linux-kernel - Re: [PATCH v2 6/7] mailbox: bcm-flexrm-mailbox: Set msg_queue

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Date:   Thu, 27 Jul 2017 09:25:28 +0530
From:   Anup Patel <anup.patel@...adcom.com>
To:     Jassi Brar <jassisinghbrar@...il.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Scott Branden <sbranden@...adcom.com>,
        Ray Jui <rjui@...adcom.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Devicetree List <devicetree@...r.kernel.org>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>
Subject: Re: [PATCH v2 6/7] mailbox: bcm-flexrm-mailbox: Set msg_queue_len for
 each channel

On Tue, Jul 25, 2017 at 9:37 PM, Jassi Brar <jassisinghbrar@...il.com> wrote:
> On Tue, Jul 25, 2017 at 11:11 AM, Anup Patel <anup.patel@...adcom.com> wrote:
>> On Mon, Jul 24, 2017 at 10:06 PM, Jassi Brar <jassisinghbrar@...il.com> wrote:
>>> On Mon, Jul 24, 2017 at 9:26 AM, Anup Patel <anup.patel@...adcom.com> wrote:
>>>> Hi Jassi,
>>>>
>>>> Sorry for the delayed response...
>>>>
>>>> On Fri, Jul 21, 2017 at 9:16 PM, Jassi Brar <jassisinghbrar@...il.com> wrote:
>>>>> Hi Anup,
>>>>>
>>>>> On Fri, Jul 21, 2017 at 12:25 PM, Anup Patel <anup.patel@...adcom.com> wrote:
>>>>>> The Broadcom FlexRM ring (i.e. mailbox channel) can handle
>>>>>> larger number of messages queued in one FlexRM ring hence
>>>>>> this patch sets msg_queue_len for each mailbox channel to
>>>>>> be same as RING_MAX_REQ_COUNT.
>>>>>>
>>>>>> Signed-off-by: Anup Patel <anup.patel@...adcom.com>
>>>>>> Reviewed-by: Scott Branden <scott.branden@...adcom.com>
>>>>>> ---
>>>>>>  drivers/mailbox/bcm-flexrm-mailbox.c | 5 ++++-
>>>>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/mailbox/bcm-flexrm-mailbox.c b/drivers/mailbox/bcm-flexrm-mailbox.c
>>>>>> index 9873818..20055a0 100644
>>>>>> --- a/drivers/mailbox/bcm-flexrm-mailbox.c
>>>>>> +++ b/drivers/mailbox/bcm-flexrm-mailbox.c
>>>>>> @@ -1683,8 +1683,11 @@ static int flexrm_mbox_probe(struct platform_device *pdev)
>>>>>>                 ret = -ENOMEM;
>>>>>>                 goto fail_free_debugfs_root;
>>>>>>         }
>>>>>> -       for (index = 0; index < mbox->num_rings; index++)
>>>>>> +       for (index = 0; index < mbox->num_rings; index++) {
>>>>>> +               mbox->controller.chans[index].msg_queue_len =
>>>>>> +                                               RING_MAX_REQ_COUNT;
>>>>>>                 mbox->controller.chans[index].con_priv = &mbox->rings[index];
>>>>>> +       }
>>>>>>
>>>>> While writing mailbox.c I wasn't unaware that there is the option to
>>>>> choose the queue length at runtime.
>>>>> The idea was to keep the code as simple as possible. I am open to
>>>>> making it a runtime thing, but first, please help me understand how
>>>>> that is useful here.
>>>>>
>>>>> I understand FlexRm has a ring buffer of RING_MAX_REQ_COUNT(1024)
>>>>> elements. Any message submitted to mailbox api can be immediately
>>>>> written onto the ringbuffer if there is some space.
>>>>> Is there any mechanism to report back to a client driver, if its
>>>>> message in ringbuffer failed "to be sent"?
>>>>> If there isn't any, then I think, in flexrm_last_tx_done() you should
>>>>> simply return true if there is some space left in the rung-buffer,
>>>>> false otherwise.
>>>>
>>>> Yes, we have error code in "struct brcm_message" to report back
>>>> errors from send_message. In our mailbox clients, we check
>>>> return value of mbox_send_message() and also the error code
>>>> in "struct brcm_message".
>>>>
>>> I meant after the message has been accepted in the ringbuffer but the
>>> remote failed to receive it.
>>
>> Yes, even this case is handled.
>>
>> In case of IO errors after message has been put in ring buffer, we get
>> completion message with error code and mailbox client drivers will
>> receive back "struct brcm_message" with error set.
>>
>> You can refer flexrm_process_completions() for more details.
>>
>>> There seems no such provision. IIANW, then you should be able to
>>> consider every message as "sent successfully" once it is in the ring
>>> buffer i.e, immediately after mbox_send_message() returns 0.
>>> In that case I would think you don't need more than a couple of
>>> entries out of MBOX_TX_QUEUE_LEN ?
>>
>> What I am trying to suggest is that we can take upto 1024 messages
>> in a FlexRM ring but the MBOX_TX_QUEUE_LEN limits us queuing
>> more messages. This issue manifest easily when multiple CPUs
>> queues to same FlexRM ring (i.e. same mailbox channel).
>>
> OK then, I guess we have to make the queue length a runtime decision.

Do you agree with approach taken by PATCH5 and PATCH6 to
make queue length runtime?

>
> BTW, is it a practical use case that needs to queue upto 1024
> requests? Or are you just testing?

Yes, we just need bigger queue length for FlexRM but we
choose 1024 (max limit) to avoid changing it again in future.

Regards,
Anup