Re: [PATCH v4] drm/i915: Add IOCTL Param to control data port coherency.

["Lis, Tomasz" <tomasz.lis@intel.com>]

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On 2018-07-09 18:37, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2018-07-09 17:28:02)
>> On 09/07/2018 14:20, Tomasz Lis wrote:
>>> +static int i915_gem_context_clear_data_port_coherent(struct i915_gem_context *ctx)
>>> +{
>>> +     int ret;
>>> +
>>> +     ret = intel_lr_context_modify_data_port_coherency(ctx, false);
>>> +     if (!ret)
>>> +             __clear_bit(CONTEXT_DATA_PORT_COHERENT, &ctx->flags);
>>> +     return ret;
>> Is there a good reason you allow userspace to keep emitting unlimited
>> number of commands which actually do not change the status? If not
>> please consider gating the command emission with
>> test_and_set_bit/test_and_clear_bit. Hm.. apart even with that they
>> could keep toggling ad infinitum with no real work in between. Has it
>> been considered to only save the desired state in set param and then
>> emit it, if needed, before next execbuf? Minor thing in any case, just
>> curious since I wasn't following the threads.
> The first patch tried to add a bit to execbuf, and having been
> mistakenly down that road before, we asked if there was any alternative.
> (Now if you've also been following execbuf3 conversations, having a
> packet for privileged LRI is definitely something we want.)
>
> Setting the value in the context register is precisely what we want to
> do, and trivially serialised with execbuf since we have to serialise
> reservation of ring space, i.e. the normal rules of request generation.
> (execbuf is just a client and nothing special). From that point of view,
> we only care about frequency, if it is very frequent it should be
> controlled by userspace inside the batch (but it can't due to there
> being dangerous bits inside the reg aiui). At the other end of the
> scale, is context_setparam for set-once. And there should be no
> inbetween as that requires costly batch flushes.
> -Chris
Joonas did brought that concern in his review; here it is, with my response:

On 2018-06-21 15:47, Lis, Tomasz wrote:
> On 2018-06-21 08:39, Joonas Lahtinen wrote:
>> I'm thinking we should set this value when it has changed, when we 
>> insert the
>> requests into the command stream. So if you change back and forth, while
>> not emitting any requests, nothing really happens. If you change the 
>> value and
>> emit a request, we should emit a LRI before the jump to the commands.
>> Similary if you keep setting the value to the value it already was in,
>> nothing will happen, again.
> When I considered that, my way of reasoning was:
> If we execute the flag changing buffer right away, it may be sent to 
> hardware faster if there is no job in progress.
> If we use the lazy way, and trigger the change just before submission 
> -  there will be additional conditions in submission code, plus the 
> change will be made when there is another job pending (though it's not 
> a considerable payload to just switch a flag).
> If user space switches the flag back and forth without much sense, 
> then there is something wrong with the user space driver, and it 
> shouldn't be up to kernel to fix that.
>
> This is why I chosen the current approach. But I can change it if you 
> wish.

So while I think the current solution is better from performance 
standpoint, but I will change it if you request that.
-Tomasz


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