[PATCH] dma-buf: Update docs for SYNC ioctl

[Tiago Vignatti]

On 03/23/2016 12:42 PM, Chris Wilson wrote:
> On Wed, Mar 23, 2016 at 04:32:59PM +0100, David Herrmann wrote:
>> Hi
>>
>> On Wed, Mar 23, 2016 at 12:56 PM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
>>> On Wed, Mar 23, 2016 at 12:30:42PM +0100, David Herrmann wrote:
>>>> My question was rather about why we do this? Semantics for EINTR are
>>>> well defined, and with SA_RESTART (default on linux) user-space can
>>>> ignore it. However, looping on EAGAIN is very uncommon, and it is not
>>>> at all clear why it is needed?
>>>>
>>>> Returning an error to user-space makes sense if user-space has a
>>>> reason to react to it. I fail to see how EAGAIN on a cache-flush/sync
>>>> operation helps user-space at all? As someone without insight into the
>>>> driver implementation, it is hard to tell why.. Any hints?
>>>
>>> The reason we return EAGAIN is to workaround a deadlock we face when
>>> blocking on the GPU holding the struct_mutex (inside the client's
>>> process), but the GPU is dead. As our locking is very, very coarse we
>>> cannot restart the GPU without acquiring the struct_mutex being held by
>>> the client so we wake the client up and tell them the resource they are
>>> waiting on (the flush of the object from the GPU into the CPU domain) is
>>> temporarily unavailable. If they try to immediately wait upon the ioctl
>>> again, they are blocked waiting for the reset to occur before they may
>>> complete their flush. There are a few other possible deadlocks that are
>>> also avoided with EAGAIN (again, the issue is more or less the lack of
>>> fine grained locking).
>>
>> ...so you hijacked EAGAIN for all DRM ioctls just for a driver
>> workaround?
>
> No, we utilized the fact that EAGAIN was already enshrined by libdrm as
> the defacto mechanism for repeating the ioctl in order to repeat the
> ioctl for a driver workaround.

Do we have an agreement here after all? David? I need to know whether 
this fixup is okay to go cause I'll need to submit to Chrome OS then.

Best Regards,

Tiago