linux-kernel - Re: [RFC PATCH 6/8] firmware: arm_scmi: add initial support for power protocol

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Date:   Thu, 8 Jun 2017 13:06:48 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        devicetree@...r.kernel.org, Roy Franz <roy.franz@...ium.com>,
        Harb Abdulhamid <harba@...eaurora.org>,
        Nishanth Menon <nm@...com>
Subject: Re: [RFC PATCH 6/8] firmware: arm_scmi: add initial support for power protocol

On Thu, Jun 8, 2017 at 11:39 AM, Sudeep Holla <sudeep.holla@....com> wrote:
>
>
> On 07/06/17 21:38, Arnd Bergmann wrote:
>> On Wed, Jun 7, 2017 at 6:10 PM, Sudeep Holla <sudeep.holla@....com> wrote:
>>
>>> +struct scmi_msg_resp_power_attributes {
>>> +       __le16 num_domains;
>>> +       __le16 reserved;
>>> +       __le32 stats_addr_low;
>>> +       __le32 stats_addr_high;
>>> +       __le32 stats_size;
>>> +} __packed;
>>> +
>>> +struct scmi_msg_resp_power_domain_attributes {
>>> +       __le32 flags;
>>> +#define SUPPORTS_STATE_SET_NOTIFY(x)   ((x) & BIT(31))
>>> +#define SUPPORTS_STATE_SET_ASYNC(x)    ((x) & BIT(30))
>>> +#define SUPPORTS_STATE_SET_SYNC(x)     ((x) & BIT(29))
>>> +           u8 name[SCMI_MAX_STR_SIZE];
>>> +} __packed;
>>
>> I think it would be better to leave out the __packed here, which
>> can lead to rather inefficient code. It's only really a problem when
>> building with -mstrict-align, but it's better to write code in a way that
>> doesn't rely on that.
>>
>
> I assume you are referring to above structure only and not general
> across all the structures ? I will have a look at this one.

I meant all of them, from my first look they all seem to have natural
alignment on all members anyway. If there is one that doesn't, I would
suggest annotating the individual unaligned members with __packed.

>>> +static int
>>> +scmi_power_domain_attributes_get(struct scmi_handle *handle, u32 domain,
>>> +                                struct power_dom_info *dom_info)
>>> +{
>>> +       int ret;
>>> +       struct scmi_xfer *t;
>>> +       struct scmi_msg_resp_power_domain_attributes *attr;
>>> +
>>> +       ret = scmi_one_xfer_init(handle, POWER_DOMAIN_ATTRIBUTES,
>>> +                                SCMI_PROTOCOL_POWER, sizeof(domain),
>>> +                                sizeof(*attr), &t);
>>> +       if (ret)
>>> +               return ret;
>>> +
>>> +       *(__le32 *)t->tx.buf = cpu_to_le32(domain);
>>> +       attr = (struct scmi_msg_resp_power_domain_attributes *)t->rx.buf;
>>
>> It seems you require a similar pattern in each caller of scmi_one_xfer_init(),
>> but it seems a little clumsy to always require those casts, so maybe there
>> is a nicer way to do this. How about making scmi_one_xfer_init() act
>> as an allocation function and having it return the buffer or a PTR_ERR?
>>
>
> Yes I agree it doesn't looks all nice. I have changed these few times
> while developing and then thought it's better to get some suggestions. I
> am open to any suggestions that will help to make these nicer.
>
>> It also seems odd to have it named 'init' but actually allocate the scmi_xfer
>> structure rather than filling a local variable that gets passed by reference.
>>
>
> It does initialise but partially. scmi_one_xfer_get does pure allocation
> while scmi_one_xfer_init initialise header variables and also tx/rx
> size. But if you think it's odd, I will looks at ways to make it better.

Yes, I'm still thinking about it, but I think we can do better. If a function
has both allocation and initialization parts in it, I would probably name
it *_alloc() rather than *_init().

What is the relation between scmi_one_xfer_get() and
scmi_one_xfer_init()? Do we need both in some callers, or
just one of the two?

       Arnd