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Subject: Re: [PATCH 2/3] venus: Limit HFI sessions to the maximum supported
To:     Alexandre Courbot <acourbot@chromium.org>
Cc:     linux-media@vger.kernel.org, linux-arm-msm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Vikash Garodia <vgarodia@codeaurora.org>,
        Mansur Alisha Shaik <mansur@codeaurora.org>,
        Dikshita Agarwal <dikshita@codeaurora.org>
References: <20201120001037.10032-1-stanimir.varbanov@linaro.org>
 <20201120001037.10032-3-stanimir.varbanov@linaro.org>
 <CAPBb6MUnXmtSKy9NwikYXjafgB+WM9TKEFjkYK16T2V7KRx=JQ@mail.gmail.com>
 <8c6231b2-61c2-d432-aa47-ddc29de8da19@linaro.org>
 <CAPBb6MXRat0g_+d04eoOL9Vpbv-2iJfqdOkzGB17=yjRMeVWXQ@mail.gmail.com>
From:   Stanimir Varbanov <stanimir.varbanov@linaro.org>
Message-ID: <59068381-43e4-559b-faa0-0e177de541f9@linaro.org>
Date:   Fri, 27 Nov 2020 00:41:59 +0200
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On 11/26/20 8:28 AM, Alexandre Courbot wrote:
> On Wed, Nov 25, 2020 at 10:01 PM Stanimir Varbanov
> <stanimir.varbanov@linaro.org> wrote:
>>
>>
>>
>> On 11/25/20 5:46 AM, Alexandre Courbot wrote:
>>> On Fri, Nov 20, 2020 at 9:12 AM Stanimir Varbanov
>>> <stanimir.varbanov@linaro.org> wrote:
>>>>
>>>> Currently we rely on firmware to return error when we reach the maximum
>>>> supported number of sessions. But this errors are happened at reqbuf
>>>> time which is a bit later. The more reasonable way looks like is to
>>>> return the error on driver open.
>>>>
>>>> To achieve that modify hfi_session_create to return error when we reach
>>>> maximum count of sessions and thus refuse open.
>>>>
>>>> Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
>>>> ---
>>>>  drivers/media/platform/qcom/venus/core.h      |  1 +
>>>>  drivers/media/platform/qcom/venus/hfi.c       | 19 +++++++++++++++----
>>>>  .../media/platform/qcom/venus/hfi_parser.c    |  3 +++
>>>>  3 files changed, 19 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/media/platform/qcom/venus/core.h b/drivers/media/platform/qcom/venus/core.h
>>>> index db0e6738281e..3a477fcdd3a8 100644
>>>> --- a/drivers/media/platform/qcom/venus/core.h
>>>> +++ b/drivers/media/platform/qcom/venus/core.h
>>>> @@ -96,6 +96,7 @@ struct venus_format {
>>>>  #define MAX_CAP_ENTRIES                32
>>>>  #define MAX_ALLOC_MODE_ENTRIES 16
>>>>  #define MAX_CODEC_NUM          32
>>>> +#define MAX_SESSIONS           16
>>>>
>>>>  struct raw_formats {
>>>>         u32 buftype;
>>>> diff --git a/drivers/media/platform/qcom/venus/hfi.c b/drivers/media/platform/qcom/venus/hfi.c
>>>> index 638ed5cfe05e..8420be6d3991 100644
>>>> --- a/drivers/media/platform/qcom/venus/hfi.c
>>>> +++ b/drivers/media/platform/qcom/venus/hfi.c
>>>> @@ -175,6 +175,7 @@ static int wait_session_msg(struct venus_inst *inst)
>>>>  int hfi_session_create(struct venus_inst *inst, const struct hfi_inst_ops *ops)
>>>>  {
>>>>         struct venus_core *core = inst->core;
>>>> +       int ret;
>>>>
>>>>         if (!ops)
>>>>                 return -EINVAL;
>>>> @@ -183,12 +184,22 @@ int hfi_session_create(struct venus_inst *inst, const struct hfi_inst_ops *ops)
>>>>         init_completion(&inst->done);
>>>>         inst->ops = ops;
>>>>
>>>> -       mutex_lock(&core->lock);
>>>> -       list_add_tail(&inst->list, &core->instances);
>>>> -       atomic_inc(&core->insts_count);
>>>> +       ret = mutex_lock_interruptible(&core->lock);
>>>> +       if (ret)
>>>> +               return ret;
>>>
>>> Why do we change to mutex_lock_interruptible() here? This makes this
>>
>> Because mutex_lock_interruptible is preferable in kernel docs, but I
>> agree that changing mutex_lock with mutex_lock_interruptible should be
>> subject of another lock related patches. I will drop this in next patch
>> version.
>>
>>> function return an error even though we could obtain the lock just by
>>> trying a bit harder.
>>
>> I didn't get that. The behavior of mutex_lock_interruptible is that same
>> as mutex_lock, i.e. the it will sleep to acquire the lock. The
>> difference is that the sleep could be interrupted by a signal. You might
>> think about mutex_trylock?
> 
> Unless that mutex can be held by someone else for a rather long time
> (i.e. to the point where we may want to give priority to signals when
> userspace opens the device, since that's where hfi_session_create() is
> called), I am not convinced this change is necessary? It may confuse

Exactly, if there is a case where the core->lock is taken (firmware
recovery) and it is not unlocked for very long time (deadlock?) then
client process cannot be interrupted with a signal.

> userspace into thinking there was a serious error while there is none.

The client should be able to handle EINTR, right?

> Granted, userspace should manage this case, and from what I can see
> this code is correct, but I'm not sure we would gain anything by
> adding this extra complexity.

The benefit is that if something wrong is happening in the driver the
client process will be killable.

> 
>>
>>>
>>>> +
>>>> +       ret = atomic_read(&core->insts_count);
>>>> +       if (ret + 1 > core->max_sessions_supported) {
>>>> +               ret = -EAGAIN;
>>>> +       } else {
>>>> +               atomic_inc(&core->insts_count);
>>>> +               list_add_tail(&inst->list, &core->instances);
>>>> +               ret = 0;
>>>> +       }
>>>> +
>>>>         mutex_unlock(&core->lock);
>>>>
>>>> -       return 0;
>>>> +       return ret;
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(hfi_session_create);
>>>>
>>>> diff --git a/drivers/media/platform/qcom/venus/hfi_parser.c b/drivers/media/platform/qcom/venus/hfi_parser.c
>>>> index 363ee2a65453..52898633a8e6 100644
>>>> --- a/drivers/media/platform/qcom/venus/hfi_parser.c
>>>> +++ b/drivers/media/platform/qcom/venus/hfi_parser.c
>>>> @@ -276,6 +276,9 @@ u32 hfi_parser(struct venus_core *core, struct venus_inst *inst, void *buf,
>>>>                 words_count--;
>>>>         }
>>>>
>>>> +       if (!core->max_sessions_supported)
>>>> +               core->max_sessions_supported = MAX_SESSIONS;
>>>> +
>>>>         parser_fini(inst, codecs, domain);
>>>>
>>>>         return HFI_ERR_NONE;
>>>> --
>>>> 2.17.1
>>>>
>>
>> --
>> regards,
>> Stan

-- 
regards,
Stan