linux-kernel - Re: [PATCH 2/2] drm/msm/hdmi: add hdmi hdcp support

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Message-ID: <250d9d48a62a2a46dba33a6f888fdd9e.squirrel@codeaurora.org>
Date:	Wed, 3 Dec 2014 17:49:19 -0000
From:	jilaiw@...eaurora.org
To:	"Bjorn Andersson" <bjorn@...o.se>
Cc:	"Jilai Wang" <jilaiw@...eaurora.org>,
	"Rob Clark" <robdclark@...il.com>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-arm-msm" <linux-arm-msm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] drm/msm/hdmi: add hdmi hdcp support

> On Wed, Dec 3, 2014 at 9:16 AM,  <jilaiw@...eaurora.org> wrote:
> [..]
>>>>> +       enum hdmi_hdcp_state hdcp_state;
>>>>> +       struct mutex state_mutex;
>>>>> +       struct delayed_work hdcp_reauth_work;
>>>>> +       struct delayed_work hdcp_auth_part1_1_work;
>>>>> +       struct delayed_work hdcp_auth_part1_2_work;
>>>>> +       struct work_struct hdcp_auth_part1_3_work;
>>>>> +       struct delayed_work hdcp_auth_part2_1_work;
>>>>> +       struct delayed_work hdcp_auth_part2_2_work;
>>>>> +       struct delayed_work hdcp_auth_part2_3_work;
>>>>> +       struct delayed_work hdcp_auth_part2_4_work;
>>>>> +       struct work_struct hdcp_auth_prepare_work;
>>>>
>>>> You shouldn't use "work" as a way to express states in your state
>>>> machine.
>>>> Better have 1 auth work function that does all these steps, probably
>>>> having
>>>> them split in functions just like you do now.
>>>>
>>>> That way you can have 1 function running the pass of authentication,
>>>> starting
>>>> by checking if you're reauthing or not then processing each step one
>>>> by
>>>> one,
>>>> sleeping inbetween them. You can have the functions return -EAGAIN to
>>>> indicate
>>>> that you need to retry the current operation and so on.
>>>>
>>>> This would split the state machine from the state executioners and
>>>> simplify
>>>> your code.
>>>
>>> As a side note (disclaimer, I'm not an hdcp expert), but I wonder if
>>> eventually some of that should be extracted out into some helpers in
>>> drm core.  I guess that is something we'll figure out when a 2nd
>>> driver gains upstream HDCP support.  One big work w/ msleep()'s does
>>> sound like it would be easier to understand (and therefore easier to
>>> refactor out into helpers).
>>>
>>> [snip]
>>>
>>
>> The reason that I break the partI/PartII work into these small works
>> because I want to avoid to use msleep in work.
>> Otherwise cancel a HDCP work may cause long delay up to several seconds.
>>
>
> I definitely think the steps are nice size and make things easy to
> understand.
>
> If you make the steps that require a retry return out to the main
> state handling function with a -EAGAIN, then you can have check if you
> should retry or abort based on cancellation. Giving you the worst case
> cancellation time of the longest sleep...
>
> As Rob suggest you could use wait_event_timeout() or something to
> improve this, but on the other hand the worst case here seems to be
> the HZ/2 after prepare_auth; others are HZ/8, HZ/20 and HZ/50; not
> "seconds".
>
> So I would start with a simple msleep() for implementation simplicity
> and then enhance that in a follow up commit (if needed)...
>
> Regards,
> Bjorn
>

The worst wait time could be 5 seconds if the downstream device is a
REPEATER. The maximum retry time is set to 100 and the delay for each time
is HZ/20, then the maximum wait time before abort will be 5 seconds.
As you suggested, I can use the flag to notify the retry process to abort
earlier in case this work needs to be canceled which can reduce the delay
to HZ/20 then. Or as Rob's suggestion to wait for hpd event.

Thanks,
Jilai


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