle finer channel granularity.
>>>>>
>>>>> Or am I missing something?
>>>>>
>>> I went over the code and Ben fix is ok with me, i need to test it a
>>> bit on radeon side.
>>>
>>> For long term solution why not just move most of the
>>> ttm_bo_handle_move_mem to the driver. It would obsolete the
>>> move_notify callback. move notify callback was introduced because in
>>> some case the driver never knew directly that a bo moved. It's obvious
>>> that driver need to know every time. So instead of having an ha-doc
>>> function for that. Let just move the handle move stuff into the
>>> driver. Yes there will be some code duplication but it will avoid
>>> anykind of weird error path and driver will be able to perform what
>>> ever make sense.
>>
>> Yes, this is a solution that eliminates the need for TTM to support private
>> GPU map setup. Code duplication can largely be avoided if we
>> collect common code in a small utility function.
> Please this, its reduces one more place TTM suffers from midlayer effects.
>
> I'd much prefer the drivers be in control and call into help common
> code instead of the driver calling in and then lots of callbacks out.
>
> (and yes the drm suffers from this a lot as well).
>
> Dave.
OK,
let's work in this direction.

/Thomas