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Subject: Re: [PATCH v2 2/8] firmware: arm_scmi: introduce protocol handles
To:     Cristian Marussi <cristian.marussi@arm.com>
Cc:     linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        sudeep.holla@arm.com, lukasz.luba@arm.com,
        james.quinlan@broadcom.com, Jonathan.Cameron@Huawei.com,
        f.fainelli@gmail.com, etienne.carriere@linaro.org,
        vincent.guittot@linaro.org, souvik.chakravarty@arm.com
References: <20201028202914.43662-1-cristian.marussi@arm.com>
 <20201028202914.43662-3-cristian.marussi@arm.com>
 <ceda764f-6cd9-9e47-edc7-2e915c920301@linaro.org>
 <20201104174427.GB24640@e120937-lin>
From:   Thara Gopinath <thara.gopinath@linaro.org>
Message-ID: <745d52d0-8578-6a25-c55e-e628d970e9fe@linaro.org>
Date:   Fri, 6 Nov 2020 11:26:44 -0500
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On 11/4/20 12:44 PM, Cristian Marussi wrote:
> Hi
> 
> On Wed, Nov 04, 2020 at 11:16:18AM -0500, Thara Gopinath wrote:
>>
>> Hi Cristian,
>>
>> On 10/28/20 4:29 PM, Cristian Marussi wrote:
>>> Add basic protocol handles definitions and helpers support.
>>> All protocols initialization code and SCMI drivers probing is still
>>> performed using the handle based interface.
>>>
>>> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
>>> ---
>>>    drivers/firmware/arm_scmi/common.h | 61 ++++++++++++++++++++++++++++
>>>    drivers/firmware/arm_scmi/driver.c | 64 ++++++++++++++++++++++++++++++
>>>    2 files changed, 125 insertions(+)
>>>
>>> diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
>>> index b08a8ddbc22a..f0678be02a09 100644
>>> --- a/drivers/firmware/arm_scmi/common.h
>>> +++ b/drivers/firmware/arm_scmi/common.h
>>> @@ -151,6 +151,67 @@ int scmi_xfer_get_init(const struct scmi_handle *h, u8 msg_id, u8 prot_id,
>>>    		       size_t tx_size, size_t rx_size, struct scmi_xfer **p);
>>>    void scmi_reset_rx_to_maxsz(const struct scmi_handle *handle,
>>>    			    struct scmi_xfer *xfer);
>>> +
>>> +struct scmi_xfer_ops;
>>> +
>>> +/**
>>> + * struct scmi_protocol_handle  - Reference to an initialized protocol instance
>>> + *
>>> + * @dev: A reference to the associated SCMI instance device (handle->dev).
>>> + * @xops: A reference to a struct holding refs to the core xfer operations that
>>> + *	  can be used by the protocol implementation to generate SCMI messages.
>>> + * @set_priv: A method to set protocol private data for this instance.
>>> + * @get_priv: A method to get protocol private data previously set.
>>> + *
>>> + * This structure represents a protocol initialized against specific SCMI
>>> + * instance and it will be used as follows:
>>> + * - as a parameter fed from the core to the protocol initialization code so
>>> + *   that it can access the core xfer operations to build and generate SCMI
>>> + *   messages exclusively for the specific underlying protocol instance.
>>> + * - as an opaque handle fed by an SCMI driver user when it tries to access
>>> + *   this protocol through its own protocol operations.
>>> + *   In this case this handle will be returned as an opaque object together
>>> + *   with the related protocol operations when the SCMI driver tries to access
>>> + *   the protocol.
>>> + */
>>> +struct scmi_protocol_handle {
>>> +	struct device *dev;
>>> +	const struct scmi_xfer_ops *xops;
>>> +	int (*set_priv)(const struct scmi_protocol_handle *ph, void *priv);
>>> +	void *(*get_priv)(const struct scmi_protocol_handle *ph);
>>> +};
>>
>> So scmi_xfer_ops are the ops that actually talks with the scmi firmware on
>> the other end , right ? IIUC, these ops are the same for all the protocols
>> of a scmi instance. Imho, this struct is not the right place for these ops
>> to reside.You are inadvertently exposing scmi internal details to the client
>> drivers. There is no reason why this should be part of scmi_handle. The
>> protocols can extract it from the handle during protocol_reigster, right?
>>
>> So, now to the second part, why do you need a scmi_protocol_handle? Again
>> IIUC, if you have set_priv and get_priv hooks and get_ops and put_ops hooks,
>> there is nothing that scmi_protocol_handle is providing extra, right? As
>> mentioned in the comments for last patch any reason all of this cannot be
>> rolled into scmi_protocol?
> 
> The basic idea for protocol_hande existence is that the protocol code
> should be able to access the core xfer ops (without EXPORTing all
> scmi_xfer ops) but protoX should NOT be allowed to mistakenly or
> maliciously build and send protoY messages: since the protocol_handle
> for protoX is embedded in a specific protocol_instance in this way you
> can call from your protocol code something like:
> 
> ph->xops->xfer_get_init(ph, ...)

I am still confused by this one... scmi_protocol_instance has a pointer 
to scmi_handle. So why not handle->xops->xfer_get_init(pi, ....). Here 
also protoX will not be allowed to send protoY messages, right? And then 
again set_priv and get_priv can be moved to scmi_protocol_instance right ?


-- 
Warm Regards
Thara