Re: [Intel-gfx] [PATCH v4 0/6] Finally fix watermarks

Maarten Lankhorst
2016-08-02T15:41:50Z

Op 01-08-16 om 13:48 schreef Ville Syrjälä:
> On Mon, Aug 01, 2016 at 10:48:37AM +0200, Maarten Lankhorst wrote:
>> Op 29-07-16 om 22:33 schreef Matt Roper:
>>> On Fri, Jul 29, 2016 at 12:39:05PM +0300, Ville Syrjälä wrote:
>>>> On Thu, Jul 28, 2016 at 05:03:52PM -0700, Matt Roper wrote:
>>>>> This is completely untested (and probably horribly broken/buggy), but
>>>>> here's a quick mockup of the general approach I was thinking for
>>>>> ensuring DDB & WM's can be updated together while ensuring the
>>>>> three-step pipe flushing process is honored:
>>>>>
>>>>> https://github.com/mattrope/kernel/commits/experimental/lyude_ddb
>>>>>
>>>>> Basically the idea is to take note of what's happening to the pipe's DDB
>>>>> allocation (shrinking, growing, unchanged, etc.) during the atomic check
>>>>> phase;
>>>> Didn't look too closely, but I think you can't actually do that unless
>>>> you lock all the crtcs whenever the number of active pipes is goind to
>>>> change. Meaning we'd essentially be back to the one-big-modeset-lock
>>>> apporach, which will cause missed flips and whanot on the other pipes.
>>>>
>>>> The alternative I think would consist of:
>>>> - make sure level 0 watermark never exceeds total_ddb_size/max_pipes,
>>>> so that a modeset doesn't have to care about the wms for the other
>>>> pipes not fitting in
>>> Unfortunately this part is the problem. You might get away with doing
>>> this on SKL/KBL which only have three planes max per pipe and a large
>>> (896 block) DDB, but on BXT you have up to four planes (we don't
>>> actually enable the topmost plane in a full-featured manner just yet,
>>> but need to soon), yet the total DDB is only 512 blocks. Sadly, the
>>> platform with more planes was given a smaller DDB... :-(
>>> We're already in trouble because users that are running setups like 3x
>>> 4K with most/all planes in use at large sizes can't find level 0
>>> watermarks that satisfy their needs and we have to reject the whole
>>> configuration. If we further limit each pipe's usage to total/maxpipes
>>> (i.e., 170 blocks per pipe on BXT), then we're going to hit similar
>>> issues when only driving one or two displays with with all of the planes
>>> in use, even though we should have had more DDB space to work with.
>>>
>>> I guess serious plane usage isn't too common in desktop setups today,
>>> but it's a very critical feature in the embedded world; we can't really
>>> afford to cripple plane usage further. Unfortunately, as you point out
>>> above, this means that we have to follow the bspec's DDB allocation
>>> method, which in turn means we need to grab _all_ CRTC locks any time
>>> _any_ CRTC is being turned on or turned off which is a BKL-style way of
>>> doing things.
>> Meh, I'm running into a similar issue on vlv/chv. I don't see a way around it. :(
> Now are you hitting it w/ vlv/chv? They don't even have a shared DDB.
Not really the same, but determining what power saving levels + values to set with possibly parallel updates.

~Maarten
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