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Subject: Re: [PATCH v12] i2c: virtio: add a virtio i2c frontend driver
To:     Viresh Kumar <viresh.kumar@linaro.org>
Cc:     linux-i2c@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, wsa@kernel.org,
        wsa+renesas@sang-engineering.com, mst@redhat.com, arnd@arndb.de,
        jasowang@redhat.com, andriy.shevchenko@linux.intel.com,
        yu1.wang@intel.com, shuo.a.liu@intel.com, conghui.chen@intel.com,
        stefanha@redhat.com
References: <f229cd761048bc143f88f33a3437bdbf891c39fd.1625214435.git.jie.deng@intel.com>
 <20210705024056.ndth2bwn2itii5g3@vireshk-i7>
 <332af2be-0fb0-a846-8092-49d496fe8b6b@intel.com>
 <20210705043841.zujwo672nfdndpg2@vireshk-i7>
From:   Jie Deng <jie.deng@intel.com>
Message-ID: <6aabc877-673a-e2bc-da2d-ec6741b4159b@intel.com>
Date:   Mon, 5 Jul 2021 14:22:15 +0800
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On 2021/7/5 12:38, Viresh Kumar wrote:
> On 05-07-21, 11:45, Jie Deng wrote:
>> On 2021/7/5 10:40, Viresh Kumar wrote:
>>> On 02-07-21, 16:46, Jie Deng wrote:
>>> The right way of doing this is is making this function return - Error on failure
>>> and 0 on success. There is no point returning number of successful additions
>>> here.
>>
>> We need the number for virtio_i2c_complete_reqs to do cleanup. We don't have
>> to
>>
>> do cleanup "num" times every time. Just do it as needed.
> If you do full cleanup here, then you won't required that at the caller site.
>
>>> Moreover, on failures this needs to clean up (free the dmabufs) itself, just
>>> like you did i2c_put_dma_safe_msg_buf() at the end. The caller shouldn't be
>>> required to handle the error cases by freeing up resources.
>>
>> This function will return the number of requests being successfully prepared
>> and make sure
>>
>> resources of the failed request being freed. And virtio_i2c_complete_reqs
>> will free the
>>
>> resources of those successful request.
> It just looks cleaner to give such responsibility to each and every function,
> i.e. if they fail, they should clean stuff up instead of the caller. That's the
> normal philosophy you will find across kernel in most of the cases.
>   
>>>> +		/*
>>>> +		 * Condition (req && req == &reqs[i]) should always meet since
>>>> +		 * we have total nr requests in the vq.
>>>> +		 */
>>>> +		if (!failed && (WARN_ON(!(req && req == &reqs[i])) ||
>>>> +		    (req->in_hdr.status != VIRTIO_I2C_MSG_OK)))
>>> What about writing this as:
>>>
>>> 		if (!failed && (WARN_ON(req != &reqs[i]) ||
>>> 		    (req->in_hdr.status != VIRTIO_I2C_MSG_OK)))
>>>
>>> We don't need to check req here since if req is NULL, we will not do req->in_hdr
>>> at all.
>>
>> It's right here just because the &reqs[i] will never be NULL in our case.
>> But if you see
>>
>> "virtio_i2c_complete_reqs" as an independent function, you need to check the
>>
>> req. From the perspective of the callee, you can't ask the caller always
>> give you
>>
>> the non-NULL parameters.
> We need to keep this driver optimized in its current form. If you see your own
> argument here, then why don't you test vq or msgs for a valid pointer ? And even
> reqs.
>
> If we know for certain that this will never happen, then it should be optimized.
> But if you see a case where reqs[i] can be NULL here, then it would be fine.
> ot the driver. And we don't need to take care of that.


This is still not enough to convince me.  So I won't change them for now 
until I see it

is the consensus of the majority.

Thank you.